Hopefully we can make it a bit less dread-inducing for you. This is Don't Panic, disaster planning for the rest of us. My name is Ronan Dowling. I'm currently the agency tools lead at Pantheon. I was the developer and maintainer, I'm the maintainer of Backup and Migrate, a backup module for Drupal. I had also led me to create Node Squirrel, which is an offsite backup service for Drupal, which was purchased by Pantheon, which is why I am now at Pantheon. So I've been thinking about and worrying about Drupal recovery for eight years, so I've done a lot of thought about it. Hopefully I can help with some advice or just perspective on it today. Who are you? The intended audience for this session is Drupal site owners and builders who are working on small to medium sites. That is not to say that if you are doing enough for these categories, this won't be interesting, it might be, but I am not going to be aiming at a very high level of technical knowledge. This is not for DevOps specialists or sys admins. If you know how to do all of these things yourself and build them all and script them all and get them all working, you've probably already done it. And if you're working on large enterprise sites, most of this advice is probably a bit too basic. So, but if you have running either for yourself or for clients, small to medium size Drupal sites, and you know you need to do more to be prepared for downtime, this is going to be a very accessible way to add some protection, a very low cost, easy way to do it. Before we talk about disaster recovery, we should probably define disaster. So over this specific case, I am defining it as any event which prevents your website's content from reaching your end user. Drupal is a content management system. That's what it's there for. The content is what we need to do. The reason we store the content is usually to get it to the end user. If anything interrupts that, it's a disaster. Now that is not a disaster on the grand scheme of things with all the things happening in the world today. To call that a disaster for a small website is a bit strong. So this is really more of a major bummer. So by the end of this session, we're going to have written a major bummer recovery plan. But I'm not just being dramatic. Disaster recovery is a term of art and a disaster recovery plan is a thing. So that's why I use the term. But whenever you see disaster, whenever I mention it, just think major bummer recovery plan. If we go to Wikipedia, they would be happy to tell us the disaster recovery plan is a documented process or a set of procedures to recover and protect business IT infrastructure in the event of a disaster. Which is a lot of words to add almost nothing descriptive beyond what's in the title. But it will tell us that most of them come down to three basic features. They have preventative measures, detective measures and corrective measures. Past, present, future. That's kind of useful. So I'm going to keep those in mind. Those will come up later. We're going to break things down by those three steps. There is not a ton of disaster recovery advice on the web for websites, specifically for websites. But that which is there usually breaks it down to three simple steps that you can do to create your plan. First step is identify all of the threats to your site. Anything that might happen that might interrupt operations. Anything that might constitute a disaster. Figure them all out, write them all down, prioritize them by likelihood and by impact. Plan the recovery of each threat. Write down a plan for when each of those happens and what your duty respond. And practice. Run through dry runs of your recovery plan so that when you have to do them in action you won't be doing them for the first time and you know they'll work. And this is actually, it's very good advice. It's solid industry standard advice and if you are doing disaster recovery planning for FEMA, you should do this. You should know what happens when the water rises a certain level in certain cities. You should know specifically what threats exist to the things you're protecting and how to deal with them specifically. But it's a bit much for your cupcake blog. It's a lot of work for a small website and which means if that's the level, if that's what you have to do to be ready for a disaster you're just not gonna do anything.  Which is where most of us are with our smaller clients or personal websites. And one of the reasons is step one, listing the risks. I had an attempt, I put together a list. We can go through them, it's interesting. It's good to know what's out there. Natural disasters, that's always the big one. That's the one that's used as an example in every disaster recovery presentation and blog post and scare tactic, that's the big bugaboo. Fact of the matter is if you're running a small website you're usually so far removed from the physical hardware that there's nothing you can do about a storm or a hurricane. You just don't, your web host needs to prepare for a hurricane on the coast but you're probably not going to run in and pull the server and bring it up above water. For small websites, this one isn't really a threat we deal with. Hackers, that's more realistic. Every site has a risk of being hacked. Whether that's an intrusion, password stolen, site defaced or denial of service where they don't need any access, they just need to point too many people at your box and now your legitimate end users can't get at your content. As a small site owner you're less at risk than a high profile website but the risk is absolutely there. User errors, this is actually the big one. If you are, if anybody here is in a web agency I can guess that probably 80 to 90 plus percent of the disaster recovery you have done is in response to something that was done by one of your clients. Somebody who has legitimate access to the site and was trying to do their job and deleted something they shouldn't have. This is the big one for us but it's not something that's usually listed in a disaster recovery plan. Imperfect services, smaller sites, usually not hosted on high availability, expensive, redundant hardware. They're usually hosted on cheap stuff and those go down and there are balances to be made and downtime is one of the things you deal with. That's a real risk for a small website and success. That's a selected denial of service but in a good way. They can be pretty rare but that's also the worst time to go down is when you go viral and everybody wants to see your website. It's good to be ready for those. That's a disaster, that's a denial of service. And dot, dot, dot, that's what makes this so hard. We could sit here all day and we could come up with a hundred more things that could break our website and we still wouldn't be done. And there's no way we could then write an individual plan for each of those even if we could categorize them, prioritize them and calculate their impact and likelihood. So who cares? Who cares why the website went down? Who cares? At the end of the day, whether a hacker broke into your website and deleted your home directory or a squirrel chewed through a cable in Chicago, your website's gone, your users can't get at the content and your recovery might be the same either way. It may not matter why it's down or where it's gone as long as you know how to get it back. Maybe we don't worry about the why so much. So I created a less intimidating approach. The major bummer recovery plan, which is identify all of the pieces that can fail. Not the why they're gonna fail but just all of those pieces that are necessary for your content to get from your author's brains to your user's brains. Figure out how to replace them, not to recover them, not to restore them, not to repair them but how to replace them. The only scenario that we are actually going to plan for is the one where your server or your database or whatever piece or more has disappeared off the face of the planet with no sign and no warning. Because if you can recover from that, if you can recover from the alien abduction scenario, you can recover from any scenario. There is nothing that can defeat you if you know how to replace every part and you know how long it's going to take you. And practice, that is still good advice. So we kept that one. So let's do it. Let's figure out all the things that can fail that can make our content not available to our end users thereby causing a disaster. Here they are, there's only six of them. Six things, that's all you have to worry about. If you're on the more medium end of the scale of site that we talked about, you may have a few more. You may have load balancer, you may have reddish cash or solar. I didn't put some of those periphery pieces in because I know you have these. You have all six of these. You may or may not have the other ones. If you do, feel free to add a couple more but most of the advice I give you for these will apply. So I'm not going to address those. But these are the six. If you were able to replace every one of these on short notice and you know how long it's going to take and you know who to contact to do it, you're bulletproof. You will have downtime but you'll get back up again. So here they are. I drew a terrible network diagram to describe the relationship. Not to scale, not realistic but it just sort of shows you the six, lets you count them. The purple cloud is not our problem. So it's purple. Or zoom in a little bit because that one was hard to read. The domain, it's the first thing you do when you want to build a website you need a domain. So you go to migratebusiness.ie. You go to buy it. You go to your registrar. You pay them anywhere from nine to $100 or euros and they say, yep, you own that now. They go engage in the DNS magic. We've nothing to do with that. We can't help that. And they tell the world that this authoritative domain name server is in charge of that domain and you are in charge of that name server. So those are two pieces that could fail. Those are two pieces you're responsible for. You need to make sure you know how to get them back. Registrar, I don't really know how to replace a registrar. If GoDaddy goes away, there's probably a process for reclaiming the domains that you own. It's probably involved, it's probably complicated. It's probably not something you want to practice. Even though I'm telling you you need to be able to replace all these parts and practice this, that one's a bit unrealistic. But it's the beginning of the process. You at least need to make sure that you are in full control of your registrar. You can access it at all times and then you just go back to hoping it doesn't go out of business and take you down with it. Name server, much more in your control. That is needed on basically every request. So that's where we are going to pay some attention. And then your users come along, these two nice people. They use more DNS magic. I dropped the cloud this time, but they eventually find out from your name server what the IP is for your web server. The contact web server, the web server uses a Drupal that you've uploaded. The files your content authors have created and the database to create a response, return it back