Re: [xwiki-devs] [proposal] Introduce a new persistence engine.

Hi, I have been working hard on filesystem attachments and I found that synchronizing manual filesystem transactions with automatic database transactions was a difficult job and I needed a new tool to do it. So I wrote what I am proposing to be the next XWiki Persistence Engine. I'll start off with the fun part of the proposal, I have been calling it xwiki-store so far but I am so excited about the capabilities of this engine that I don't think it does it justice to name it "store" after the place on the corner with milk and eggs. I am proposing it be named "XWiki Persistence Engine", the directory will be renamed xwiki-persistence, the artifact name xwiki-core-persistence, and the package name org.xwiki.persistence. Persistence is an attribute of castles, mountains and redwood trees which I think is fitting for a conservatively designed storage engine. Now a little explanation of what I'm so excited about: The common and error prone way of saving things in the database is to open a transaction, enter a try clause, do something then commit. If we catch an exception, then we rollback. something like this: begin transaction; try { do something; do something else; commit; } catch (Any exception which may occur) { rollback; } There are 3 things which can go wrong. 1 we forget to begin the transaction, 2 we forget to commit and 3 we do not rollback properly. What makes things worse is often the database will "assume we meant to..." and things will work ok most of the time which makes things much worse because bugs will hide very well. My answer to this problem is a class called TransactionRunnable. It provides a set of 5 empty methods to override: onPreRun(), onRun(), onCommit(), onRollback(), and onComplete(). the exact circumstances under which each are called is documented in the javadoc comments here: http://svn.xwiki.org/svnroot/xwiki/contrib/sandbox/xwiki-store/xwiki-store-… I wrote TransactionRunnable twice, I wrote it, used it for attachments, then after having real experience as a user, I wrote it again. To repeat our original example with TransactionRunnable you might say this: public class DoSomethingTransactionRunnable extends TransactionRunnable { public void onRun() { do something; do something else; } } Now we can use another TransactionRunnable which opens and closes the transaction for us. StartableTransactionRunnable transaction = new HibernateTransactionRunnable(); new DoSomethingTransactionRunnable().runIn(transaction); transaction.start(); the runIn() function allows us to run one TransactionRunnable inside of another. Supposing we wanted to reuse "do something else" in other places, we can make it a separate TransactionRunnable and use the runIn() function to hook it to our DoSomethingTransactionRunnable ie: public class DoSomethingTransactionRunnable extends TransactionRunnable { public DoSomethingTransactionRunnable() { new DoSomethingElseTransactionRunnable().runIn(this); } .. The only limitations on running TransactionRunnables inside of one another are they cannot run more than once and they cannot call themselves (this would be an infinite loop). This pattern makes each job which is done on storage easily isolated and, as I have so far experienced, trivial to test. However, it still leaves the possibility that we might forget that DoSomethingTransactionRunnable must be run inside of a hibernate transaction. I have devised a solution for this too. Using generics, I offered a means for the author of a TransactionRunnable to communicate to the compiler what other TransactionRunnable their runnable must be run in and without explicit casting or defining of an intermediary runnable, this requirement cannot be violated or else it wouldn't compile! Finally we have the issue of starting the runnable. Who's to say I won't be tired one day and just write new DoSomethingTransactionRunnable().start() without opening a transaction first? If DoSomethingTransactionRunnable cannot be safely run outside of a transaction all it needs to do is not extend StartableTransactionRunnable and it won't have any start function. I have taken a multitude of very easy mistakes and given the author of a TransactionRunnable the tools to make it very hard for the user to make them. Also, since a TransactionRunnable has no reason to be very long (it can just branch off into another runnable) this will make testing and code review easy in the place where it is most important. This part of the code is entirely generic and has no dependence on hibernate or anything else. I propose we move: contrib/sandbox/xwiki-store/xwiki-store-transaction/ to: platform/core/xwiki-persistence/xwiki-persistence-transaction And I will propose moving each additional piece in the coming days. WDYT? Caleb