[xwiki-devs] Transaction API (was Re: [proposal] Introduce a new persistence engine.)

On 01/19/2011 12:13 PM, Vincent Massol wrote: Yes, this proposal only covers the transaction sub-module of the persistence engine. The so far un-proposed modules include xwiki-store-serialization, xwiki-store-filesystem and a legacy attachment storage module: xwiki-store-filesystem-attachments.

Anything which requires transactions could use the TransactionRunnable although I'm at a loss to think of anything other than storage which would require transactions. I don't like to propose an interface until I have tried to implement it. Also I do not like to propose an implementation until I have tried to use it. At this point it's far enough off that I would rather wait than propose APIs blind. My experience with attachment store has shown that what we want is a set of functions which provide TransactionRunnables to do various things: aka: TransactionRunnable<T> getDocumentSaveTransactionRunnable(XWikiDocument toSave); In a hibernate implementation it would return TransactionRunnable<HibernateTransaction> and in a JCR it would return TransactionRunnable<JCRTransaction>. We cannot have APIs like this until TransactionRunnable is agreed upon these will return instances of it. IMO we should change the persistence engine and implement the same schema, once the persistence engine is rebuilt, then we can consider modifying the schema. The schema is a specification, it may not be perfect but it is something to comply with. It is important to me that a we prove that a new persistence engine is able comply with existing specifications before we start designing new ones around it.

Everything proposed still holds true but I did add 2 new features. 1. There is a way for a TransactionRunnable<DBTransaction> to be passed an instance of DBTransaction using a new method called getContext(). 2. There is a new class which serves what I believe is an edge use case. Suppose you want to define a TransactionRunnable (we will call it YourTransactionRunnable) which must run inside of a DBTransaction but it must _also_ run after an instance of MyTransactionRunnable. You can make MyTransactionRunnable a "ProvidingTransactionRunnable<DBTransaction, MyInterface>" and then MyTransactionRunnable must run inside of a DBTransaction and we define YourTransactionRunnable as a TransactionRunnable<MyInterface>. This also allows MyTransactionRunnable to share information since YourTransactionRunnable.getContext() will provide an implementation of MyInterface. Of course this feature must be used with care as it provides the tools to write horrible constructs but IMO it is the type of feature which when you need it, there is no other way around. Caleb > > Thanks > -Vincent > > On Jan 10, 2011, at 2:15 PM, Caleb James DeLisle wrote: > >> 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