RE: logical replication empty transactions

On Tuesday, January 11, 2022 6:43 PM From: Ajin Cherian <itsajin@gmail.com> wrote:
> Minor update to rebase the patch so that it applies clean on HEAD.
Hi, let me share some additional comments on v16.


(1) comment of pgoutput_change

@@ -630,11 +688,15 @@ pgoutput_change(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
                                Relation relation, ReorderBufferChange *change)
 {
        PGOutputData *data = (PGOutputData *) ctx->output_plugin_private;
+       PGOutputTxnData *txndata = (PGOutputTxnData *) txn->output_plugin_private;
        MemoryContext old;
        RelationSyncEntry *relentry;
        TransactionId xid = InvalidTransactionId;
        Relation        ancestor = NULL;

+       /* If not streaming, should have setup txndata as part of BEGIN/BEGIN PREPARE */
+       Assert(in_streaming || txndata);
+

In my humble opinion, the comment should not touch BEGIN PREPARE,
because this patch's scope doesn't include two phase commit.
(We could add this in another patch to extend the scope after the commit ?)

This applies to pgoutput_truncate's comment.

(2) "keep alive" should be "keepalive" in WalSndUpdateProgress

        /*
+        * When skipping empty transactions in synchronous replication, we need
+        * to send a keep alive to keep the MyWalSnd locations updated.
+        */
+       force_keepalive_syncrep = send_keepalive && SyncRepEnabled();
+

Also, this applies to the comment for force_keepalive_syncrep.

(3) Should finish the second sentence with period in the comment of pgoutput_message.

@@ -845,6 +923,19 @@ pgoutput_message(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
        if (in_streaming)
                xid = txn->xid;

+       /*
+        * Output BEGIN if we haven't yet.
+        * Avoid for streaming and non-transactional messages

(4) "begin" can be changed to "BEGIN" in the comment of PGOutputTxnData definition.

In the entire patch, when we express BEGIN message,
we use capital letters "BEGIN" except for one place.
We can apply the same to this place as well.

+typedef struct PGOutputTxnData
+{
+       bool sent_begin_txn;    /* flag indicating whether begin has been sent */
+} PGOutputTxnData;
+

(5) inconsistent way to write Assert statements with blank lines

In the below case, it'd be better to insert one blank line
after the Assert();

+static void
+pgoutput_begin(LogicalDecodingContext *ctx, ReorderBufferTXN *txn)
+{
        bool            send_replication_origin = txn->origin_id != InvalidRepOriginId;
+       PGOutputTxnData *txndata = (PGOutputTxnData *) txn->output_plugin_private;

+       Assert(txndata);
        OutputPluginPrepareWrite(ctx, !send_replication_origin);


(6) new codes in the pgoutput_commit_txn looks messy slightly

@@ -419,7 +455,25 @@ static void
 pgoutput_commit_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
                                        XLogRecPtr commit_lsn)
 {
-       OutputPluginUpdateProgress(ctx);
+       PGOutputTxnData *txndata = (PGOutputTxnData *) txn->output_plugin_private;
+       bool            skip;
+
+       Assert(txndata);
+
+       /*
+        * If a BEGIN message was not yet sent, then it means there were no relevant
+        * changes encountered, so we can skip the COMMIT message too.
+        */
+       skip = !txndata->sent_begin_txn;
+       pfree(txndata);
+       txn->output_plugin_private = NULL;
+       OutputPluginUpdateProgress(ctx, skip);

Could we conduct a refactoring for this new part ?
IMO, writing codes to free the data structure at the top
of function seems weird.

One idea is to export some part there
and write a new function, something like below.

static bool
txn_sent_begin(ReorderBufferTXN *txn)
{
    PGOutputTxnData *txndata = (PGOutputTxnData *) txn->output_plugin_private;
    bool            needs_skip;

    Assert(txndata);

    needs_skip = !txndata->sent_begin_txn;

    pfree(txndata);
    txn->output_plugin_private = NULL;

    return needs_skip;
}

FYI, I had a look at the v12-0002-Skip-empty-prepared-transactions-for-logical-rep.patch
for reference of pgoutput_rollback_prepared_txn and pgoutput_commit_prepared_txn.
Looks this kind of function might work for future extensions as well.
What did you think ?

Best Regards,
    Takamichi Osumi