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help / color / mirror / Atom feedFrom: [email protected] <[email protected]>
To: Amit Kapila <[email protected]>
Cc: Euler Taveira <[email protected]>
Cc: Masahiko Sawada <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Peter Smith <[email protected]>
Cc: Fabrice Chapuis <[email protected]>
Cc: Simon Riggs <[email protected]>
Cc: Petr Jelinek <[email protected]>
Cc: [email protected] <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Ajin Cherian <[email protected]>
Subject: RE: Logical replication timeout problem
Date: Mon, 11 Apr 2022 06:38:59 +0000
Message-ID: <OS3PR01MB62755D216245199554DDC8DB9EEA9@OS3PR01MB6275.jpnprd01.prod.outlook.com> (raw)
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On Wed, Apr 7, 2022 at 1:34 PM Amit Kapila <[email protected]> wrote:
> On Wed, Apr 6, 2022 at 6:30 PM [email protected]
> <[email protected]> wrote:
> >
> > On Wed, Apr 6, 2022 at 1:58 AM Amit Kapila <[email protected]> wrote:
> > On Wed, Apr 6, 2022 at 4:32 AM Amit Kapila <[email protected]> wrote:
> > > Also, let's try to evaluate how it impacts lag functionality for large
> transactions?
> > I think this patch will not affect lag functionality. It will updates the lag
> > field of view pg_stat_replication more frequently.
> > IIUC, when invoking function WalSndUpdateProgress, it will store the lsn of
> > change and invoking time in lag_tracker. Then when invoking function
> > ProcessStandbyReplyMessage, it will calculate the lag field according to the
> > message from subscriber and the information in lag_tracker. This patch does
> > not modify this logic, but only increases the frequency of invoking.
> > Please let me know if I understand wrong.
> >
>
> No, your understanding seems correct to me. But what I want to check
> is if calling the progress function more often has any impact on
> lag-related fields in pg_stat_replication? I think you need to check
> the impact of large transaction replay.
Thanks for the explanation.
After doing some checks, I found that the v13 patch makes the calculations of
lag functionality inaccurate.
In short, v13 patch lets us try to track lag more frequently and try to send a
keepalive message to subscribers. But in order to prevent flooding the lag
tracker, we could not track lag more than once within
WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS (see function WalSndUpdateProgress).
This means we may lose informations that needs to be tracked.
For example, suppose there is a large transaction with lsn from lsn1 to lsn3.
In HEAD, when we calculate the lag time for lsn3, the lag time of lsn3 is
(now - lsn3.time).
But with v13 patch, when we calculate the lag time for lsn3, because there
maybe no informations of lsn3 but has informations of lsn2 in lag_tracker, the
lag time of lsn3 is (now - t2.time). (see function LagTrackerRead)
Therefore, if we lose the informations that need to be tracked, the lag time
becomes large and inaccurate.
So I skip tracking lag during a transaction just like the current HEAD.
Attach the new patch.
Regards,
Wang wei
Attachments:
[application/octet-stream] v14-0001-Fix-the-logical-replication-timeout-during-large.patch (10.5K, ../OS3PR01MB62755D216245199554DDC8DB9EEA9@OS3PR01MB6275.jpnprd01.prod.outlook.com/2-v14-0001-Fix-the-logical-replication-timeout-during-large.patch)
download | inline diff:
From 7f14fb91f39467cae80a310d34d2d72bf15a94fd Mon Sep 17 00:00:00 2001
From: wangw <[email protected]>
Date: Mon, 11 Apr 2022 11:52:31 +0800
Subject: [PATCH v14] 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.
---
src/backend/replication/logical/logical.c | 7 +--
src/backend/replication/pgoutput/pgoutput.c | 57 ++++++++++++++++++---
src/backend/replication/walsender.c | 23 +++++++--
src/include/replication/logical.h | 3 +-
src/include/replication/output_plugin.h | 3 +-
5 files changed, 77 insertions(+), 16 deletions(-)
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 788769dd73..d3fe045c84 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -669,17 +669,18 @@ OutputPluginWrite(struct LogicalDecodingContext *ctx, bool last_write)
}
/*
- * Update progress tracking (if supported).
