A place to be (re)educated in Newspeak

Saturday, October 31, 2009

Atomic Install

A few months ago, I wrote a post about the become: operation in the Smalltalk family of languages. The post elicited a good discussion. One of the interesting points that came up, was that you could implement become: by changing the class of objects, provided all the changes were atomic - changing the class, modifying the object’s schema accordingly, and copying the data between the objects.

Setting the discussion of become: aside, I observe that in general, reflective change should be atomic. What I mean by this, is that one should be able to group an arbitrary number of distinct reflective changes to the program, and install them into the running program all at once, in one atomic operation. I will use the term atomic install to refer to this ability.

Oddly enough, this point is not well understood. Most languages that support reflective change do not provide a way for atomically performing a set of changes.

Smalltalk provides a reflective API centered around classes. You can ask a class to modify itself in a variety of ways - adding or removing variables, methods or changing its superclass. There is, however, no way to change several classes at once.

The CLOS APIs are similar in this respect.

Scripting languages don’t really have a reflective API as such. Instead, the reflective changes may come about as the result of executing some code (e.g., an assignment may add a variable). In other cases, the reflective capacity of the language comes about directly from exposing the data structures of the language implementation back to the language.

In all these cases, one cannot perform a set of program changes as an atomic unit.

Why do you need to atomically apply a set of changes instead of sequentially applying one change after another?

One reason is that you’d like to apply the changes as a transaction. If a change fails (say, because you created a cycle in the class graph, or duplicated an instance variable) you don’t want any further changes to take place. It’s easier if you don’t have to catch exceptions etc. after each step.

Another reason is that the changes are often dependent on each other. Applying one change without the other leaves your program in a broken state. The fact that intermediate states may be inconsistent even though the overall set of changes is correct means that it isn't sufficient to wrap a series of reflective changes in a transaction.

Of course, most people don’t rely on reflective modification to develop their programs. Rather, they suffer through the classic edit-compile-run-debug cycle. In the absence of anything better, you typically edit source code in files. You then compile these files and load the resulting program. This actually has one big advantage: the load is atomic - all the changes that resulted from your edits are loaded as a unit.

Evolving your program reflectively has the advantage that you can make corrections to the running program. Often, this is done during fix-and-continue debugging. Even then, in most cases, the “program” in question is an application that is stopped in the debugger, and you can apply fixes sequentially. But as noted above, it’s still easier if you can apply the changes as a transaction.

More interesting cases are when you are modifying a program, and need to run it between the individual modifications. Unlike most people, I run into this often, when modifying the IDE I am using. However, there are additional situations of this nature.

Long lived applications that must provide continuous service have this flavor. Erlang allows you to replace an entire module as a unit in these situations. In Java, you use class loaders to get the desired effect; it’s complicated, but it’s your only way out. A scenario I'm especially interested in is software services: the service updates applications on the fly without shutting them down - and in particular, I may want to update the update mechanism itself.

If you can’t make all the changes in one go, you find that you have to break the transition into a series of steps, each of which leaves the system in a consistent state while leading to the desired final program. This is tricky and error prone: you need to apply the right changes in just the right order.

Scripting languages like Ruby often go through a series of program changes during program start up. Different modules are loaded, modifying the program in the process. This process is also order dependent, and therefore brittle. In many cases, this sort of reflective change isn’t actually essential; rather it’s an artifact of the language semantics. However, I suspect there are situations where it is used to real advantage.

Overall, one can live for a long time without the ability to atomically apply a set of program changes. And yet it seems that there are some situations where atomic install seems to be very useful. It also has another advantage: batching the changes is more performant. Often one doesn’t care, but it doesn’t hurt to be faster, and on some occasions it actually can matter. One might as well see if one can come up with a reflective API that supports atomic install.

In Strongtalk, the VM supported atomic install as a primitive operation in the VM. More recently, in the latest Newspeak update release, I added an atomic install facility written entirely in Newspeak. Exactly 372 lines of code (whitespace and copious comments included). It is a tribute to the Squeak design that this is doable without any privileged access to the VM .

Tangent1: Thanks to Peter Ahe and Eliot Miranda for the discussions that led to this scheme; and to Lars Bak, for the discussions that led to the original notion of atomic install.

Tangent2: Squeak’s existing mechanism for changing classes, the ClassBuilder, is, on the other hand, rather unattractive. It’s three times as long, vastly more complicated, and provides only a subset of the functionality. It shows how tricky this kind of reflective change can get if you don’t conceptualize it the right way.

Naturally, the actual atomic step here is done using a variant of become:. Specifically, it’s a one-way become: on an array of objects. Given two object arrays a and b, both of size n, all references to a[i] are changed to refer to b[i], for i =1 .. n.

The atomic installation process nice and simple. The input is a list of mirrors describing mixins, and a namespace listing which of those mixins already exist in the system. The namespace parameter is crucial BTW, as there is no global namespace in Newspeak.

We then produce a fresh set of mixins based on the input list. For any existing mixins, we locate all classes that are invocations of those mixins, and all subclasses thereof. We make fresh versions of these as well, reflecting the changed mixins involved. For each such class, we locate all its instances (does your system have allInstances?) and produce new instances based on the new descriptions. Each new object is associated with the old object by keeping them in parallel arrays. Then we just do the become: and we’re done.

Unlike Smalltalk, in Newspeak we never have to recompile code that hasn’t been modified at the source level - because Newspeak code is representation independent.