to them. That's the other pieces. Those are the pieces that can fail. And if any one of those goes away, so does your website. So there they are. Green and blue, you're responsible for. Purple is somebody else's problem. The blue is blue because it's iffy. And we'll get to that a bit later. So back to our measures. If we, for following Wikipedia, we know that we need preventative measures, detective measures and corrective measures. We need to see it coming or we need to prevent it. We need to see it coming and we need to get back when it happens, whatever the disaster or bummer it might be. Because of the twisted, the way I'm twisted the question here, we're mostly going to worry about corrective measures. This is to figure out how to replace them part of the plan. We're not going to ignore preventative or detective, but no matter how many preventative measures you put in place, you will not be 100% free of downtime. So spend some time, do what you can, but don't sweat it. Detective measures will let you start your recovery faster, but they're not going to recover anything any more thoroughly or any better than you would if you didn't have them. Again, it's cheap, it's easy to do them, but really what we're concentrating on is replacing. So we'll deal with the other two, preventative measures. There are some basic steps you can do ahead of time to lower the chance of downtime. Again, it's never going to be zero. So we're not going to worry too much about it. These are all pretty basic. I bet you're all doing them. So I'm not going to spend too much time here. Security best practices. Do your best to prevent hackers getting into the first place. You don't have to worry about this a bit less. Use good vendors, smaller sites. Again, come with lower cost hosts, lower cost vendors, and those come with more downtime. That's what you pay when you've got enterprise redundancy, when you've got, when you're paying $200,000 a year for hosting, that's going to stay up a lot more than when you're paying $60 a year from Bluehost or whoever. But while that's great advice, and I stick to that advice, if you really are talking about a small website with low budget, the fact that you, by the end of this, you will know what it will take to replace every one of your vendors in an emergency, what it'll cost, and how long it'll take, you might be able to cheap out. Because you might be willing to accept a half a day of downtime for your non-critical site in exchange for hundreds or thousands of dollars worth of savings. And you'll be able to actually quantify it and say that half a day will cost me X. I feel like it might happen once a year, might not. You take that risk. So while I would always say, you know, spend a little upfront to avoid the headache, if you're going to have a high risk setup, a non-redundant setup, it's good to know what the cost, the actual cost will be if something goes wrong. And then, yeah, cheap out, who cares. Building redundancy, redundancy is expensive, so I'm not gonna dwell on that, but wherever you can, wherever you can find cheap ways of having a backup, a hot spare, an easy replacement, look for it. You'll always be thinking about, it's more to maintain, so it's not, even if it's free, it's not free, but something to think of, something to keep your eyes open for, and it'll be more clear once you've practiced replacing something.   Things like that, that's preventative. There are some tools we can use, cheap and free, which is great, that's what we like for cheap disaster recovery. CDN, these used to be the domain of very expensive high-end websites. They are no longer, they are now free or very, very cheap. We normally think of these as a performance enhancement or, you know, distribution for world. They also do offer us some disaster recovery advantages. Cloudflare and Fastly are the two I've picked on. There's a few more, these are the big ones. Cloudflare, at least, they have different services, but they do offer some protection from hackers. They will try and detect malicious access attempts and just block them, so that's really nice. That's preventative. They will prevent DDOS by either detecting malicious traffic from locations you're not usually receiving traffic and just blocking them all together, or unintentional DDOS, you get that slash dot or that reddit hug and they can help keep your site up during those spikes, which is great. So that's preventing the downtime.  They'll give you a little bit more breathing room if something does go wrong. They will serve old content, day old content when your site's not responding.  Maybe there won't be any downtime at all if you get it done quickly enough, if you've practiced this, if you're okay with a little bit of older content staying up. So that's another little advantage. It's kind of more of a recovery thing than it, but it's great for disaster recovery. So free is the lowest level of Cloudflare. It's almost a no-brainer if their terms and services don't disagree with your site or your client. You can use a third-party host to DNS. The thing about the name servers, it's one of your pieces. It's usually tied, married to a different one of your pieces. You either do it with your registrar or you do it with your web host. That's very, very common. You can't actually outsource this. There are ways to do it. None of these are actually free, but they're all quite inexpensive. They will give you some protection from DDOS. They will do some of that same traffic, about that same traffic analysis.  These guys don't. These guys are very, very, very reliable for the cost. And then there's actual redundancy. When you registered that domain, it gave you two name servers or it told you to put in two name servers and s1.migratesite.com and s2.migratesite.com. But in a lot of cases, if you are using a VPS from a sort of budget host, not only are both of those domains, those name servers pointing at the same network, they're possibly pointing at the same box. And that box might be your web server. So even though you're supposed to pick a primary and a secondary name server for redundancy, you're not actually getting redundancy if you're cheaping out in this case. This will let you have actual redundancy and inexpensive redundancy. And that's what we're looking for. It is one more point of failure. And that's one more thing to be monitoring. It's one more thing to be checking. It's one more thing that can go down. So I don't necessarily say that this is a no-brainer. You might wanna have a reason for this. I will say if you're choosing between your web host or your registrar for hosting your name server, I would go with registrar over web host, unless you were really, really, really, really cheaped out on a registrar and found somebody really iffy. The chances are the major guys have pretty good uptime. They are hosting a lot of domains. If they go down, your site is probably at least to your worries. Also, if your web server goes down, it can't be replaced if it's also tied in with the network where your name server is. Replacing the web server requires switching your name server to point to a new web server. So it's harder. Again, I'm punting on the registrar a bit, but as long as you're going to trust them, trust them with both. On the other hand, if you took my advice and installed Cloudflare, that's now your DNS. So this whole slide is moot. They're detective measures. We can figure out what's happening before we start getting angry calls from clients. So subscribe to Security Advisories. Know what's coming down the line, what risks you might be at. If you stay abreast of that, you're going to have a better job fighting it. Set up Twitter alerts when your site goes down. If you've got any kind of traffic at all, you're going to hear about it from your loyal and now angry followers. If you're doing this for clients, set up alerts for their names too, just so that you know, so that when you get that call, you're not super surprised. Audit site users and content is a hard practice to be in because there's not really a lot of great, simple, easy tools. This is actually hard work. I'm trying to create a system here that involves no hard work and certainly no ongoing hard work because if you were to come away, if I was to give you a plan that required you to do something every week, it would get done done. So, but maybe if you've got an admin who's in every day, maybe you make a habit of checking the user list and the node list for irregularities because that can be an indication of something getting corrupted. Somebody who's in there that they shouldn't be, it's good to keep in heads up. It's a bit more manual, but it's usually not a lot of effort and set up automatic monitoring, which is basically the crux of our detective tools. Cheap and free tools that will let us figure out when we have downtime as quickly as possible. Uptime monitoring, this is the one. This is the big daddy of web monitoring tools, probably already using them because this is not new news. If in case you aren't, these things go and visit your website periodically, usually every few minutes, try and load your homepage. If the site's down, you get an email. Simple as that. It is that simple. It's so simple that there are literally thousands of these services. They start at free. I don't think Pingdom has a free plan anymore, but it is very cheap. It's extremely worth the money if you care at all about downtime because you'll find out sooner and that's all there is to it. You can step it up a little bit. Application monitoring. These services will check the health of the server in a deeper way than simply is it up or is it down. It'll do resource usage, memory, time of response, all of these things. And that will let you detect problems before they happen a little bit. If you're starting to see things starting to spike and things are starting to get slowed, it's not down, it's not working, you can sort of dig in and figure out before there's a disaster. They do have to be installed on your server. These all come with very, very expensive contracts, which is why five years ago I probably would never have included this in a session like this, because I'm looking for cheap, easy to set up, no brainer solutions. However, these are now included because talk to your host, you may be getting them for free. Acquia gives you the light version of New Relic for free. Pantheon gives you the pro version. I've been hearing around the floor lots of other people giving away free versions of these things or maybe it's like stripped down version or it's a full version or it's limited time, but you may have it and not know it. You're not gonna get it with the super, super cheap host, but it's worth asking and it's worth knowing it's there. And then the meat of it, the corrective measures. What do we do when it happens? We've got our six pieces, one or more of them disappeared. It was abducted, it's gone, we can't find it. We can't rely on it to help us get the new one up. So what do we need to do? How do we do that? Well, obviously, the reason I'm here, backup. Backup is redundancy for your data. All of your work, this is informational technology. So all of your work results in data. Some of it can't be reproduced. Some of it would take a long time to reproduce. And if your data doesn't exist in three places, it doesn't exist in all, at all. That's the old adage. So that means back it up, copy that backup. And then you know when you have to recover that at least the data is there, seriously. So what do we back up? Well, that's the question. It breaks down, again, quite simply, to four different things. There's four pieces you will need to back up if you want your site to come back up.  