+ * Update progress tracking and try to send a keepalive message (if supported).
*/
void
OutputPluginUpdateProgress(struct LogicalDecodingContext *ctx,
- bool skipped_xact)
+ bool skipped_xact,
+ bool end_xact)
{
if (!ctx->update_progress)
return;
ctx->update_progress(ctx, ctx->write_location, ctx->write_xid,
- skipped_xact);
+ skipped_xact, end_xact);
}
/*
diff --git a/src/backend/replication/pgoutput/pgoutput.c b/src/backend/replication/pgoutput/pgoutput.c
index fe5accca57..67a87be7a2 100644
--- a/src/backend/replication/pgoutput/pgoutput.c
+++ b/src/backend/replication/pgoutput/pgoutput.c
@@ -91,6 +91,7 @@ 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_progress(LogicalDecodingContext *ctx, bool skipped_xact, bool end_xact);
/*
* Only 3 publication actions are used for row filtering ("insert", "update",
@@ -558,7 +559,7 @@ 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_progress(ctx, !sent_begin_txn, true);
pfree(txndata);
txn->output_plugin_private = NULL;
@@ -597,7 +598,7 @@ static void
pgoutput_prepare_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
XLogRecPtr prepare_lsn)
{
- OutputPluginUpdateProgress(ctx, false);
+ update_progress(ctx, false, true);
OutputPluginPrepareWrite(ctx, true);
logicalrep_write_prepare(ctx->out, txn, prepare_lsn);
@@ -611,7 +612,7 @@ static void
pgoutput_commit_prepared_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
XLogRecPtr commit_lsn)
{
- OutputPluginUpdateProgress(ctx, false);
+ update_progress(ctx, false, true);
OutputPluginPrepareWrite(ctx, true);
logicalrep_write_commit_prepared(ctx->out, txn, commit_lsn);
@@ -627,7 +628,7 @@ pgoutput_rollback_prepared_txn(LogicalDecodingContext *ctx,
XLogRecPtr prepare_end_lsn,
TimestampTz prepare_time)
{
- OutputPluginUpdateProgress(ctx, false);
+ update_progress(ctx, false, true);
OutputPluginPrepareWrite(ctx, true);
logicalrep_write_rollback_prepared(ctx->out, txn, prepare_end_lsn,
@@ -1361,6 +1362,8 @@ pgoutput_change(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
TupleTableSlot *old_slot = NULL;
TupleTableSlot *new_slot = NULL;
+ update_progress(ctx, false, false);
+
if (!is_publishable_relation(relation))
return;
@@ -1593,6 +1596,8 @@ pgoutput_truncate(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
Oid *relids;
TransactionId xid = InvalidTransactionId;
+ update_progress(ctx, false, false);
+
/* Remember the xid for the change in streaming mode. See pgoutput_change. */
if (in_streaming)
xid = change->txn->xid;
@@ -1656,6 +1661,8 @@ pgoutput_message(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
PGOutputData *data = (PGOutputData *) ctx->output_plugin_private;
TransactionId xid = InvalidTransactionId;
+ update_progress(ctx, false, false);
+
if (!data->messages)
return;
@@ -1848,7 +1855,7 @@ pgoutput_stream_commit(struct LogicalDecodingContext *ctx,
Assert(!in_streaming);
Assert(rbtxn_is_streamed(txn));
- OutputPluginUpdateProgress(ctx, false);
+ update_progress(ctx, false, true);
OutputPluginPrepareWrite(ctx, true);
logicalrep_write_stream_commit(ctx->out, txn, commit_lsn);
@@ -1869,7 +1876,7 @@ pgoutput_stream_prepare_txn(LogicalDecodingContext *ctx,
{
Assert(rbtxn_is_streamed(txn));
- OutputPluginUpdateProgress(ctx, false);
+ update_progress(ctx, false, true);
OutputPluginPrepareWrite(ctx, true);
logicalrep_write_stream_prepare(ctx->out, txn, prepare_lsn);
OutputPluginWrite(ctx, true);
@@ -2362,3 +2369,41 @@ 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 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_progress(LogicalDecodingContext *ctx, bool skipped_xact, bool end_xact)
+{
+ static int changes_count = 0;
+
+ if (end_xact)
+ {
+ /* Update progress tracking at xact end. */
+ OutputPluginUpdateProgress(ctx, skipped_xact, end_xact);
+ changes_count = 0;
+ return;
+ }
+
+ /*
+ * After continuously processing CHANGES_THRESHOLD changes, update
+ * progress which will also try to send a keepalive message if required.
