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Re: Perform streaming logical transactions by background workers and parallel apply

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Peter Smith <smithpb2250@gmail.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-26T13:02:24Z
Lists: pgsql-hackers 
On Mon, Dec 26, 2022 at 1:22 PM houzj.fnst@fujitsu.com
<houzj.fnst@fujitsu.com> wrote:
>
> On Friday, December 23, 2022 5:20 PM houzj.fnst@fujitsu.com <houzj.fnst@fujitsu.com> wrote:
> >
> > I noticed a CFbot failure in one of the new testcases in 015_stream.pl which
> > comes from old 032_xx.pl. It's because I slightly adjusted the change size in a
> > transaction in last version which cause the transaction's size not to exceed the
> > decoding work mem, so the transaction is not being applied as expected as
> > streaming transactions(it is applied as a non-stremaing transaction) which cause
> > the failure. Attach the new version patch which fixed this miss.
> >
>
> Since the GUC used to force stream changes has been committed, I removed that
> patch from the patch set here and rebased the testcases based on that commit.
> Here is the rebased patch set.
>

Thank you for updating the patches. Here are some comments for 0001
and 0002 patches:


I think it'd be better to write logs when the leader enters the
serialization mode. It would be helpful for investigating issues.

---
+        if (!pa_can_start(xid))
+                return;
+
+        /* First time through, initialize parallel apply worker state
hashtable. */
+        if (!ParallelApplyTxnHash)
+        {
+                HASHCTL                ctl;
+
+                MemSet(&ctl, 0, sizeof(ctl));
+                ctl.keysize = sizeof(TransactionId);
+                ctl.entrysize = sizeof(ParallelApplyWorkerEntry);
+                ctl.hcxt = ApplyContext;
+
+                ParallelApplyTxnHash = hash_create("logical
replication parallel apply workershash",
+
             16, &ctl,
+
             HASH_ELEM |HASH_BLOBS | HASH_CONTEXT);
+        }
+
+        /*
+         * It's necessary to reread the subscription information
before assigning
+         * the transaction to a parallel apply worker. Otherwise, the
leader may
+         * not be able to reread the subscription information if streaming
+         * transactions keep coming and are handled by parallel apply workers.
+         */
+        maybe_reread_subscription();

pa_can_start() checks if the skiplsn is an invalid xid or not, and
then maybe_reread_subscription() could update the skiplsn to a valid
value. As the comments in pa_can_start() says, it won't work. I think
we should call maybe_reread_subscription() in
apply_handle_stream_start() before calling pa_allocate_worker().

---
+static inline bool
+am_leader_apply_worker(void)
+{
+        return (!OidIsValid(MyLogicalRepWorker->relid) &&
+                        !isParallelApplyWorker(MyLogicalRepWorker));
+}

How about using !am_tablesync_worker() instead of
!OidIsValid(MyLogicalRepWorker->relid) for better readability?

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix invalid memory access during the shutdown of the parallel apply worker.

  2. Fix assertion failure in apply worker.

  3. Use elog to report unexpected action in handle_streamed_transaction().

  4. Use appropriate wait event when sending data in the apply worker.

  5. Allow the logical_replication_mode to be used on the subscriber.

  6. Rename GUC logical_decoding_mode to logical_replication_mode.

  7. Display the leader apply worker's PID for parallel apply workers.

  8. Improve the code to decide and process the apply action.

  9. Document the newly added wait events added by commit 216a784829.

  10. Perform apply of large transactions by parallel workers.

  11. Wake up a subscription's replication worker processes after DDL.

  12. Add copyright notices to meson files

  13. Better document logical replication parameters

  14. Add a common function to generate the origin name.

  15. Harmonize parameter names in storage and AM code.

  16. Avoid using list_length() to test for empty list.

  17. Improve two comments related to a boolean DefElem's value

  18. Fix partition table's REPLICA IDENTITY checking on the subscriber.

  19. Fix data inconsistency between publisher and subscriber.

  20. Fix cache look-up failures while applying changes in logical replication.