On Monday, November 7, 2022 6:18 PM Masahiko Sawada <sawada.m...@gmail.com> wrote: > > On Thu, Nov 3, 2022 at 10:06 PM houzj.f...@fujitsu.com > <houzj.f...@fujitsu.com> wrote: > > > > On Wednesday, November 2, 2022 10:50 AM Masahiko Sawada > <sawada.m...@gmail.com> wrote: > > > > > > On Mon, Oct 24, 2022 at 8:42 PM Masahiko Sawada > > > <sawada.m...@gmail.com> wrote: > > > > > > > > On Wed, Oct 12, 2022 at 3:04 PM Amit Kapila > <amit.kapil...@gmail.com> > > > wrote: > > > > > > > > > > On Tue, Oct 11, 2022 at 5:52 AM Masahiko Sawada > > > <sawada.m...@gmail.com> wrote: > > > > > > > > > > > > On Fri, Oct 7, 2022 at 2:00 PM Amit Kapila > <amit.kapil...@gmail.com> > > > wrote: > > > > > > > > > > > > > > About your point that having different partition structures for > > > > > > > publisher and subscriber, I don't know how common it will be once > we > > > > > > > have DDL replication. Also, the default value of > > > > > > > publish_via_partition_root is false which doesn't seem to indicate > > > > > > > that this is a quite common case. > > > > > > > > > > > > So how can we consider these concurrent issues that could happen > only > > > > > > when streaming = 'parallel'? Can we restrict some use cases to avoid > > > > > > the problem or can we have a safeguard against these conflicts? > > > > > > > > > > > > > > > > Yeah, right now the strategy is to disallow parallel apply for such > > > > > cases as you can see in *0003* patch. > > > > > > > > Tightening the restrictions could work in some cases but there might > > > > still be coner cases and it could reduce the usability. I'm not really > > > > sure that we can ensure such a deadlock won't happen with the current > > > > restrictions. I think we need something safeguard just in case. For > > > > example, if the leader apply worker is waiting for a lock acquired by > > > > its parallel worker, it cancels the parallel worker's transaction, > > > > commits its transaction, and restarts logical replication. Or the > > > > leader can log the deadlock to let the user know. > > > > > > > > > > As another direction, we could make the parallel apply feature robust > > > if we can detect deadlocks that happen among the leader worker and > > > parallel workers. I'd like to summarize the idea discussed off-list > > > (with Amit, Hou-San, and Kuroda-San) for discussion. The basic idea is > > > that when the leader worker or parallel worker needs to wait for > > > something (eg. transaction completion, messages) we use lmgr > > > functionality so that we can create wait-for edges and detect > > > deadlocks in lmgr. > > > > > > For example, a scenario where a deadlock occurs is the following: > > > > > > [Publisher] > > > create table tab1(a int); > > > create publication pub for table tab1; > > > > > > [Subcriber] > > > creat table tab1(a int primary key); > > > create subscription sub connection 'port=10000 dbname=postgres' > > > publication pub with (streaming = parallel); > > > > > > TX1: > > > BEGIN; > > > INSERT INTO tab1 SELECT i FROM generate_series(1, 5000) s(i); -- streamed > > > Tx2: > > > BEGIN; > > > INSERT INTO tab1 SELECT i FROM generate_series(1, 5000) s(i); -- > streamed > > > COMMIT; > > > COMMIT; > > > > > > Suppose a parallel apply worker (PA-1) is executing TX-1 and the > > > leader apply worker (LA) is executing TX-2 concurrently on the > > > subscriber. Now, LA is waiting for PA-1 because of the unique key of > > > tab1 while PA-1 is waiting for LA to send further messages. There is a > > > deadlock between PA-1 and LA but lmgr cannot detect it. > > > > > > One idea to resolve this issue is that we have LA acquire a session > > > lock on a shared object (by LockSharedObjectForSession()) and have > > > PA-1 wait on the lock before trying to receive messages. IOW, LA > > > acquires the lock before sending STREAM_STOP and releases it if > > > already acquired before sending STREAM_START, STREAM_PREPARE and > > > STREAM_COMMIT. For PA-1, it always needs to acquire the lock after > > > processing STREAM_STOP and then release immediately after acquiring > > > it. That way, when PA-1 is waiting for LA, we can have a wait-edge > > > from PA-1 to LA in lmgr, which will make a deadlock in lmgr like: > > > > > > LA (waiting to acquire lock) -> PA-1 (waiting to acquire the shared > > > object) -> LA > > > > > > We would need the shared objects per parallel apply worker. > > > > > > After detecting a deadlock, we can restart logical replication with > > > temporarily disabling the parallel apply, which is done by 0005 patch. > > > > > > Another scenario is similar to the previous case but TX-1 and TX-2 are > > > executed by two parallel apply workers (PA-1 and PA-2 respectively). > > > In this scenario, PA-2 is waiting for PA-1 to complete its transaction > > > while PA-1 is waiting for subsequent input from LA. Also, LA is > > > waiting for PA-2 to complete its transaction in order to preserve the > > > commit order. There is a deadlock among three processes but it cannot > > > be detected in lmgr because the fact that LA is waiting for PA-2 to > > > complete its transaction doesn't appear in lmgr (see > > > parallel_apply_wait_for_xact_finish()). To fix it, we can use > > > XactLockTableWait() instead. > > > > > > However, since XactLockTableWait() considers PREPARED TRANSACTION > as > > > still in progress, probably we need a similar trick as above in case > > > where a transaction is prepared. For example, suppose that TX-2 was > > > prepared instead of committed in the above scenario, PA-2 acquires > > > another shared lock at START_STREAM and releases it at > > > STREAM_COMMIT/PREPARE. LA can wait on the lock. > > > > > > Yet another scenario where LA has to wait is the case where the shm_mq > > > buffer is full. In the above scenario (ie. PA-1 and PA-2 are executing > > > transactions concurrently), if the shm_mq buffer between LA and PA-2 > > > is full, LA has to wait to send messages, and this wait doesn't appear > > > in lmgr. To fix it, probably we have to use non-blocking write and > > > wait with a timeout. If timeout is exceeded, the LA will write to file > > > and indicate PA-2 that it needs to read file for remaining messages. > > > Then LA will start waiting for commit which will detect deadlock if > > > any. > > > > > > If we can detect deadlocks by having such a functionality or some > > > other way then we don't need to tighten the restrictions of subscribed > > > tables' schemas etc. > > > > Thanks for the analysis and summary ! > > > > I tried to implement the above idea and here is the patch set. I have done > some > > basic tests for the new codes and it work fine. > > Thank you for updating the patches! > > Here are comments on v42-0001: > > We have the following three similar name functions regarding to > starting a new parallel apply worker: > --- > /* > * Exit if any parameter that affects the remote connection > was changed. > - * The launcher will start a new worker. > + * The launcher will start a new worker, but note that the > parallel apply > + * worker may or may not restart depending on the value of > the streaming > + * option and whether there will be a streaming transaction. > > In which case does the parallel apply worker don't restart even if the > streaming option has been changed?
Sorry, I forgot to reply to this comment. If user change the streaming option from 'parallel' to 'on' or 'off', the parallel apply workers won't be restarted. Best regards, Hou zj