They're separate because they have different levels of difficulty of backup, different levels of frequency of change and different levels of difficulty of restore. So you probably don't want to treat them all the same. You don't want to have one backup to rule them all. You need to treat these individual parts as separate backupable data. If you want to have the easiest way, the easiest and most success, so we'll go through those. Server configuration, HPI and I, NGINX, DNS, zone files, anything that's unique to your website. If it's standard out of the box in Fedora, you might not need to back it up because you can get that back pretty easily. But if it's a MIME file change in your patchy configuration, you need that for your website. That's data, it needs to be backed up somewhere. The good news is it almost never changes. For most sites, especially smaller sites, we do this once, it starts running. We don't touch it again until something breaks and that's usually because of some security update at some point. It's not too hard to recover with that backup. We don't spend dozens and dozens and dozens and dozens of hours configuring servers for small sites. So if we have to do it again from scratch, we usually can, it's stressful. We don't all love doing it. If we're using cheaper hosts, it can be a bit tough to get those changes in.   It is difficult to back up because each of those changes usually goes to a different file, possibly on a different piece of hardware, possibly that you have limited access to if you're not running your own hardware. You have to ask your host what their plan is for backup. How do we get this configuration back? You need to know, but you also wanna make sure you're keeping your own records of any custom configuration, whether you're going through a managed host, a shared host, or if you have a systems operations person you work with at a school or at a client or whatever. Every time you ask them to, hey, can you bump the memory on PHP? Can you install this Apache module that's on every other server but somehow wasn't on this one? Every time you have to do that, you need to keep a record of that, keep it together, because you may be asking for all of those things again, and you may be asking for them in an emergency. So you don't wanna have to go remember what was the thing we have to do to make that one module work on this one site. Just keep a record, keep it safe, and then you know how to get back from scratch. And the other thing you can do is adopt DevOps. This is what the track is. You're taking all of that server configuration that we only state to do, all those buttons that we had to check, all those things that were impossible to back up, and you're turning them into code which we know how to deal with. So that might be a bit of a big ask for small websites, but it's a great future because it means we can treat our server configuration like code. And code is easier. It changes rarely. Not much more frequently than server configuration, but for small websites, you're probably not pushing changes every day. You may not be pushing them every month. You may not be pushing them every six months. It changes pretty rarely. It's sometimes possible to recover your code without a backup. It all lives on, most of it lives on Drupal.org. You get it off from GitHub. You might have bought a theme if it really is a very small website. Chances are you have some custom code and some custom beaming, even on the very smallest of websites. So don't rely on not having a backup. It's a huge pain to gather everything and remember what it was and all that. So treat it as something to be backed up, but it's not, for some people it might not be the end of the world. If they're using it pretty out of the box, it always should be in version control. That might be enough for you if you've got version control and you've got hosted version control and you're comfortable that GitHub has more availability and uptime than you could possibly ever need. Maybe you don't have to worry about having a second copy of it. I still would if you can, but this is not sort of end of the world stuff if you're using decent providers. And try and automate your deployments. Again, if you can go to fully integrated DevOps, that would be great, but even if you're just hand managing code, pushing it by FTP, make sure that that's automated because that will mean there are fewer places for you to be the user error. And it also means that in that emergency, when you've got to recover, it's quicker. You've got one button to push, you switch your deployment over, it goes to the new server or it goes to the same server that just got recovered. Makes it a little bit quicker. If you don't have a sophisticated DevOps team setting you up with continuous integration, there's services like deploy bot that literally read your git, push it over to an FTP server, make a log. They're inexpensive, they're well worth it. Uploaded files, these are the files your users create. Videos, images, documents, upload them into Drupal. They don't change that frequently. Usually for smaller sites, people aren't churning out dozens of videos a day. These are pretty low changes. They're pretty difficult to recover without backup. This is, I mean, I should take the issue away here. They're difficult to recover, but they're not always impossible. Remember that there's always two copies of a file when you upload it to Drupal, it doesn't disappear. So it could be a research project to get back files if you're three weeks out of date, but you may only have three or four files uploaded in those three weeks and they may be available. So you need to backup, but you don't necessarily need to be obsessive about it and that's good because they're quite difficult to backup. They usually are, even a small site will have hundreds of megabytes of files that can go up to gigabytes, that could go up to terabytes. That's not easy to backup and restoring is slow. It might be easier to go find the one video than it is to restore gigabytes and gigabytes and gigabytes. Some tools to help you do that, backup and migrate works on files, but it's going to cap out at gigabytes. Our sync will be a little bit more work, but a little bit more efficient and you can tie them all together with custom scripts. The size of your backup directory is going to dictate what exactly you need to do to back it up and how frequently, if you can use something really simple and easy like backup and migrate and do, but if not, then try and have something in place, remembering that it's difficult to recover that backup and then use your database. This is all of the hard work you or your client has done to the website. This is the critical one. It changes very frequently, usually daily, sometimes hourly, sometimes less. It's impossible to recover that backup. If you're a blogger, you're probably writing your blog posts directly into the web and that's the only place that text has ever existed. This is not, this is impossible. If that's gone, you have to rewrite your entire website. So back it up. That's the priority. It's easy to backup. It's a small amount of data. There are lots of tools. Backup and migrate, I wrote it, it's great, use it. It will work for almost every site because your database is so small. You could be backing up hourly and on most places you won't feel the hit of the extra work that's being done. You won't feel the cost of the extra files that are being stored because it's a few megabytes of data. So that should be the most frequent. It should be the easiest, it should be the cheapest. But as well as the pieces we can back up, we can also, we can sort of turn the backup on its side and look at the different layers, the levels I'm calling them, of things that can be backed up. We have server level backup, we have application level backup and we have content level backup. Server level backup means backing up your whole server. It's usually provided by hosts. This is the kind of like C panel backup tool or most hosts have something, a lot of them promise they do, they don't always have it, they don't always have it working, they don't always have it as frequently as they say they're going to. But it will back up the entire server configuration, database, code, files, everything. It should be, it's quite slow to recover. You may need to file a ticket, it may take a week and the first three they try won't work. They'll have to find one that's a month old. So it's dependent on your hosts, cheap hosts are worse, better hosts are good. But it is pretty good for total system failure. If that server is completely wiped out, it's no good just having your files and your database. You kind of want everything to be restored all at once. So, look into it, make sure your host has it or you have it set up, make sure it gets tested once before you need it because it won't be, just because they say they have it, doesn't mean they do. Application level backup, this is a layer above. This is just your database and your files, you control this, so there's no reason not to do it. You're not relying on anybody else. You can recover a database in a couple seconds, which makes it a really easy operation. You don't need to open a ticket, but it only really works. It's best for user error, homepage got deleted, you're gonna back up, you're gonna restore your database and partial failure. If it's not completely dead, if the aliens didn't literally take the server, then you may be able to use an application level backup and that's backup and migrate or PHP might admin or whatever. And then content level backup, we don't usually think of as backup because it isn't, but it helps for disaster recovery and for our context or bummer recovery at least. It's version control, it's revisioning in Drupal. It's in core, turn it on, it's very easy, that's couldn't get any cheaper than that. And it means if someone messes up a page, you can be the hero by reverting it. It's not good for things that aren't entities, which is quite a bit of data and it's not good for deleting because the revisions get deleted too. But have it only because it costs almost nothing to turn it on and just to know that it's there and then be a superhero. It's also important where you back up. This one seems kind of obvious. You can back up onsite, offsite. Onsite backup can be very good. This means backing up to the same server or at least the same network that your site is hosted on. It's quick, it's quick so you can do it every hour and not worry too much about pushing a lot of data over the wire. It's quick, recovery's quick. Quickest around, it literally can take seconds. But it's not good for system failure.  