3:36 a.m.
First of all, please let's separate "functional API" and "async
API".
Functional API already provides only the async API, but this discussion
should not be specific to functional calls.
More inline.
On 10/26/2016 06:03 PM, Dan Berindei wrote:
In theory something like this should work, resuming the transaction
after each asynchronous operation:
https://paste.fedoraproject.org/461451/77496416/raw/
I'm not sure about the usability, or performance... The
TransactionManager API doesn't allow asynchronous commit(), so one
might as well bite the bullet and spawn a new thread to run their
transaction in.
I don't think that the commitTransactionAsync() is correct, because it
assumes that continueTransactionAsync() was invoked beforehand. If you
run just
cache.commitTransactionAsync(cache.putAsync(...))
the thread calling the handler may not be associated with given tx. So
you would have to use:
default <T> TxCompletableFuture<T>
continueTransactionAsync(CompletableFuture<T> asyncOperation) { ... }
default <T> CompletableFuture<T>
commitTransactionAsync(TxCompletableFuture<T> asyncOperation) { ... }
to enforce invoking continue.
I am starting to worry our LocalTransaction, TxInvocationContext and
other classes that currently assume single-threaded access. Once we
start executing multiple async operations using the same context, we'll
see tons of race conditions. It's not hard to handle them within
execution of single command (e.g. when doing getAll, we already have to
sync the responses if using invokeRemotelyAsync), but with multiple
commands it's a different cup of coffee.
I would consider single big lock on the context/transaction that will
guarantee that commands for the same transaction won't run in parallel
(so put(keyA, ...) will get to a point when it's waiting for an entry
lock/doing RPC, and only then a put(keyB, ...) can continue. As the RPC
responses come, these will be executed serially). While this is
non-trivial to implement, it will simplify the reasoning - I am worried
that just changing all collections to concurrent ones won't do the trick.
I've checked JTA spec about thread-safety and it permits multiple
threads working on the same transaction but it's a bit vague how to
achieve that:
"Multiple threads may concurrently be associated with the same global
transaction."
"Note that some transaction manager implementations allow a suspended
transaction to be resumed by a different thread. This feature is not
required by JTA."
So it's probably a question to Narayana guys if you can suspend a
transaction in one thread and resume it in two threads.
JTA is not too much async-friendly, as XAResource docs says:
The XAResource interface, in order to be better integrated with the Java
environment, differs from the standard X/Open XA interface in the
following ways:
* Asynchronous operations are not supported. Java supports
multi-threaded processing and most databases do not support asynchronous
operations.
* The DTP concept of “Thread of Control” maps to all Java threads that
are given access to the XAResource and Connection objects. For example,
it is legal (although in practice rarely used) for two different Java
threads to perform the start and end operations on the same XAResource
object.
* ...
A more pie-in-the-sky idea for the the functional API would be to
support "batching" multiple evals over the same cache, not involving
the transaction manager. That should allow us to catch
WriteSkewExceptions and retry the batch ourselves, assuming the
lambdas don't have side-effects. And if the transaction is
Infinispan-only, we can also make the commit asynchronous.
I am not sure if I follow... you're assuming that all the async ops are
fired without any logic based on intermediate results.
WriteSkewException is thrown when you've already read something, and
then decided based on the value.
R.
There's always the option of forcing the functional operations to be
synchronous when they're joining a running TransactionManager
transaction...
Cheers
Dan
On Tue, Oct 11, 2016 at 8:16 PM, Radim Vansa <rvansa(a)redhat.com> wrote:
> Hi Galder,
>
> I've looked to the Narayana thread but I also have some trouble
> understanding the R ret = CompletableFuture.supplyAsync(...) - that
> method returns CF<R>, not directly R... so the answer that guys on the
> forum give you makes sense to me. Maybe you could revive that thread and
> clarify what you had in mind.
>
> You're describing general issue of async calls, not just functional API.
> Functional API is about moving some lambda to be executed in situ. Your
> question is no different as if you were calling putAsync and trying to
> chain it with another operations. TBH I haven't dealt with this problem
> at all, the tests I've written use the operations in sync way.
>
> If you try to do something transactionally-sensitive as a part of the
> lambda itself, you're doing it wrong - the lambda can be executed on
> different node which has no link to the original transaction. And the
> lambda should be purely functional (no side effects), otherwise you're
> poking the devil.
