RE: logical replication empty transactions

On Tuesday, January 11, 2022 6:43 PM Ajin Cherian <itsajin@gmail.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.

(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);
+               }


(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.

(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
 * ===========================================================
 */

(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.

Best Regards,
    Takamichi Osumi