So that's why you need offsite backup as well. This means backing up to a different server. It's slower. It will slow down your website if you do it too much and you're using something like backup and migrate which uses web resources to do the backup. You're gonna have a problem with gigabytes and gigabytes going over the wire every so often. It is more effort to set up because you have to have something, some place to send it to. You have to have that other thing that's offsite and that is independent and that takes set up. But it's there when total and utter disaster comes, when that piece is missing, the backup is not tied to it. Some of the solutions, NodeSquirrel, I built it. It's free, so for smaller sites, I highly recommend that. S3, not free but extremely cheap. So if NodeSquirrel doesn't work for you for whatever reason, it's not that hard to set up S3. It needs a little bit more maintenance because you gotta make sure that bill keeps getting paid. That's another big source of disasters for small sites is the credit card runs out and you forget to renew your domain. Or you can FTP to another host. If you found a cheap host, you can keep running as a hot spare. That's a bit iffy because two cheap hosts is not necessarily better than one cheap host. But if this is the only thing you take away from this backup section of this thing, then I hope that you take it away because offsite backups from your host are not offsite. Even if they tell you they're using S3 and they've got it redundant in six data centers and it's protected from anything that might affect them, if you need to access it through them, if you're reliant on them to get at the backups, then it's not offsite for you. And case in point, NodeSquirrel now owned by Pantheon. If you're not on Pantheon, use NodeSquirrel because it's free and it's awesome and it's offsite. But if you are on Pantheon, don't consider that offsite backup. Even though they are independent systems, we control them both. I mean, I trust us, you shouldn't trust us, right? That's not having independent offsite backup. So I hate to tell you not to use my product, but don't use them both and think you've got two because you don't. There have definitely been pretty high profile incidents of servers going down and backups going with them. So don't remember this part. And then restoring, obviously, what good is backing up if you can't restore?  They're different. They're all different, slightly different restores. Depends on how down your site is. You really can do partial recoveries. You don't have to assume that everything's gone just because someone deleted a Node doesn't mean you should blow up your server and start the whole process of scratch even though I said you can and you're bulletproof. Layer your levels. Server level is going to restore everything but it might be a week old or it might be a month old. Your application level is only going to work after your server gets recovered but it could save you by having data that's an hour old. So make sure you know to go from the bottom up, keep applying backups all the way up the stack until everything looks great. Practice, I know you're not actually going to but feel guilty for not practicing and then time your practice. Again, if you know how long it's going to take you to do that recovery then you've got a better idea of whether it's worth it, whether it's worth spending more for this service or that service or whatever and you can tell your clients, hey, don't make any promises unless you're willing to put a contract in but you can have a good sense of what the recovery's going to look like. And there's a few more little pieces and then we should have time for some questions. There's a few more little sort of wrap up pieces that could help. I've identified six parts of your website in a lot of setups that's four or five different third party services, who you are paying, who have given you a login credentials. I've also recommended about four more so now you're going to come away with seven or eight accounts that all need to be accessible to you while you're panicking. So think about that before you start panicking, which means keep all your logins together. For a particular site, not just for your whole company, not for, you know, not just your key chain manager, keep all the logins for each particular site together. Web host, registrar, CDN, all the stuff I recommended. Make sure it's all in the same place. Make store copy online for when you're on holiday and something bad happens and offline for when something bad happens to the internet. Keep them all together and listed by site. Tech support contacts, all those same services. They, the good ones have good tech support but don't assume that their ticketing system is going to be up when they go down. Don't assume that you're going to be able to look up their phone number when their website goes down. Store their email, store their phone number, store their Twitter, store their status page because it's usually hosted somewhere else. It usually stays up when they go down. That's a good idea for them. Make sure you know where those are without having to go to the homepage of the thing that might be dead right now. Email, password, reset. Each of these accounts, I don't know if any of these have any exception here, so I'm going to assume every single one of these accounts has a user or an email based account and they use password reset via email. That's the standard right now. So you really should know how to do a recovery. If somebody hacks your stuff, you need to get those passwords back if you just forgot them or if they got changed. You're going to find that out at the worst possible time. Have them all go to the same email. They don't have to go rooting around and figuring out who owns the domain and who owns this, who owns Fastly and who owns the server. Have them all go to the same place. Takes a little time to coordinate that but it's well worth it. Don't use a real user's email address. People leave companies. People go on holidays. People get annoyed and people lose their own passwords. And so don't use a real person. Use some address that's designed for this purpose. Don't use an email that uses your website's domain. Your name server is one of the pieces. It's the second piece. If it goes away, you need to be able to replace it and if you can't replace it without getting an email sent to something on that domain, you're in big trouble. So use a different domain. There's nothing to do with the website. There's security issues with forwarding but you need to make sure that anybody who may need to recover, if it's just you, it's just you but if you've got a team, if you've got clients, you need to make sure that anybody who needs to recover can recover and there's no single human point of failure. And since we've just created this absolutely huge security risk by creating one email account that can now give access to every part of your operation to anybody who gets access to it, consider using two factor authentication and everything else you can do to make sure this email is secure because this really is. That's the downside of this convenience. It will be great in an emergency but it means if anybody figures out how to get in there, you've just created the biggest emergency you ever saw. So those four points tell me that you should really use a Gmail account. That's free, some of the best availability of any host and they have free features like two factor off and forwarding and they're not on your domain. Don't use your Google for works account. Use a Gmail account even if it feels cheesy. No, you don't have to tell anybody. And then test resetting your passwords. This is the practice piece. Test it, maybe you see how long it takes to reset a password. If it takes six hours, and you'll be a little less panicked when it takes six hours in an emergency, you'll just know that that's the process. If there's suddenly a security question that sometimes you do the reset and it's asking you a security question as well. I don't know. Make sure you know that ahead of time. And the best part about testing resetting your passwords is now you just rotated your passwords, which you knew you needed to do, but you haven't done it in years. But you just did because you tested it. And there we go. I told you you'd have a written plan. It doesn't look much like a plan. It doesn't have scenarios. It doesn't have responses. But what it does have and it's written down, it's not just you know these things exist.  It's going to have a list of all of the people, all of the parts, all of the services that you employ, either free or paid to make your website run. It's going to have the login credentials. It's going to have support contacts. It's going to have them all together. It's going to have a list of people internally in your company, in your client's company, your brother, whoever's going to help you at this, who's responsible so that there isn't a lot of confusion when things go down. And if you're working for clients, they need to know who to contact. You're probably their contact support.  And you're going to have the location type and frequency of every backup and any gotchas, anything that you may need to know to do a restore on those that wouldn't be obvious. You don't need to get into too much detail, but if you have this written down, it's going to help you a lot in an emergency and you're going to feel a lot better knowing that that's there. And you can write it down and you can leave it there and now pretty much everything I told you to do does not need to be done every week or every month or every year.  So that's my session. Thank you, we will have time. Yeah, we got plenty of time for questions. I do need to remind you to please evaluate this and all of these sessions. And before I go, we got to make sure you guys join us for sprints to their tomorrow. Start at nine, go pretty much all day. So thank you. And if anybody does have questions, the microphone.  And then you said also off-site, on-site don't consider it with the host. So ideally, what are those three places? Well, if it doesn't exist in three places, it doesn't exist at all. So the original data, the data that you care about that's on your Drupal website, that's one. Then you should probably have, for as much as possible, it's not practical for everything. For as much as possible, you should have an on-site backup, something that's on your server, quick to restore. When your client calls, it's not an emergency, they just screwed something up, you just do a restore, and they feel bad about themselves. And then you should have something off-site. And that can be less frequent because it's going to be less critical, it's gonna be used less often. The chances of a disastrous thing happening to your server are a lot lower than something pedestrian-like deletions. So you can consider those time-based. So those would be the three I would do for at least your Drupal database, your files, and your code. Your server config, it's if-year. I don't know where the third place would be, but you should at least have that in two places. Anybody else? All right, well, thank you very much.


