Fix the logical replication timeout during large transactions.

The problem is that we don't send keep-alive messages for a long time
while processing large transactions during logical replication where we
don't send any data of such transactions. This can happen when the table
modified in the transaction is not published or because all the changes
got filtered. We do try to send the keep_alive if necessary at the end of
the transaction (via WalSndWriteData()) but by that time the
subscriber-side can timeout and exit.

To fix this we try to send the keepalive message if required after
processing certain threshold of changes.

Reported-by: Fabrice Chapuis
Author: Wang wei and Amit Kapila
Reviewed By: Masahiko Sawada, Euler Taveira, Hou Zhijie, Hayato Kuroda
Backpatch-through: 10
Discussion: https://postgr.es/m/CAA5-nLARN7-3SLU_QUxfy510pmrYK6JJb=bk3hcgemAM_pAv+w@mail.gmail.com

1 files changed, 48 insertions, 6 deletions

diff --git a/src/backend/replication/pgoutput/pgoutput.c b/src/backend/replication/pgoutput/pgoutput.c
index b197bfd565d..406ad84e1d6 100644
--- a/src/backend/replication/pgoutput/pgoutput.c
+++ b/src/backend/replication/pgoutput/pgoutput.c
@@ -91,6 +91,8 @@ static void send_relation_and_attrs(Relation relation, TransactionId xid,
 static void send_repl_origin(LogicalDecodingContext *ctx,
 							 RepOriginId origin_id, XLogRecPtr origin_lsn,
 							 bool send_origin);
+static void update_replication_progress(LogicalDecodingContext *ctx,
+										bool skipped_xact);
 
 /*
  * Only 3 publication actions are used for row filtering ("insert", "update",
@@ -558,7 +560,7 @@ pgoutput_commit_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	 * from this transaction has been sent to the downstream.
 	 */
 	sent_begin_txn = txndata->sent_begin_txn;
-	OutputPluginUpdateProgress(ctx, !sent_begin_txn);
+	update_replication_progress(ctx, !sent_begin_txn);
 	pfree(txndata);
 	txn->output_plugin_private = NULL;
 
@@ -597,7 +599,7 @@ static void
 pgoutput_prepare_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 					 XLogRecPtr prepare_lsn)
 {
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_prepare(ctx->out, txn, prepare_lsn);
@@ -611,7 +613,7 @@ static void
 pgoutput_commit_prepared_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 							 XLogRecPtr commit_lsn)
 {
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_commit_prepared(ctx->out, txn, commit_lsn);
@@ -627,7 +629,7 @@ pgoutput_rollback_prepared_txn(LogicalDecodingContext *ctx,
 							   XLogRecPtr prepare_end_lsn,
 							   TimestampTz prepare_time)
 {
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_rollback_prepared(ctx->out, txn, prepare_end_lsn,
@@ -1360,6 +1362,8 @@ pgoutput_change(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	TupleTableSlot *old_slot = NULL;
 	TupleTableSlot *new_slot = NULL;
 
+	update_replication_progress(ctx, false);
+
 	if (!is_publishable_relation(relation))
 		return;
 
@@ -1592,6 +1596,8 @@ pgoutput_truncate(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	Oid		   *relids;
 	TransactionId xid = InvalidTransactionId;
 
+	update_replication_progress(ctx, false);
+
 	/* Remember the xid for the change in streaming mode. See pgoutput_change. */
 	if (in_streaming)
 		xid = change->txn->xid;
@@ -1655,6 +1661,8 @@ pgoutput_message(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	PGOutputData *data = (PGOutputData *) ctx->output_plugin_private;
 	TransactionId xid = InvalidTransactionId;
 
+	update_replication_progress(ctx, false);
+
 	if (!data->messages)
 		return;
 
@@ -1847,7 +1855,7 @@ pgoutput_stream_commit(struct LogicalDecodingContext *ctx,
 	Assert(!in_streaming);
 	Assert(rbtxn_is_streamed(txn));
 
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_stream_commit(ctx->out, txn, commit_lsn);
@@ -1868,7 +1876,7 @@ pgoutput_stream_prepare_txn(LogicalDecodingContext *ctx,
 {
 	Assert(rbtxn_is_streamed(txn));
 
-	OutputPluginUpdateProgress(ctx, false);
+	update_replication_progress(ctx, false);
 	OutputPluginPrepareWrite(ctx, true);
 	logicalrep_write_stream_prepare(ctx->out, txn, prepare_lsn);
 	OutputPluginWrite(ctx, true);
@@ -2361,3 +2369,37 @@ send_repl_origin(LogicalDecodingContext *ctx, RepOriginId origin_id,
 		}
 	}
 }
+
+/*
+ * Try to update progress and send a keepalive message if too many changes were
+ * processed.
+ *
+ * For a large transaction, if we don't send any change to the downstream for a
+ * long time (exceeds the wal_receiver_timeout of standby) then it can timeout.
+ * This can happen when all or most of the changes are either not published or
+ * got filtered out.
+ */
+static void
+update_replication_progress(LogicalDecodingContext *ctx, bool skipped_xact)
+{
+	static int	changes_count = 0;
+
+	/*
+	 * We don't want to try sending a keepalive message after processing each
+	 * change as that can have overhead. Tests revealed that there is no
+	 * noticeable overhead in doing it after continuously processing 100 or so
+	 * changes.
+	 */
+#define CHANGES_THRESHOLD 100
+
+	/*
+	 * If we are at the end of transaction LSN, update progress tracking.
+	 * Otherwise, after continuously processing CHANGES_THRESHOLD changes, we
+	 * try to send a keepalive message if required.
+	 */
+	if (ctx->end_xact || ++changes_count >= CHANGES_THRESHOLD)
+	{
+		OutputPluginUpdateProgress(ctx, skipped_xact);
+		changes_count = 0;
+	}
+}