{"action":"created","comment":{"url":"https://api.github.com/repos/opf/openproject/pulls/comments/40182453","id":40182453,"diff_hunk":"@@ -98,6 +98,8 @@ gem 'gon', '~> 4.0'\n # don't require by default, instead load on-demand when actually configured\n gem 'airbrake', '~> 4.1.0', require: false\n \n+gem 'transactional_lock', git: 'https://github.com/finnlabs/transactional_lock.git', branch: 'master'","path":"Gemfile","position":4,"original_position":4,"commit_id":"79e72e7556b4bfe02309d079a6afea1604718a68","original_commit_id":"d787b427192b2633c2a9b42255917aee2dd787bc","user":{"login":"NobodysNightmare","id":453584,"avatar_url":"https://avatars.githubusercontent.com/u/453584?v=3","gravatar_id":"","url":"https://api.github.com/users/NobodysNightmare","html_url":"https://github.com/NobodysNightmare","followers_url":"https://api.github.com/users/NobodysNightmare/followers","following_url":"https://api.github.com/users/NobodysNightmare/following{/other_user}","gists_url":"https://api.github.com/users/NobodysNightmare/gists{/gist_id}","starred_url":"https://api.github.com/users/NobodysNightmare/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/NobodysNightmare/subscriptions","organizations_url":"https://api.github.com/users/NobodysNightmare/orgs","repos_url":"https://api.github.com/users/NobodysNightmare/repos","events_url":"https://api.github.com/users/NobodysNightmare/events{/privacy}","received_events_url":"https://api.github.com/users/NobodysNightmare/received_events","type":"User","site_admin":false},"body":"Okay, after some back and forth, I've now left the gem where it was, because it does not depend on any specific database backend itself, there is no gem dependency.\r\n\r\nIt will (as of today) only work correctly for a MySQL backend, but this is no design choice, but rather a limitation of the implementation (of the gem).","created_at":"2015-09-23T09:00:04Z","updated_at":"2015-09-23T09:00:04Z","html_url":"https://github.com/opf/openproject/pull/3482#discussion_r40182453","pull_request_url":"https://api.github.com/repos/opf/openproject/pulls/3482","_links":{"self":{"href":"https://api.github.com/repos/opf/openproject/pulls/comments/40182453"},"html":{"href":"https://github.com/opf/openproject/pull/3482#discussion_r40182453"},"pull_request":{"href":"https://api.github.com/repos/opf/openproject/pulls/3482"}}},"pull_request":{"url":"https://api.github.com/repos/opf/openproject/pulls/3482","id":44427899,"html_url":"https://github.com/opf/openproject/pull/3482","diff_url":"https://github.com/opf/openproject/pull/3482.diff","patch_url":"https://github.com/opf/openproject/pull/3482.patch","issue_url":"https://api.github.com/repos/opf/openproject/issues/3482","number":3482,"state":"open","locked":false,"title":"Ensure that concurrent version updates play well together","user":{"login":"NobodysNightmare","id":453584,"avatar_url":"https://avatars.githubusercontent.com/u/453584?v=3","gravatar_id":"","url":"https://api.github.com/users/NobodysNightmare","html_url":"https://github.com/NobodysNightmare","followers_url":"https://api.github.com/users/NobodysNightmare/followers","following_url":"https://api.github.com/users/NobodysNightmare/following{/other_user}","gists_url":"https://api.github.com/users/NobodysNightmare/gists{/gist_id}","starred_url":"https://api.github.com/users/NobodysNightmare/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/NobodysNightmare/subscriptions","organizations_url":"https://api.github.com/users/NobodysNightmare/orgs","repos_url":"https://api.github.com/users/NobodysNightmare/repos","events_url":"https://api.github.com/users/NobodysNightmare/events{/privacy}","received_events_url":"https://api.github.com/users/NobodysNightmare/received_events","type":"User","site_admin":false},"body":"## OpenProject work package\r\n\r\nhttps://community.openproject.org/work_packages/21300\r\n\r\n## Description\r\n\r\nThis PR adds locking around the assignment of a journal version. This ensures that there are no intermittent `INSERT`s into the journals table between determining the latest version of a journable and actually adding the next version.\r\n\r\n### PostgreSQL\r\n\r\nFor PSQL we can use a `SHARE ROW EXCLUSIVE` lock on the journals table. While held, this locks prevents:\r\n\r\n* concurrent `INSERT`/`DELETE`/`UPDATE` statements\r\n* concurrent acquisition of a `SHARE ROW EXCLUSIVE` lock (i.e. it is self-exclusive)\r\n\r\nThe lock is released upon `COMMIT` or `ROLLBACK` of the surrounding transaction, which is neccessary, because otherwise a concurrent transaction would not be able to see the changes of the lock bearer and therefore still try to insert an outdated value for `version`.\r\n\r\n### MySQL\r\n\r\nOpposed to PostgreSQL, in MySQL there are no locks that are released at the end of a transaction. Table locks of all kinds implicitly commit running transactions (and starting a transaction releases all table locks).\r\nAdvisory locks (the ones without meaning for the DBMS, but usable by applications) at least don't commit your transactions, but you must release them independently of your transactions.\r\n\r\nThis means we can use advisory locks to implement self exclusiveness, but we have to release it after our transaction finished (`COMMIT` or `ROLLBACK`) manually. Since the code starting the transaction should not know about the implementation details of the code relying on the advisory lock (those are oftentimes different parts of our application), I extracted the release-on-transaction-end behaviour into a separate gem called [transactional_lock](https://github.com/finnlabs/transactional_lock).\r\n\r\nNote: Please see the transactional_lock gem as a part of this PR and include it in your review.\r\n\r\nAfter review, we might want to publish it like any regular gem on rubygems.org and use it from there instead of using a git 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