Hi,
Now that I work for Neo Technology I've started looking more in detail
into the Neo4j API, and found something very interesting with regard to
how transaction completion is handled.
The idea is to simplify exception handling. Example code:
Transaction tx = ...;
try
{
...
tx.success();
} finally
{
tx.finish();
}
---
At first the above looked a little bit funny to me, but I got it
explained that it relates to how you handle exceptions. In the above, if
there is any exception (Exception, RuntimeException, Error) then the
finish() call will do a rollback, since success() has not been called.
This way you don't have to do tx.rollback() in catch statements, since
theoretically you would have to catch Throwable to be sure you have
covered all cases. I.e. this is unsafe:
UnitOfWork uow = ...;
try
{
uow.complete();
} catch (Exception ex)
{
uow.discard();
}
---
... since it does not handle RuntimeException or Error.
So how does this affect us? Not much it turns out, because I realized
that uow.discard() on a closed UoW is a no-op. In other words, the
following becomes best practice:
UnitOfWork uow = ...;
try
{
...
uow.complete();
} finally
{
uow.discard();
}
---
With this template you are sure that the UoW is always handled properly
after the block exits.
So, is this the same as Neo4j? Turns out, no actually not. The
difference is that in Neo4j the work happens in finish(), and so if that
method throws a TransactionFailureException, then you need a new
try/catch outside of it to handle. In Qi4j, the complete() method does
the work (and discard() never throws an exception), and so any
exceptions from complete() can be handled in the catch blocks of that
try block.
This also works if you have multiple UnitOfWork's in one try-block:
UnitOfWork uow = uowf.newUnitOfWork();
try
{
...
uow.complete();
uow = uowf.newUnitOfWork();
...
uow.complete();
} finally
{
uow.discard();
}
Does this seem like a good strategy? Do you prefer the Qi4j variant or
the Neo4j variant? Any other pros and cons I've missed?
/Rickard
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