we will move on to radiation 2. So we are going to introduce concept of black body. So in the black body what is black body?   We have understood that any body which is at a finite temperature is going to emit radiation. Now the radiation is dependent on various other things that is material of the body whether the body is surface, the surface is rough or smooth because that is going to decide the reflectivity and the absorptivity. And the amount of the radiation emitted by a surface depends on its temperature, material and the roughness of the surface. We will worry about these parameters little later on. But first thing is what we have to consider the ideal case always I take this example in the class in thermodynamic cycles whenever we are teaching we always refer back to Carnot cycle. Why Carnot cycle? Because the efficiency of the Carnot cycle is highest. I cannot get anything more than that that is the ideal situation. Similarly here I have to define an ideal situation or an ideal body which cannot emit any other body should not be able to emit more than this or any other body should not be able to observe more than this that is the ideal situation that is the black body. Black body is serving as a standard. Black body serves as a standard against which radiative properties of real surfaces can be compared. Why because black body? Now let us see what is black body? What is so ideal about a black body? Black body absorbs all the incident radiation which falls onto it regardless of the wavelength whether it is smaller wavelength or larger wavelength all the incident radiation is absorbed by a black body. The thing reflected that is the point what we understand from this and for a prescribed temperature and wavelength no surface can emit more energy than a black body. So although the radiation emitted by a black body is a function of wavelength and temperature it is independent of direction that is black body is independent of direction. There is no directional sensitivity so black body is a diffuse emitter three points that is it absorbs all the incident radiation second point it no body can emit more than black body for a particular wavelength and temperature that is very important and it is not having any directional preference or dependence it is directionally independent. Black body is a diffuse emitter these are the three points which we need to note down for black body. Now the question is do we come across any black body in real life? Yes, yes what is that? That is I think the cavity. Cavity can be I will cover this and then go back to spectral distribution of Planck's distribution that is if I take a large cavity and now let us say there is a small hole in this cavity what is happening whatever is entering into this first let us see how will become a perfect absorber whatever is entering into this whatever radiation is entering into this cavity what should happen? What should it happen? It has to be absorbed if it has to be a perfect absorber that is the first definition of our black body what will happen if it comes into the cavity it undergoes multiple reflections and it gets absorbed completely so nothing is lost so nothing so the cavity can be visualized as a perfect black body. Now how can it become a perfect emitter? Now whatever is coming out in the same same way with the way we argued for absorption same way whatever is getting multiply reflected can also get reflected and get out through this hole so that is why it is called as a perfect I mean it can act as a perfect emitter that is I will rephrase this radiation coming in through the opening area undergoes multiple reflection and thus it has several chances to get absorbed by the interior surfaces of the cavity before any part of it can possibly escape. If the surface of the cavity is isothermal the radiation emitted by the interior surfaces streams through opening undergoing multiple reflections that is why it is diffused in nature number one and it is a perfect emitter number two it has no preference in direction that is why cavity acts as a perfect absorber or emitter and I also take this example always if I have to calibrate heat flux sensors radiative heat flux sensors what do we do? We create a cavity we create a cavity and that is a thin metal foil I try to keep it spherical I try to keep it spherical and apply and try to maintain that at constant temperature and put my heat flux sensor somewhere here this is my sensor. So whatever this sensor is seeing is essentially because of radiation of course this chamber is now evacuated why do I say evacuated because I need to exclude natural convection if there is no medium inside there is no natural convection then this cavity can be considered as a perfect black body against which why I say perfect black body because I can get whatever voltage into current upon the surface area I have this is my net heat flux which is falling on to my body so that is how I can calibrate my heat flux sensor against this black body against this black body so that is one of the examples where in which cavity can be used for calibrating heat flux sensors and cavity can be used as a perfect black body. So with this we will move on and try to understand what is called as Stefan Boltzmann law. So what does Stefan Boltzmann law say that Stefan Boltzmann law says that radiation emitted by a black body per unit time per unit surface area is that is given by sigma t to the power of 4 that is eb equal to sigma t to the power of 4 what is sigma sigma is 5.67 into 10 to the power of minus 8 what is the unit watts per meter square Kelvin to the power of 4 watts per meter square Kelvin to the power of 4. So little later we will understand how did we come across this 5.67 for now we can just take it as a relation which was given by Stefan Boltzmann okay actually this was first found empirically that is through experiments that was found by Stefan and Boltzmann found out this through theory that indeed the sigma is 5.67 into 10 to the power of minus 8. So it came through experiments first and it was explained through Boltzmann statistics through later on by Boltzmann himself and Boltzmann happens to be student of Stefan and Stefan is his teacher. So I just want to stop here for a minute and tell who is Stefan and who is Boltzmann Stefan is an Austrian physicist and of course Boltzmann is also an Austrian physicist and Boltzmann worked with Stefan. Stefan took all the Sun's data whatever was available at that time and did the simple Karoo fitting and got that eb equal to sigma t to the power of 4 but why others could not do this no they could not do this why because in all the experimental data there was conduction losses which was involved he was the first person to measure the thermal conductivity closest to what we have thermal conductivity of air closest to what we have today that is k equal to 0.02635 he got 0.025 because he could measure the thermal conductivity of air he could rule out the conduction losses which were there in the experimental measurement he removed those conduction losses convection losses and got the radiation alone and did the Karoo fitting and he could get that sigma as close to not exactly 5.65 little lower than 5.65 5.67 and Boltzmann derived this as 5.67 and Boltzmann was the student of Stefan so through statistical mechanics or Boltzmann statistics he could arrive with this to this number sigma of 5.67 into 10 to the power of minus 8. So with this we will move on what is the distinction between idealized black body and an ordinary black body so point here is in this transference I will not read this the point here is all black bodies that is black painted bodies need not be black and all white surfaces visible through visible eye whatever white surfaces are visible whatever white surface we see or perceive through eye or not we cannot consider them as non black bodies no snow is a perfect black snow is emissivity is one that is it can act as a perfect emitter so it cannot be thought of as non non black body so whatever we see the color of the surface does not mean anything about idealized black body or all black bodies need not be ideal black body that is the point what I want to tell through this transference okay and we have seen this perfect cavity now what is spectral distribution here we need to go little slow I want to write this relation that is I want all of you to write along with me so I lambda comma B that is I lambda comma B equal to I lambda comma B equal to 2 h c naught squared 2 h c naught squared upon lambda to the power of 5 lambda to the power of 5 upon lambda to the power of 5 lambda to the power of 5 exponential of h c naught by exponential of h c naught by lambda k t minus 1 okay this was given by Planck's distribution that is why it is called as Planck's distribution what is h this is universal Planck's constant I have presented this in the morning 6.6256 into 10 to the power of minus 34 joule second and k k is not thermal conductivity 1.3805 into 10 to the power of minus 23 joule per Kelvin this is universal Boltzmann constant c naught is velocity of electromagnetic wave in perfect vacuum that is 2.998 into 10 to the power of 8 meters per second t is in absolute temperature in radiation one important thing we need to remember is that temperature is always in Kelvin temperature is always in Kelvin if we take degree Celsius we are gone so we are going to give way off from our actual result so with these constants what is that we see from this intensity of radiation coming out from a black body is a function of only wavelength and temperature the function is looking something like this and of course wavelength is sitting here also it is a non-linear function now this was given by Planck now what do we do with this let us plot this so who is Planck of course in radiation there are no many noble prices okay everyone has got a noble price Planck has also got noble price for giving the Planck's distribution so anyway I will not spend too much of time because of velocity of time you but I usually give lot of time for historical perspective in the class but I would request you also to give lot of historical perspective because the students understand only when we give the historical perspective so he got noble price in 1988 for this Planck's distribution now what did he do what did Boltzmann do so e lambda, b lambda, t equal to pi i lambda comma b lambda comma t what does this lambda comma