It’s clearly harder to do atomic install in a system that does sophisticated JIT compilation and inlining - but as I said, Strongtalk supported it (albeit at the VM level, largely because of the extar complexity involved) in the mid-90s.

How do we get this kind of reflective power on mainstream platforms? It is harder, because the widely used platforms don’t support the needed abstractions as well as Smalltalk VMs do.

At least on the web browser, I expect to be able to get similar effects with Javascript, albeit with a very different implementation.

In Java, the absence of the necessary primitives tends to force one to build one’s own custom object representation, which is costly in both developer time and machine time.

Ironically (but not coincidentally), the bulk of the necessary machinery already exists in the Hotspot JVM, which is capable of changing at least method implementations on the fly (via JVMDI), including deoptimizing compiled code that may have inlined a method that has been modified. The problem is exposing it to user - and especially exposing it securely. Mirrors can help here - but that is for a future post.

On .Net, the DLR helps one construct one’s own custom representation. Conversely, there’s little support for deoptimization etc. in the CLR itself.

Of course, one goal of this post is to encourage implementors to add such support, and do it in the right way.

Tuesday, October 06, 2009

An Image Problem

Imagine a library, say in Java, that provided a universal facility for saving the exact runtime state and configuration of any application. It would essentially capture the heap, all threads and their stacks, and save them to a file.

You could then load this file from disk and reconstitute your application just as it was when it was saved: For example, windows would re-open just where they were (adjusting their location if the screen size differed). The system would cope with open file handles and sockets in a reasonable fashion, re-opening them if they were available. All this would work irrespective of the underlying platform, and it would be fast as well.

The facility would be transparent to the programmer - you wouldn’t need to change your program in any way, beyond calling the library function. Pretty neat.

I don’t know of such a facility in Java or .Net. However, Smalltalk has had such a beast for over 30 years. In Smalltalk parlance, it’s called an image. Most Smalltalks are inseparably coupled to the notion of such an image.

Tangent: There are exceptions of course, notably Strongtalk.

The idea hasn’t caught on. Why? Should it catch on, and if so, how?

To be sure, the image is a very powerful mechanism, and has significant advantages. For example, in the context of the IDE, it gives the ability to save and share debugging sessions.

One advantage of images is rapid start-up. If your program builds up a lot of state as it starts, it can be intolerably slow coming up. This is one the problems that killed Java on the client. One should avoid computing that state at startup; one easy way to do this is to precompute most of it and store it in an image. It is typically much faster to read in an image than to try and compute its contents from scratch.

The problem begins when one relies exclusively on the image mechanism. It becomes very difficult to disentangle one’s program from the state of the ongoing computation.

If your program misbehaves (and they always do) and corrupts the state of the process, things get tedious. Say you have a test that somehow failed to clean up properly. In a conventional setting, you fix the problem with your code, and run the test again. With an image, you have the added burden of cleaning up the mess your program made.

In general, traditional environments tend to force you to start over with each run of the program: the edit-compile-link-load-throw-the-application-off-the- cliff-let-it-crash-and-start-all-over-again cycle, in the words of the original Java whitepaper.

Tangent: That whitepaper is a masterpiece of technical rhetoric, and was visionary in its day. Alas, Java never fully realized that vision.

The thing is, sometimes it’s good to be able to start afresh. It may be easier to start from scratch than to mutate your existing process.

Mega-tangent: incidentally, this is an argument for sexual reproduction as well.

Of course sometimes starting anew isn’t so nice: think about fix-and-continue debugging.

In some cases it is even more critical to separate your code from the computation. You often save your image just to save your program. It may take you a while to find out that your image has been corrupted. Now you need to go back to a correct image, and yet you need to extract your code safely from the corrupt image. To be sure, Smalltalk IDEs provide a variety of tools that can help you with that, but I have never been really happy with them.

Tangent: this is where irate Smalltalkers berate me about change sets and logs and Envy and Monticello etc. etc. Sorry, I don’t think it’s good enough.

In general, Smalltalk makes it hard to modularize one's code, and especially to separate the application from the IDE. The exclusive reliance on the image model greatly aggravates these difficulties.

Traditional development tools, primitive as they often are, naturally provide a persistent, stateless representation of the program. In fact they provide two: the source code, in a text file, and a binary.

Semi-tangent: source code seems the most obvious thing in the world; but traditional Smalltalk’s have no real syntax above the method level! Classes are defined via the evaluation of reflective expressions, which rely on the reflective API. This is very problematic: the API often varies from one implementation to another. By the way, this is one of the ways Newspeak differs from almost every Smalltalk (the late, great Resilient being the only exception I can recall). Newspeak has a true syntax. Furthermore, because Newspeak module declarations are fully parametric in all their external dependencies, they can be compiled at any time in any order - unlike code in most languages (say Java packages) where there are numerous constraints on compilation order (e.g., imports must be defined).

A binary is a stateless representation of the program code, but one that does not require a compiler to decode it. Smalltalk doesn’t usually have a binary form. Code is embedded in the image. There are exceptions, and some Smalltalk flavors have ways of producing executables, but the classic approach ties code and computation together in the image and makes it very hard to pry them apart.

None of this means you can’t have an image as well as a binary format. What is important is that you do not have just an image. Ideally, you have images and a binary format. This is one of my goals with Newspeak, and we are pretty close.

In Newspeak, serialized top level classes can serve as a binary format. I will expand on how serialization can serve as a binary format in an upcoming post. At the same time, we continue to use images, though I hope they will become much less central to our practice as time goes by.