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All of what BrowserUk says is true, although most journaled filesystems will limit your liability when using regular files. Couple this with (at least on Linux) sync-ed writes (which you *don't* want to do a lot of, as they are dreadfully slow), and you might get by. A transactional DB is better, but you do have to remember that the two-phase commit is designed to ensure that multiple operations on the DB itself are either all done, or not done (ie rolled back). When part of what you are trying to 'commit' has nothing to do with the database (ie, transition a server to a new state), then you are still not atomic. In BrowserUk's example, if you
  1. INSERT the command msg
  2. perform the command
  3. commit the INSERT
but the system crashes before step 3 completes, the DB will rollback the INSERT, but the DB has no knowledge of the command you performed. You would have to take the additional step of looking at the DB's transaction log (which many DBs allow you to record in a readable format). Upon crash recovery, if you see a 'command rollback', you would want to check the state of the execution of that command, and try to 'roll that back' too...


In reply to Re^2: Atomic Config Updater with History? by Illuminatus
in thread Atomic Config Updater with History? by pileofrogs

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