b represent lambda means that intensity is a function of wavelength and what are whose we are talking about we are talking about black bodies and it is a function of wavelength and temperature that is given by just now whatever I had written that is h c naught squared upon lambda 2 h c naught squared upon lambda to the power of 5 exponential of blah blah blah but where did this pi come from pi i lambda comma b we said that black body is a diffuse emitter so emissive power in the morning professor Arun has derived it for us e equal to pi i if it is a diffuse emitter so black body is a diffuse emitter so I can take emissive power equal to pi i lambda comma b and that pi is sitting here so 2 h c naught squared I am calling that as c 1 and h c naught by k I am calling it as c 2 so that it looks little easily it looks amenable little bit so c 1 upon lambda to the power of 5 exponential of c 2 upon lambda t minus 1 so that means what all these constants I know that is c 1 and c 2 I know and they are represented here all that I get is emissive power is a function of wavelength and temperature now let us plot this as a function of wavelength and temperature that these plants distribution so what is this y axis the y axis is emissive power that is wad per meter spectral emissive power sorry it is spectral emissive power e lambda comma b that is wad per meter squared micrometer and x axis is wavelength you see and I am plotting it for different temperatures I said that emissive power is a function of both wavelength and temperature so for different temperatures if you see any temperature one of the things which we see here at any given wavelength emissive power is increasing with the increase of temperature number one and for a given temperature emissive power increases initially there is a maxima and subsequently decreases all these maxima if I connect them then I if I that is a linear line then if I curve fit that I get lambda t that is lambda maximum that is the wavelength that is it is maximum lambda maximum into t is a constant that is what is called as Wien's displacement law lambda maximum t equal to 2897.8 micrometer Kelvin now here I want to stop and I am answering a question perhaps you have this question here when we said in electromagnetic spectrum we said that only infrared waves and visible light and a little bit of ultraviolet light ultraviolet light is responsible for thermal energy that is what we said students usually ask the question why only this why not x-rays no you see here they cannot other than these waves they do not carry any thermal energy there is no emissive power contribution after this wavelength what is the emissive power 0 after this wavelength what is the emissive power 0 that is the reason okay so here we have been told to just do some upload a different file and get back to this file and what I am doing is I am opening radiation one it is for good it is for good in radiation one I want you to show the spectrum that is you see in the spectrum what did we say the thermal radiation blessing in disguise what you said is right I was lazy to go come here actually so thermal radiation is contributed by ultraviolet infrared so this only why these two are contributing for thermal radiation I said I will come back to this question later on this can be answered through Planck's distribution in Planck's distribution in Planck's distribution only within a particular wavelength in a particular wavelength you see that is for all wavelengths lower than around 0.0 what is that 0.08 I is that 0.008 yeah yeah 0.08 and may be about 1000 micrometer the contributions for thermal radiation is negligibly small that is why we do not say that that is why we say that only these waves contribute for thermal radiation this can be explained only through Planck's distribution otherwise it is quite difficult to explain why only these waves are contributing for thermal radiation no matter whatever is the temperature you take an x-ray there is x-rays there is no contribution for thermal energy that is the point you can understand from this Planck's distribution with this with these insights let us move on to of course again Wien also has got Nobel Prize so no so many Nobel Prizes in radiation okay he has also got Nobel Prize in 1911 actually Wien's displacement law was found before then before the Planck's distribution so always student asked why Wien has got although it is a very trivial thing compares to Planck's but he had done that first compared to Planck's distribution that's why Wien has got Nobel Prize okay now if we take this if we take the bull this Planck's distribution and integrate this if you are lost what did we do this is what the emissive power we got by putting the intensity of radiation through Planck's distribution if I integrate this for all wave length what wave length the wave lengths which are contributing for the thermal radiation let me let me go there and get that wave length because I am not able to remember that wavelength but I just want to go and do that otherwise I will be just hand waving that that is 0.3 micrometer to 3 micrometer that is from 0.3 micrometer to 3 micrometer that is if I have to be very clear up to 10 to the power of minus 5 micrometers to oh there is a problem in this transparency okay so everywhere it is 10 to the power of minus 5 so it is between 0.3 to not 3 15 micrometers it is 0.32 I think I can answer this the previous transparency yeah that's right so that is from 0.3 micrometer to around 10 to the power of minus 8 micrometer is the wavelength range in which thermal radiation contribution is there okay so with this we will come back to radiation 2 now what do I do is I will take the spectral distribution and integrate over complete wavelength if I integrate that I am going to get sigma t to the power of 4 that's what I am going to get I think that you can give as an exercise for the students we have not put that you can give that as an integral that integration can be given as exercise that is you take although it is 0 to infinity what my point is it is not 0 to infinity even if you take 0 to infinity you will be getting automatically but it is between only that thermal radiation range band that is 0.3 to 10 to the power of minus 8 micrometer so now if you get that 10 to the power of minus 8 meters not micrometer so if you integrate that you are going to get eb equal to sigma t to the power of 4 that is ib equal to eb by pi this is what is what we use as Stefan Boltzmann now you see why two names are there Stefan found it experimentally Boltzmann did this integration and he could find that this is indeed sigma equal to 5.67 into 10 to the power of minus 8 so this is how we can I think this is the equation which is used all over the radiation okay so with this so professor Arun wants to add some point here see this area eb lambda when you integrate with respect to lambda what do you get you get eb so this area under the curve under each of these curve represents the emissive power of the black body at that temperature so higher the temperature greater is the emissive power and that is logical right body black body or any body black emissive power of black body is easy to understand 500 degree Kelvin it has a certain amount of emission 1000 Kelvin it has a greater emission and it is obvious from this diagram because at 1000 Kelvin it encompasses a larger spectrum of wavelengths associated okay okay so what I would request all the participants is that we will be visiting several centers to the extent possible but limit your questions only to the radiation we are not going to take any questions on convection if you have we will take the questions on convection during tutorial session please restrict your questions on radiation number one because professor Arun is not going to be there in the afternoon number two because subsequently if your questions are clear you will be able to understand all whatever we are going to teach radiation downstream you will be able to understand very easily if you have understood the definition so over to participants for questions sir the so the assumption of diffuse surfaces when is it applicable sir or when does it fail what kind of surfaces will the diffuse assumption fail okay so the question is when does a surface is diffuse or when does a surface is not diffuse that is the question so that surface which is a black body is a diffuse surface all surfaces which are closer to black body are diffuse surfaces see in the definition of the black body itself we said three constraints three conditions one is perfect emitter perfect absorber and diffuse when will it be diffuse I will tell you in a very what to say in a very layman terms if you take how do you make a black surface in your lab let me ask that question you take a plate and typically you make a surface black by making by putting lamb black that is by taking a candle and putting the plate below the candle so that all the suit gets deposited on my plate okay so now after getting depositing after depositing if you just see the surface it looks diffuse okay I shouldn't be using the word diffuse it looks very what to say not shining it is not shining it is not at all shining it looks it it's not it is dull it is dull it is dull why the dullness gives it diffuseness that is it is not having any preference if it is shining then it becomes it is not diffuse so to answer your question all shining surfaces are not diffuse surfaces mostly dull surfaces are diffuse surfaces as long as you paint them with black black paint and there are there are certain black paint for example there is a paint called pyromark if you paint that if you paint that paint if you paint any surface with that paint generally emissivity and absorptivity are equal to closer to one not just one it's roughly around 0.9495 and even with lamb black you are going to get an emissivity of the absorptivity of 0.9596 I am not going to answer you the question emissivity and absorptivity how do I measure when I touch upon emissivity and absorptivity I am going to tell you how do we measure emissivity and absorptivity I have answered you just when does a surface is a diffuse okay sir is it just a surface property or yeah we wake we will we wake we will understand this when we when we get to emissivity and absorptivity emissivity is a function of surface absorptivity is a function of source we will understand this when we go to emissivity