Re: logical replication empty transactions

On Wed, Jan 26, 2022 at 8:33 PM osumi(dot)takamichi(at)fujitsu(dot)com
<osumi(dot)takamichi(at)fujitsu(dot)com> wrote:
>
> On Tuesday, January 11, 2022 6:43 PM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
> > Minor update to rebase the patch so that it applies clean on HEAD.
> Hi, thanks for you rebase.
>
> Several comments.
>
> (1) the commit message
>
> "
> transactions, keepalive messages are sent to keep the LSN locations updated
> on the standby.
> This patch does not skip empty transactions that are "streaming" or "two-phase".
> "
>
> I suggest that one blank line might be needed before the last paragraph.

Changed.

>
> (2) Could you please remove one pair of curly brackets for one sentence below ?
>
> @@ -1546,10 +1557,13 @@ WalSndWaitForWal(XLogRecPtr loc)
> * otherwise idle, this keepalive will trigger a reply. Processing the
> * reply will update these MyWalSnd locations.
> */
> - if (MyWalSnd->flush < sentPtr &&
> + if (force_keepalive_syncrep ||
> + (MyWalSnd->flush < sentPtr &&
> MyWalSnd->write < sentPtr &&
> - !waiting_for_ping_response)
> + !waiting_for_ping_response))
> + {
> WalSndKeepalive(false);
> + }
>
>

Changed.

> (3) Is this patch's reponsibility to intialize the data in pgoutput_begin_prepare_txn ?
>
> @@ -433,6 +487,8 @@ static void
> pgoutput_begin_prepare_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn)
> {
> bool send_replication_origin = txn->origin_id != InvalidRepOriginId;
> + PGOutputTxnData *txndata = MemoryContextAllocZero(ctx->context,
> + sizeof(PGOutputTxnData));
>
> OutputPluginPrepareWrite(ctx, !send_replication_origin);
> logicalrep_write_begin_prepare(ctx->out, txn);
>
>
> Even if we need this initialization for either non streaming case
> or non two_phase case, there can be another issue.
> We don't free the allocated memory for this data, right ?
> There's only one place to use free in the entire patch,
> which is in the pgoutput_commit_txn(). So,
> corresponding free of memory looked necessary
> in the two phase commit functions.
>

Actually it is required for begin_prepare to set the data type, so
that the checks in the pgoutput_change can make sure that
the begin prepare is sent. I've also added a free in commit_prepared code.

> (4) SyncRepEnabled's better alignment.
>
> IIUC, SyncRepEnabled is called not only by the walsender but also by other backends
> via CommitTransaction -> RecordTransactionCommit -> SyncRepWaitForLSN.
> Then, the place to add the prototype function for SyncRepEnabled seems not appropriate,
> strictly speaking or requires a comment like /* called by wal sender or other backends */.
>
> @@ -90,6 +90,7 @@ extern void SyncRepCleanupAtProcExit(void);
> /* called by wal sender */
> extern void SyncRepInitConfig(void);
> extern void SyncRepReleaseWaiters(void);
> +extern bool SyncRepEnabled(void);
>
> Even if we intend it is only used by the walsender, the current code place
> of SyncRepEnabled in the syncrep.c might not be perfect.
> In this file, seemingly we have a section for functions for wal sender processes
> and the place where you wrote it is not here.
>
> at src/backend/replication/syncrep.c, find a comment below.
> /*
> * ===========================================================
> * Synchronous Replication functions for wal sender processes
> * ===========================================================
> */

Changed.
>
> (5) minor alignment for expressing a couple of messages.
>
> @@ -777,6 +846,9 @@ pgoutput_truncate(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
> Oid *relids;
> TransactionId xid = InvalidTransactionId;
>
> + /* If not streaming, should have setup txndata as part of BEGIN/BEGIN PREPARE */
> + Assert(in_streaming || txndata);
>
>
> In the commit message, the way you write is below.
> ...
> skip BEGIN / COMMIT messages for transactions that are empty. The patch
> ...
>
> In this case, we have spaces back and forth for "BEGIN / COMMIT".
> Then, I suggest to unify all of those to show better alignment.

fixed.

regards,
Ajin Cherian