[hibernate-dev] [OGM] Transaction-aware

[Pawel Kozlowski]

Hi Sanne, All,

On Mon, Mar 26, 2012 at 3:38 PM, Sanne Grinovero <sanne at hibernate.org> wrote:

> Are you interested in implementing JTA for a specific NoSQL store, or
> on the general problem?

Both actually. Just to give you more context I'm looking into OGM as a
JPA interface for 2 NoSQL data stores:

(1) Neo4j - from what I can see it should already support JTA
transactions so not much problem here. BTW: is there any ongoing
effort integrating Neo4j into OGM? If so I would be keen on helping
out (or start looking into it if there are no such efforts).

(2) Git (used as a versioned key-value store, based on jgit). I know
it might sound a bit crazy but I'm using jgit as an embeded, versioned
key value store. It works great in situations where:

a) most of the content stored is text-like and strict versioning /
history is needed
b) a storage is mostly-read (I mean, there are far more reads as
compared to writes)

So it is really cool in situations like CMS / WIKI where content
versioning / auditing is stricly needed. Combine it with OGM +
hibernate search and you get a very interesting data storage.

Now, for the jgit-backed storage there is no JTA support (althought
people were thiniking about it:
http://git.661346.n2.nabble.com/Any-plans-to-support-JTA-and-XA-in-jgit-td1495190.html)
but from my knowledge of jgit it would be relativelly easy to
implement (having concurent transactions would be next to impossible
but this is not a use case I'm targetting here - sequential
transactions would be fine for rare-write use case like CMS).

So once again, given the above idea of intgrating git storage, the
transactional behaviour would look like this:

1) on transaction's start I would rember a git's commit id and create
an in-memory git index (easy with jgit);
2) any operation inside of a transaction would write objects to git's
repo + update in-memory index;
3) on transaction's commit I would simply create a new commit in git
and move rev pointer to this new commit (here is the place were it is
very easy to detect any concurent modifications);
4) transaction's rollback is trivial - I would just drop the in-memory
index (git object written to a git database would be removed during a
standard cleanup / garbage collection of a git repository).

Given the above it wonder if I should take a route of implementing an
XAResource for jgit or maybe there is another place in OGM I could
plug into to be notified on transaction's start/commit/rollback.

> Some NoSQL databases support JTA, for example both EHCache and
> Infinispan do participate in JTA transactions; for this reason the
> currently supported databases by Hibernate OGM have JTA "for free" and
> you don't find much code about it (other than binding JTA).

Yes, I see. Digging more into the EHCache and Infinispan
implementation at the moment to make sure that I understand all the
details.

Thnx for your input so far, it would be also interesting to know if
other people would be interested in a such versioned key-value store.
It works perfectly for my particular use case but I wonder if others
would be interested as well.

Cheers,
Pawel