and absorptivity yeah hello sir I have question regarding comparison between radiation and conduction suppose I am taking two body at a finite temperature difference A and B now I am taking other losses 0 now if I want to thermal equilibrium in which case in heat transfer by either conduction or heat transfer by other radiation I will get earlier thermal equilibrium condition the question is if I have two surfaces if I have two surfaces and now between the two surfaces in order to reach thermal equilibrium whether conduction makes me reach faster or radiation reaches me faster I cannot answer this in terms of time but all that I can say is that conduction takes place conduction is very good in solid and radiation is very good in vacuum for conduction I said there is a medium required if there is no medium there is no conduction they one molecule has to get in touch with the other molecule if there is no molecule itself then there is no radiation sorry then there is no conduction so for radiation I do not need any molecule for conduction I need a molecule so I cannot compare the way you asked in terms of time from one surface to another surface because from one surface to another surface something has to be filled for the conduction to take place and nothing should be filled for radiation if it is filled there is no radiation or minimal radiation between the two surfaces if there is a media there is a medium then what happens radiation gets stunted because the medium which is there starts participating and conduction overtakes so that is all I can answer in a case of conduction I assume that both the body are in a contact that is precisely what we are saying if the both the bodies are in contact then there is no question of radiation to take place if when they see the question separate case where temperature difference is same the question is between the two plates if I if I have two plates if I have two plates and both are in contact with each other and one plate is higher temperature than the other one is lower temperature which is the mode of the heat transfer which is taking place it is conduction what his question is is that if I have two plates at two different temperatures will bringing them together and keeping them next to each other make them reach thermal equilibrium faster or will radiative heat transfer give the thermal equilibrium faster radiative heat transfer between surfaces depends on several things geometry orientation how far how close are you from each other all these things are parameters if I keep them one meter apart it is going to be much slower than if I keep them one feet apart okay so these things are all very variables actually I will pitch in now actually we will be able to appreciate how much closer or farther when we when we get to resistance concept and when we study view factors will you please hold on until then sir in radius we have defined the term radio city so in that radio city we have taken the emission radiation which is emitted plus the reflected rays so actually in case of a body suppose a suppose a ray is incident on a body so it will be some part of it will be absorbed some will be reflected and some will be transmitted so in case of radio city why do we take only the emission and the reflected rays why don't we take the transmitted rays transmitted has gone away no the question asked is in case of the definition of radiosity for the case of definition of radiosity what we are taking is only the emission plus reflection we are not taking transmission why is it so that we don't account transmission into account where is the transmission going the answer is where is the transmission going transmission is going downwards the surface I am not interested what is going downwards the surface I am interested what is coming out from the surface what is coming out from the surface is only emission and reflection nothing else so transmission is what goes down not what is coming out it is what it is just going inside what is what is transmitted passing through. Passing through the plate I am not interested in that what is getting out of the plate again back is what is I am interested. Because I am interested heat transfer between that surface and another surface. So, that surface is giving out energy and also reflecting something correct. So, this part is what is interacting with the other surface. That is what we need to account for. In fact, we this radiosity is going to be the major potential when we study the furnace problems or the enclosure problems. We are just going to keep the radiosities into account. Those are the potentials for us in radiation like temperature was the potential. Any other questions quickly? Sir, how the radiation energy emitted by a surface is changing with respect to dry and moist surface? Is it place any road for a dry and moist surfaces? How the energy emitted will be? The question, it is little the question asked is if I have a surface dry surface and another surface which is moist surface. How do these two surfaces who are radiation intensity from these surfaces going to get affected? Does it get affected at all or not? It is little premature, but still I will go ahead and answer your question. The emissivity of the moisture is usually closer to 1. That is emissivity of water is 1. In fact, if you take the emissivity of the human body, emissivity of the human body is 1. Why? Because we are all made up of water. So moisture, the plate which is having moisture on it will have higher emissivity compared to that of the emissivity of the dry plate. That is all the answer for your question. Hello sir, good afternoon sir. When we have derived the expression for differential solid angle in the morning, we have taken that area in the hemisphere and drawn the right angle triangle. Whether it is approximation or it is exact right angle, one figure we have drawn on the white board sir. Correct. So the angle subtended at point A, whether it is a exact right angle or it is approximation because that area can be anywhere in the hemisphere sir. This figure, this figure. Here the question is actually the triangle, this is perpendicular sign is missing here. Let me put the pointer. The perpendicular sign is missing here. That is not an approximation. It is indeed right angle triangle. That is not an approximation. The question asked is, is this triangle which we have taken, it is a right angle triangle which is taken, is it an approximation? It is not an approximation. It is indeed a right angle to triangle. I think your question is, because we have drawn it on the surface you are asking. What if the point is on the inside, maybe your, if that is the doubt, then we do not consider this outside hemisphere. We will consider a hemisphere on which this point lies. So still always that will be a right angle. So it will be a smaller hemisphere if you are inside. If you think of the fruit, if you are inside the fruit you will consider the smaller sphere. That is all it is. So the angle is always right angle. The question is, refrigerator backside of the refrigerator is always painted black and the condenser and compressor is always painted black and the air conditioning is not having the same criteria. So is it something related to radiation or it is just the normal convention what we are following? Professor, the question asked is for the always the refrigerator backside is painted black and for evaporators and condensers in air conditioning it is not done so. This is an application oriented question. I have no answer. Please put up this in the model. We will think and get back. I want a question pertaining to basics which are taught since morning. The question is if I take a cavity for defining a black body I took isothermal. That is a very good question. Isothermal body, why do we take it as an isothermal body? Let us say the radiation we said that it is a function of temperature. Now if I take a cavity and in the cavity half of the cavity is half one temperature, another cavity is another temperature let us say 1000 and 300 then there is radiation between one wall to the half of the cavity to the other cavity. No it is not so. We have to take complete cavity as one temperature so that the emission and absorption is same. Otherwise I have involved one more parameter that is the temperature. I do not want to take the temperature as a parameter when I am taking the black body. Another thing is if different parts of the cavity have different temperature interaction we will bring them to a thermal equilibrium eventually. So, it will come and settle down at 750 degree Kelvin or whatever, but it will have to come and settle to a unique temperature isothermally completely then only this thing will be valid otherwise it is no longer correct. One last question from for this session. Question is regarding, yes sir basic question as you told us while answering one of the questions that air is medium is not participating in the radiation effect. As I know that polar gas is like CO2 and NO2 is taking part in the radiation effect. As the air contains such in polar gases why miss air is not a participating is not participating. I think the question asked is the air is combination of carbon monoxide, carbon dioxide, oxygen, nitrogen and all etcetera, but why is it that air is not participating. You see usually the inert gases the air is combined is a combination of nitrogen, oxygen and then carbon monoxide and carbon dioxide. Carbon monoxide and carbon dioxide are participating, but oxygen and nitrogen are thoroughly non participating. So, that is the reason why we can consider that as air as a non participating medium. Major portion that is almost 90 percent of the air is consisting of nitrogen and oxygen. Left out is only carbon monoxide and carbon dioxide the extraneous gases. While quoting that examination as you told that air will get heated up certain temperature. That is because of carbon, air is getting slightly getting heated up because air is having slight amount of carbon monoxide and carbon dioxide because of which it is getting heated up that is all. Otherwise because of oxygen and nitrogen it is not going to get heated. For all practical purposes we can take that air is a non participating medium. That is what we do while designing furnaces for that matter. That is non participating, but it is taking participation in the very minimum. I think we will all break for lunch and come back.


