We're not using EJBs and we don't need ThreadLocal variables. OFbiz has
the Delegator, and as my code fragment shows, if the framework needs to
determine what transaction is currently running, it fetches the
transaction from the Delegator.
Adrian Crum
Sandglass Software
www.sandglass-software.com
On 8/27/2014 7:14 AM, Jacopo Cappellato wrote:
This is an interesting conversation and I don't have yet a strong opinion, and
I am happy to discuss new ideas, but I would be careful in changing the
behavior of this core part of the system because the way transactions are
managed in OFBiz has proven to be good and well suited for most of the
applications built in OFBiz. There are defects, some of them are described by
Adrian, but I would recommend to analyze them closely and figure out if they
can be fixed with the current system.
As regards our usage of ThreadLocal variables, please see below:
On Aug 27, 2014, at 3:01 AM, Adrian Crum <[email protected]>
wrote:
That doesn't solve the problem with using ThreadLocal variables.
this is probably not too bad and it may be inline with what other framework do; see for
example this quote from the (excellent) book "Java Concurrency in Practice" by
Brian Goetz:
"ThreadLocal is widely used in implementing application frameworks. For example,
J2EE containers associate a transaction context with an executing thread for the duration
of an EJB call. This is easily implemented using a static ThreadLocal holding the
transaction context: when framework code needs to determine what transaction is currently
running, it fetches the transaction context from this ThreadLocal."
Regards,
Jacopo
Adrian Crum
Sandglass Software
www.sandglass-software.com
On 8/26/2014 11:06 PM, Scott Gray wrote:
I don't think that's an issue with our transaction handling, it's simply a
problem that the cache isn't transaction aware.
If the cache were to be transaction aware it would need to implement the
XAResource interface or perhaps even the simpler Synchronization interface and
push cache entries to the global cache only upon commit or discard them on
rollback. I'm loathe to suggest XAResource because we don't implement it
properly in GenericXaResource/ServiceXaWrapper/DebugXaWrapper and it breaks
some transaction managers (Atomikos is the only one I've tried). I have a
strong feeling it could be implemented using the Synchronization interface
without too much trouble though.
Regards
Scott
On 26/08/2014, at 9:46 pm, Adrian Crum <[email protected]>
wrote:
The concepts of "suspend" and "resume" are implemented by a ThreadLocal stack. A
"suspend" pushes the current transaction on the stack, and a "resume" pops a transaction off the
stack.
If you read the Jira issue, I point out another problem with this clunky implementation - calling
"commit" doesn't really commit the transaction. That is why we end up with invalid data
in the entity cache - because developers are fooled into thinking the "commit" calls in
Delegator code actually commit the data, but they don't. The transaction is committed by the first
bit of code that began the transaction - either a request event or a service invocation.
This is an arcane problem and it is difficult to describe, but I will try to
diagram it:
Request Event
Service Dispatcher
Begin Transaction (actual begin)
Begin Service
Some service logic
Delegator calls "commit" - nothing happens
Delegator puts uncommitted values in cache
More service logic
Delegator calls "commit" - nothing happens
Delegator puts uncommitted values in cache
End Service
Commit Transaction (actual commit)
Return service results to event handler
If something goes wrong in the service and the transaction is rolled back, the
uncommitted values in the cache are still there!
You really have to spend time in Entity Engine code to fully appreciate how
awful the transaction implementation really is.
My approach keeps a Transaction reference in the Delegator. Instead of calling the fake
"commit", the Delegator notifies the Transaction about changed values. The
Transaction saves the changed values locally. After the transaction is committed, the
Transaction instance copies the saved values to the cache.
If you look at my previous code fragment, there will be no more "suspend" or
"resume" - if you want a new transaction, you just get another instance and use it.
Adrian Crum
Sandglass Software
www.sandglass-software.com
On 8/26/2014 9:02 PM, Scott Gray wrote:
Okay so I guess I don't really understand what you're suggesting, or how it
really differs much from what we have now. It's also not clear what your
suggested API changes have to do with the ThreadLocal usages?
Thanks
Scott
On 26/08/2014, at 3:22 pm, Adrian Crum <[email protected]>
wrote:
Just use the Delegator factory.
Adrian Crum
Sandglass Software
www.sandglass-software.com
On 8/26/2014 2:43 PM, Scott Gray wrote:
Hi Adrian,
I'll probably have plenty of questions, but the first that comes to mind is:
how would you use a delegator outside of a transaction with this approach?
Thanks
Scott
On 25/08/2014, at 10:51 am, Adrian Crum <[email protected]>
wrote:
One persistent problem with the current Entity Engine implementation is the use
of ThreadLocal variables in the Delegator and Transactions. Their use makes it
difficult (and sometimes impossible) to fix Entity Engine bugs. They also make
it impossible to multi-thread a Delegator instance.
Here is what I have had percolating in my head the last few months:
Transaction tx = TransactionFactory.newTransaction();
Delegator delegator = tx.getDelegator("default");
// Do stuff with delegator
Transaction nestedTx = TransactionFactory.newTransaction();
Delegator nestedDelegator = nestedTx.getDelegator("default");
// Do stuff with nestedDelegator
nestedTx.commit();
tx.commit();
A Delegator instance always references the transaction it is running in.
The advantage to this approach is we gain the ability to hand off Delegator
instances to other threads. Other threads can even commit/rollback a
transaction:
Transaction tx = delegator.getTransaction();
tx.commit();
After a commit, the Delegator instance is discarded. Any attempt to use it
after a commit throws an exception (the same is true with the Transaction
instance).
Another problem is Delegator localization - which also uses ThreadLocal
variables. We can localize Delegator instances like this:
Transaction tx = TransactionFactory.newTransaction();
Delegator delegator = tx.getDelegator("default", locale);
Finally, the current implementation has a caching problem:
https://issues.apache.org/jira/browse/OFBIZ-5534
With the new design, the Delegator instance, Transaction instance, and entity
cache are tightly coupled - so that problem is easy to solve.
What do you think?
--
Adrian Crum
Sandglass Software
www.sandglass-software.com
