Re: logical replication empty transactions

On Mon, Apr 19, 2021 at 6:22 PM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:

>
> Here are a some review comments:
>
> ------
>
> 1. The patch v3 applied OK but with whitespace warnings
>
> [postgres(at)CentOS7-x64 oss_postgres_2PC]$ git apply
>
> ../patches_misc/v3-0001-Skip-empty-transactions-for-logical-replication.patch
>
> ../patches_misc/v3-0001-Skip-empty-transactions-for-logical-replication.patch:98:
> indent with spaces.
> /* output BEGIN if we haven't yet, avoid for streaming and
> non-transactional messages */
>
> ../patches_misc/v3-0001-Skip-empty-transactions-for-logical-replication.patch:99:
> indent with spaces.
> if (!data->xact_wrote_changes && !in_streaming && transactional)
> warning: 2 lines add whitespace errors.
>
> ------
>

Fixed.

>
> 2. Please create a CF entry in [1] for this patch.
>
> ------
>
> 3. Patch comment
>
> The comment describes the problem and then suddenly just says
> "Postpone the BEGIN message until the first change."
>
> I suggest changing it to say more like... "(blank line) This patch
> addresses the above problem by postponing the BEGIN message until the
> first change."
>
> ------
>
>
Updated.

> 4. pgoutput.h
>
> Maybe for consistency with the context member, the comment for the new
> member should be to the right instead of above it?
>
> @@ -20,6 +20,9 @@ typedef struct PGOutputData
> MemoryContext context; /* private memory context for transient
> * allocations */
>
> + /* flag indicating whether messages have previously been sent */
> + bool xact_wrote_changes;
> +
>
> ------
>
> 5. pgoutput.h
>
> + /* flag indicating whether messages have previously been sent */
>
> "previously been sent" --> "already been sent" ??
>
> ------
>
> 6. pgoutput.h - misleading member name
>
> Actually, now that I have read all the rest of the code and how this
> member is used I feel that this name is very misleading. e.g. For
> "streaming" case then you still are writing changes but are not
> setting this member at all - therefore it does not always mean what it
> says.
>
> I feel a better name for this would be something like
> "sent_begin_txn". Then if you have sent BEGIN it is true. If you
> haven't sent BEGIN it is false. It eliminates all ambiguity naming it
> this way instead.
>
> (This makes my feedback #5 redundant because the comment will be a bit
> different if you do this).
>
> ------
>

Fixed above comments.

>
> 7. pgoutput.c - function pgoutput_begin_txn
>
> @@ -345,6 +345,23 @@ pgoutput_startup(LogicalDecodingContext *ctx,
> OutputPluginOptions *opt,
> static void
> pgoutput_begin_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn)
> {
>
> I guess that you still needed to pass the txn because that is how the
> API is documented, right?
>
> But I am wondering if you ought to flag it as unused so you wont get
> some BF machine giving warnings about it.
>
> e.g. Syntax like this?
>
> pgoutput_begin_txn(LogicalDecodingContext *ctx, ReorderBufferTXN * txn) {
> (void)txn;
> ...
>

Updated.

> ------
>
> 8. pgoutput.c - function pgoutput_begin_txn
>
> @@ -345,6 +345,23 @@ pgoutput_startup(LogicalDecodingContext *ctx,
> OutputPluginOptions *opt,
> static void
> pgoutput_begin_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn)
> {
> + PGOutputData *data = ctx->output_plugin_private;
> +
> + /*
> + * Don't send BEGIN message here. Instead, postpone it until the first
> + * change. In logical replication, a common scenario is to replicate a set
> + * of tables (instead of all tables) and transactions whose changes were
> on
> + * table(s) that are not published will produce empty transactions. These
> + * empty transactions will send BEGIN and COMMIT messages to subscribers,
> + * using bandwidth on something with little/no use for logical
> replication.
> + */
> + data->xact_wrote_changes = false;
> + elog(LOG,"Holding of begin");
> +}
>
> Why is this loglevel LOG? Looks like leftover debugging.
>

Removed.

>
> ------
>
> 9. pgoutput.c - function pgoutput_commit_txn
>
> @@ -384,8 +401,14 @@ static void
> pgoutput_commit_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
> XLogRecPtr commit_lsn)
> {
> + PGOutputData *data = ctx->output_plugin_private;
> +
> OutputPluginUpdateProgress(ctx);
>
> + /* skip COMMIT message if nothing was sent */
> + if (!data->xact_wrote_changes)
> + return;
> +
>
> In the case where you decided to do nothing does it make sense that
> you still called the function OutputPluginUpdateProgress(ctx); ?
> I thought perhaps that your new check should come first so this call
> would never happen.
>

Even though the empty transaction is not sent, the LSN is tracked as
decoded, hence the progress needs to be updated.

> ------
>
> 10. pgoutput.c - variable declarations without casts
>
> + PGOutputData *data = ctx->output_plugin_private;
>
> I noticed the new stack variable you declare have no casts.
>
> This differs from the existing code which always looks like:
> PGOutputData *data = (PGOutputData *) ctx->output_plugin_private;
>
> There are a couple of examples of this so please search new code to
> find them all.
>
> -----
>

Fixed.

> 11. pgoutput.c - function pgoutput_change
>
> @@ -551,6 +574,13 @@ pgoutput_change(LogicalDecodingContext *ctx,
> ReorderBufferTXN *txn,
> Assert(false);
> }
>
> + /* output BEGIN if we haven't yet */
> + if (!data->xact_wrote_changes && !in_streaming)
> + {
> + pgoutput_begin(ctx, txn);
> + data->xact_wrote_changes = true;
> + }
>
> If the variable is renamed as previously suggested then the assignment
> data->sent_BEGIN_txn = true; can be assigned in just 1 common place
> INSIDE the pgoutput_begin function.
>
> ------
>

Updated.

>
> 12. pgoutput.c - pgoutput_truncate function
>
> @@ -693,6 +723,13 @@ pgoutput_truncate(LogicalDecodingContext *ctx,
> ReorderBufferTXN *txn,
>
> if (nrelids > 0)
> {
> + /* output BEGIN if we haven't yet */
> + if (!data->xact_wrote_changes && !in_streaming)
> + {
> + pgoutput_begin(ctx, txn);
> + data->xact_wrote_changes = true;
> + }
>
> (same comment as above)
>
> If the variable is renamed as previously suggested then the assignment
> data->sent_BEGIN_txn = true; can be assigned in just 1 common place
> INSIDE the pgoutput_begin function.
>
> 13. pgoutput.c - pgoutput_message
>
> @@ -725,6 +762,13 @@ 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 */
> + if (!data->xact_wrote_changes && !in_streaming && transactional)
> + {
> + pgoutput_begin(ctx, txn);
> + data->xact_wrote_changes = true;
> + }
>
> (same comment as above)
>
> If the variable is renamed as previously suggested then the assignment
> data->sent_BEGIN_txn = true; can be assigned in just 1 common place
> INSIDE the pgoutput_begin function.
>
> ------
>

Fixed.

>
> 14. Test Code.
>
> I noticed that there is no test code specifically for seeing if empty
> transactions get sent or not. Is it possible to write such a test or
> is this traffic improvement only observable using the debugger?
>
>
The 020_messages.pl actually has a test case for tracking empty messages
even though it is part of the messages test.

regards,
Ajin Cherian
Fujitsu Australia