Today I'm going to show you how to get free games on your Nintendo Switch.  You see, we did a similar guide to this back in 2022 but the problem is that Nintendo removed a lot of the quick and dirty ways to get to all of this free content so I'm going to show you the only viable no matter what year we're in ways to get this free content and of course we first begin by going to the Nintendo Switch eShop and remember, remember everything we do is going to be in this eShop to get your free games. We could talk about one other method that actually works but you know what? Because Nintendo keeps changing the quick and dirty methods, I don't really want to focus on that. Alright, so let's get into this method right now and open up the eShop so you will have to have an account already made, a Nintendo account already made. In order to access the eShop you can create Nintendo accounts online. This isn't a guide on how to set up your Switch and create accounts. This presumes you already have a Nintendo account created. If not, there is a little guide that comes with your Switch to show you how to do it. Now once you're in here, there's really a few places we can go to find some free games to play. The first one that we want to mention is a more obvious one and that's by going to the search and browse. Okay, we have two things we're going to do here. First off, we're going to go search by price range and then we're going to go down all the way down. They bury this way at the bottom. There's your free games and you can go over here to view more to see all of them. Now we have a hundred and six free games here and there's a lot of good stuff. I mean Fortnite's obviously one of the most popular games you guys know about that, Fall Guys. We got Rocket League in here, Pac-Man 99, Overwatch 2, Just Dance, Tetris 99, Super Kirby Clash is really fun, etc. There's a whole lot of good games, but because there's a hundred and nine of them, maybe it's hard for you to find exactly what you want. Maybe you know what kind of genres you're into. Well, you can go over here and with the price range at free, you can change it to a different genre. So let's say you're really into all racing games. You want to see if there's any free racing games you can play. So let's go ahead and sort to racing games and there you go. Rocket League, Ash Fault, Mario Kart Live, Home Circuit, which technically requires a purchase by the way. So it's a little tricky that this is in the free software area because it's free to download but without buying Mario Kart Live. Like it's useless, but anyway it's Chocobo, GP Light, etc. And they have this for everything. So you see there's five games in there. Let's say you want to go play some arcadey games. We'll select that and you could find some more arcadey style games. Rocket League fits in multiple categories. You've got Pac-Man 99 sitting there, Brawlhalla, Namco Museum, Super Animal Royale, etc. So it's just kind of a neat way to find free games to play on your Switcher with a hundred and nine titles. That's a lot of content for you to enjoy. Now, what we can do as well is go back to where we were previously and instead of searching my price range, we can also go to games with demos. Now, while these are not obviously free to play games, they are free to try. And as you see, they're 645. And like before, you can sort it by genre, price, etc. But yeah, all these games will have demos. So let's scroll down and find maybe a game like Harvestella. Let's say you're wondering if Harvestella is for you. It is a Switch exclusive. You can go right on in here. And when it opens up, you can go over to here and instead of proceed to purchase, go down one spot to download demo. And then it'll download a demo of the game for you to play. This is a great way to try games before you buy them.  Now, there's one other thing we need to talk about, because this is something that is exclusive to Nintendo Switch Online members. And again, this isn't a guide on how to join Nintendo Switch Online, but it is nice to know. And that is this Nintendo Switch Online section right here. Now what Nintendo likes to do once in a while are these things called game trials. So with a Nintendo Switch Online subscription only costing just 20 bucks per year for the basic, you get access to game trials. And Nintendo doesn't announce them all the time ahead of time enough sometimes they just shadow drop them on social media. So I suggest probably checking out this section once per, I don't know, once per week or something like that, because sometimes they'll give you access to an entire game like say access to, I don't know, all those Splatoon three and they'll give you like a whole free weekend. Yeah, you get this expansion impact content. Also, we got to know you get access the N64 and Genesis are tied to the top tier, but then we have the NES and SNES as well. Again, these are more free games that you can absolutely enjoy. But yeah, there'll be game trials listed in there as well. There's no current game trials running right now as you'll see here. There's no game trials. It'll be in this list when there are game trials. So we don't have any game trials right now. So pretty much in here, you're just going to find your your NES Super Nintendo and if you have to have the bigger package, your N64 and Genesis games that you can play for free as well. Now one last thing I know I said that was it, but you can actually share games with friends. So all you need to do is have a friend basically add their account to your account.  Now, you are going to want to create a new user when you do this. I'll show you quick, create a new user to select anything you want. I don't know, let's say we got picked before coming out. This isn't going to matter because when you connect it to a Nintendo account, it's actually going to change everything. So let's just go who remembers the Wii? Let's just call this account Wii. Alright, so nothing matters now because this is the point we get to where you link a Nintendo account. For those who don't know, your Nintendo Switch account can be used across multiple switches as can your friends. So let's say your friend has a bunch of digital games because this is what we're talking about. Physical games are still, hey, ask a friend, borrow a physical game to you. But if someone has a huge digital library, a friend of yours, someone in school, a parent, whatever, you can have them link their Nintendo account on your system, and you will be able to access their entire digital library of games.  Thankfully, a lot of games have multiple save files, you don't have to overwrite anyone's save data, but it does give you access to their games. The big thing is you can't play the same game at the same time. So let's say they have a digital copy of Breath of the Wild, you both can't play Breath of the Wild at the same time. But it is a way to share free games between everyone. Anyways, that is my updated guide for 2023 on how to download free games. If you guys found this useful, I would appreciate it if you liked the video, subscribe, have any questions, leave them down in the comments below, and I will definitely respond to as many as I can. If there's any confusion, I want to thank you guys so much for checking out Nintendo Prime's video today. And I'll catch you later.


I'm Aaron Holman, 12 Whittemore Street and a town resident for 23 years. I oppose Article 35 because it's a big step towards entirely eliminating Arlington's shrunken industrial area, turning it into housing and turning Arlington into a bedroom only community. I have four related concerns which I hope you share. Growing Arlington's business sector, creating more decent paying jobs in town, creating more affordable housing and limiting Arlington's ever growing dependence on its financially burdensome residential property tax. In every one of these aspects, Article 35 goes in the opposite direction or is significantly inferior to better alternatives. Article 35 has a glib language about allowing restaurants breweries wine bars and artists. Despite that Arlington's article 35 primary purpose is to turn Arlington's industrial area into another residential district, and that its primary use as measured by space will be for relative market rate housing. Any business use, first floor or otherwise, will be a flimsy facade. The purpose of an industrial area is to provide space for business activities that are incompatible with housing to mix in housing violates that purpose. And once housing is built there, any complaints by residents about business activities will trump the otherwise legitimate needs of businesses. This won't happen all at once. Instead, the residential camel will start by poking its nose into the industrial tent, a bit at a time, and then intruding itself more and more. Here's how it also shows how its proponents tried to sneak in housing. The original article mentioned residential housing only as a footnote. This was unacceptable and explicit language was added. However, the explicit language allowed twice as much residential space as light industrial use light industrial. That's the glib category name for restaurants wine bars etc. There's not too much by any standard and really does make the area primarily residential. It was too blatant, and it was turned back to allow only as much space as light industrial, not twice as much. This sounds better, but if you read on there's a loophole. All the redevelopment board needs to do is to find out that the lower limit is financially infeasible that's a quote for the developer and the ARB can raise that residential limit back up to twice as much as for business. Presto residential at last. This is sneaky and underhanded, because it gives the redevelopment board the discretion to allow what was previously removed, precisely because it was objectionable, and any developer will easily be able to cook up numbers that support a claim of financial infeasibility. So article 35 drafting and current languages flawed and you should vote against it, but that brings up to bigger issues. Why is Arlington's industrial area so underutilized and what kind of future Arlington do you want. Consider our low cost business space around a wife costs far less than space and Kendall Square or the seaport and Arlington space costs even less than a wife. We're nearby. We're less cramped. We have good infrastructure. So why is there such a lack of interest in our space. Two reasons. The landowners have not been interested in developing their land for business. They have let their land sit vacant for decades, waiting and wanting the kind of zoning change you now have before you. The other reason is that Arlington's town Arlington's town government has passively acquiesced making insincere, half hearted attempts to plan for these sites, as if all of a sudden the town, not the landowner could decide what would go there. They largely consist of plans for again housing with little or no business. Why follow the money. It does that because that's the easiest way to simply let more revenue roll in town budgets keep going up and keeping town officials well paid. Of course, that can't go on forever. But hey, that's problem for the future. Even a bedroom only community as this would facilitate is both economically and socially unhealthy for Arlington economically our share of property tax revenue from business, only 5% is way less than that of surrounding towns, typically 20 to 50% excessively depend on residential property tax that raises the cost to those who would move here makes diversity as a goal less achievable, and tends to push out longtime residents who can no longer afford it. How many of you are now paying $10,000 per year or more. Those of you who aren't soon will. Don't you deserve a break today. Socially it's unhealthy. Most jobs in Arlington are low paying retail jobs. Over 90% of Arlington residents have to travel elsewhere for work. Thus, people who work here can't afford to live here, and most people who live here can't afford to work here. This makes a mockery of the language in our master plan which describes Arlington as a town where residents live, work and play. Again, I urge you to vote against article 35. Should an amendment be proposed which would strip out its language allowing residential use, you should vote for that, but only as a way to mitigate the damage that article 35 would cause. Again, I urge you to vote against article 35. Thank you for listening.