>
> Regarding RC vs. RR., in [1] I've decided to attach version on which the
> functional command operated with the return value if versions (and WSC)
> is used. Then, if the version changes during the transaction (say,
> you've did a functional read, and it changes before second read),
> WriteSkewException is thrown as we're not operating on the same value.
> However, this does not work after you modify the entry with a functional
> command; then your reads still work on the non-modified entry... I have
> to tweak this a bit yet, basically sending all the modifications to a
> given entry along with the read :-/
> Note: RR without WSC is just broken, I should add a warning log somewhere...
>
> Radim
>
> [1] https://github.com/infinispan/infinispan/pull/4608
>
> On 10/10/2016 05:49 PM, Galder Zamarreño wrote:
>> Hey Radim,
>>
>> Sorry for late reply. Indeed, I think you should start by getting RC done first.
>>
>> When I first experimented with this, I had bigger issues than the ones explained
here. For example, how to pass down transaction context when multiple async operations are
being executed within a transaction.
>>
>> In particular, say you have two async operations, chained, executed with T1 and
T2 respectively, both part of the the same transaction. Suspending tx in T1 and resuming
it when T2 starts is easy, the problem I had was how to get T1 to resume the transaction
when T2 has completed async transactional operation. At the time I created a forum post in
the transactions forum about it [1]. I was not able to able to try out the solution in
[1], but it doesn't feel very idiomatic to me (e.g. needing to call
CompletableFuture.get()).
>>
>> Did you consider this issue? Or are you avoiding it somehow?
>>
>> Cheers,
>>
>> [1] https://developer.jboss.org/thread/265595
>> --
>> Galder Zamarreño
>> Infinispan, Red Hat
>>
>>> On 27 Sep 2016, at 16:51, Radim Vansa <rvansa(a)redhat.com> wrote:
>>>
>>> Hi,
>>>
>>> seems I'll have to implement the functional stuff on tx caches [1][2] if
>>> I want to get rid of DeltaAware et al.
>>>
>>> The general idea is quite simple - ReadOnly* commands should behave very
>>> similar to non-tx mode, WriteOnly* commands will be just added as
>>> modifications to the PrepareCommand and ReadWrite* commands will be both
>>> added to modifications list, and sent to remote nodes where the result
>>> won't be stored yet.
>>> The results of operations should not be stored into transactional
>>> context - the command will execute remotely (if the owners are remote)
>>> unless the value was read by Get* beforehand.
>>>
>>> With repeatable-reads isolation, the situation gets more complicated. If
>>> we use ReadOnly* that performs identity lookup (effectively the same as
>>> Get*) and the entry was modified in during the transaction, we can
>>> return two different results - so a read committed semantics. With write
>>> skew check enabled, we could at least fail the transaction at the end
>>> (the check would be performed reads as well if the transaction contains
>>> functional reads), but we cannot rely on WSC always on with RR.
>>>
>>> Retrieving the whole entry and applying the functional command is not a
>>> viable solution, IMO - that would completely defy the purpose of using
>>> functional command.
>>>
>>> A possible solution would be to send the global transaction ID with
>>> those read commands and keep a remote transactional context with read
>>> entries for the duration of transaction on remote nodes, too. However,
>>> if we do a Read* command to primary owner, it's possible that further
>>> Get* command will hit backup. So, we could go to all owners with Read*
>>> already during the transaction (slowing down functional reads
>>> considerably), or read only from primary owner (which slows down Get*s
>>> even if we don't use functional APIs - this makes it a no-go). I am not
>>> 100% sure how a transaction transfer during ST will get into that.
>>>
>>> We could also do it the ostrich way - "Yes we've promissed RR but
Func
>>> will be only RC". I'll probably do that in the first draft anyway.
>>>
>>> Comments & opinions appreciated.
>>>
>>> Radim
>>>
>>> [1] https://issues.jboss.org/browse/ISPN-5806
>>> [2] https://issues.jboss.org/browse/ISPN-6573
>>>
>>> --
>>> Radim Vansa <rvansa(a)redhat.com>
>>> JBoss Performance Team
>>>
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>
> --
> Radim Vansa <rvansa(a)redhat.com>
> JBoss Performance Team
>
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Radim Vansa <rvansa(a)redhat.com>
JBoss Performance Team