On 01/20/2011 04:24 AM, Vincent Massol wrote:
On Jan 20, 2011, at 10:12 AM, Denis Gervalle wrote:
> On Thu, Jan 20, 2011 at 09:23, Vincent Massol <vincent(a)massol.net> 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?
"Introduce a generic transaction API independent of the underlying storage
implementation"
so boring compared to "The XWiki Persistence Engine" ;)
Indeed you are correct but this proposal is for the foundation of the engine.
>>
>> 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 The original answer was
that I defined my requirements and wrote rather than looking for something.
Now looking at the JTA information, I still can't tell if it meets the requirements
which
TransactionRunnable was designed to meet.
1. Can multiple jobs be chained together inside of a transaction?
Can each job be made atomic? Can they be tested without mocking?
2. Are try/catch completely handled?
I notice a lot of transact.begin() and transact.commit() in the code.
3. Is is possible to run a job outside of the proper type of transaction?
IE: Can a "save document in JCR" job accidentally be run in a Hibernate
transaction?
4. Can JTA run on any storage platform? Hibernate, JCR, raw JDBC, Filesystem, Memcached,
etc.
>> * 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?
TransactionRunnable
is designed to run on top of absolutely anything.
>> * 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)
Yes, anything. Also
I notice in the example code given at that page, they are using the same (anti)
pattern. begin, do stuff, commit. That code can't be incorporated into a larger
transaction or else
it would be rolled back when begin() is called. This is the kind of code which begs to be
made into
a TransactionRunnable.
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...
From my perspective we need to rewrite the engine so
it's not a mess, get the new engine working
well and with all the kinks worked
out, then when each job is in a TransactionRunnable, we can
easily swap them out for new ones one at a time. To redesign the schema now means we need
to
interact with the old XWikiHibernateStore code in order to do migration, that's not
something I'd
like to do.
>> 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.
"be clear that the schema is not part of the API" yes which is why I want to
change one at a time.
If we design a new API and a new schema at the same time, we risk making an API with
limitations and
not noticing it because we make a schema which hides those limitations.
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.
Boring but more importantly unsafe.
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?]
There is... Now ;)
Caleb
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-…
>>>> 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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