On Jan 20, 2011, at 10:12 AM, Denis Gervalle wrote:
> On Thu, Jan 20, 2011 at 09:23, Vincent Massol <[email protected]> wrote:
>
>> Hi Caleb,
>>
>> On Jan 19, 2011, at 7:54 PM, Caleb James DeLisle wrote:
>>
>>>
>>>
>>> On 01/19/2011 12:13 PM, Vincent Massol wrote:
>>>> Hi Caleb,
>>>>
>>>> I see you're excited, that's good! :)
>>>>
>>>> Some general comments:
>>>> * This looks more like a design for a transaction module than for a
>> persistence engine. I don't see anything related to persistence in your
>> proposal below. Your proposal could work on stuff others than storage,
>> right?
>>> 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.
>>
>> Ok I was misled by the title of your mail "Introduce a new persistence
>> engine". I guess it could/should have been "Introduce a generic transaction
>> API independent of the underlying storage implementation", right?
>>
>> So if we focus purely on the transaction part here are some questions:
>>
>> * Why don't we use existing standards such as JTA/JTS? See
>> http://en.wikipedia.org/wiki/Java_Transaction_API
>> * If we were to define our own API, would we be able to implement it using
>> JTA, ie is it a higher level transaction API than JTA?
>> * Imagine we decide to use JCR as the storage implementation, how would
>> this transaction API integrate with it knowing that JCR integrates with JTA?
>> (http://www.day.com/specs/jcr/2.0/21_Transactions.html)
>>
>>> 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 was expecting to see some Store/Storage/Persistence interfaces with
>> proposed APIs and explanation on how they could be implemented both with
>> Hibernate and JCR for example. And the relationship with the proposed new
>> Model defined.
>>> 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.
>>>
>>>> * I was also expecting a strategy defined to migrate users from the
>> current implementation of the storage to the new one
>>> 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.
>>
>> I don't quite agree with this since it assumes this schema is universal
>> which it definitely is not. It's only a schema that works with RDBMS. It
>> wouldn't work with an ODBMS or a file system implementation.
>>
>
> The proposal of Caleb is not RDBMS related at all. He first use it for the
> filesystem persistence of attachement.
I was responding to Caleb's point about keeping the schema...
>> Also if there's one thing we shouldn't care about it's the schema. It's
>> supposed to be opaque for the user and the user has to use the storage API
>> to access it (and never go directly to the DB). In other words we should be
>> clear that the schema is not part of the API since that would prevent any
>> modification of it. I don't think we need this barrier.
>>
>> Conclusion:
>>
>> We're doing something really difficult here, which is defining a
>> transaction API without defining the storage implementation we want to use
>> and thus without defining how this transaction API would integrate with it.
>> Right now the most common (if not the only one!) known and standard
>> transaction API in the java world is JTA and most if not all known storage
>> implementation support it. Thus if we really want a transaction API separate
>> of the storage implementation I'd be in favor of looking at JTA and see
>> whether it would fit our needs.
>
>
>> WDYT?
>>
>
> Proposal of Caleb is over anything, including JTA, it is absolutely not a
> concurrent of JTA. What he proposed is to helps managing transactional
> processing properly without having to write boring try/finally code and
> ensuring we will commit or rollback and we do not mix incompatible
> "sub"-transactions.
BTW there are other ways of doing this. In JEE servers it's done with
annotations (no "boring" try/finally as you say :)). It's the container-managed
part of JTA.
Normally transactions should be demarcated at the level of the services so that
several calls to the DB can be in the same Tx for example. At least that's what
I used to do in my J2EE days...
> I would like to see it extended to support transmission
> of datas between these "sub"-transaction. In regards to a DBMS, there will
> be a single transactions and what is proposed by Caleb helps allowing it to
> be customarily subdivide in smaller steps without having to take care of the
> overall process of committing/rollbacking and collecting errors. You
> concentrate on the work that should be done, and the rest is taken care by
> your TransactionRunnables.
> This is very interesting in my opinion, but this is not yet mature enough to
> manage all situations.
Question: Since this looks generic, isn't there frameworks out there that do
this? If not, why?
Thanks
-Vincent
> Denis
>
>
>>
>> Thanks
>> -Vincent
>>
>>>>
>>>> I noticed some discussions between Denis and you on IRC about all this.
>> Does you latest findings change the proposal below?
>>> 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-transaction/src/main/java/org/xwiki/store/TransactionRunnable.java
>>>>> 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
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