+ *
+ * We don't want to try sending a keepalive message or updating progress
+ * after processing each change as that can have overhead. Testing reveals
+ * that there is no noticeable overhead in doing it after continuously
+ * processing 100 or so changes.
+ */
+#define CHANGES_THRESHOLD 100
+ if (++changes_count >= CHANGES_THRESHOLD)
+ {
+ OutputPluginUpdateProgress(ctx, skipped_xact, end_xact);
+ changes_count = 0;
+ }
+}
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 75400a53f2..8d2191694e 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -251,7 +251,7 @@ static void WalSndWait(uint32 socket_events, long timeout, uint32 wait_event);
static void WalSndPrepareWrite(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid, bool last_write);
static void WalSndWriteData(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid, bool last_write);
static void WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
- bool skipped_xact);
+ bool skipped_xact, bool end_xact);
static XLogRecPtr WalSndWaitForWal(XLogRecPtr loc);
static void LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time);
static TimeOffset LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now);
@@ -1461,21 +1461,28 @@ ProcessPendingWrites(void)
* Write the current position to the lag tracker (see XLogSendPhysical).
*
* When skipping empty transactions, send a keepalive message if necessary.
+ *
+ * If too many changes are processed then try to send a keepalive message to
+ * receiver to avoid timeouts.
*/
static void
WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
- bool skipped_xact)
+ bool skipped_xact, bool end_xact)
{
static TimestampTz sendTime = 0;
TimestampTz now = GetCurrentTimestamp();
+ bool pending_writes = false;
/*
* Track lag no more than once per WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS to
* avoid flooding the lag tracker when we commit frequently.
+ *
+ * we do not track lag during a transaction to prevent flooding the lag
+ * tracker.
*/
#define WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS 1000
- if (TimestampDifferenceExceeds(sendTime, now,
- WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS))
+ if (end_xact && TimestampDifferenceExceeds(sendTime, now,
+ WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS))
{
LagTrackerWrite(lsn, now);
sendTime = now;
@@ -1501,8 +1508,14 @@ WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId
/* If we have pending write here, make sure it's actually flushed */
if (pq_is_send_pending())
- ProcessPendingWrites();
+ pending_writes = true;
}
+
+ /* process pending writes if any or try to send a keepalive if required */
+ if (pending_writes || (!end_xact &&
+ now >= TimestampTzPlusMilliseconds(last_reply_timestamp,
+ wal_sender_timeout / 2)))
+ ProcessPendingWrites();
}
/*
diff --git a/src/include/replication/logical.h b/src/include/replication/logical.h
index a6ef16ad5b..e4c50a193e 100644
--- a/src/include/replication/logical.h
+++ b/src/include/replication/logical.h
@@ -27,7 +27,8 @@ typedef LogicalOutputPluginWriterWrite LogicalOutputPluginWriterPrepareWrite;
typedef void (*LogicalOutputPluginWriterUpdateProgress) (struct LogicalDecodingContext *lr,
XLogRecPtr Ptr,
TransactionId xid,
- bool skipped_xact
+ bool skipped_xact,
+ bool end_xact
);
typedef struct LogicalDecodingContext
diff --git a/src/include/replication/output_plugin.h b/src/include/replication/output_plugin.h
index 539dc8e697..82929db313 100644
--- a/src/include/replication/output_plugin.h
+++ b/src/include/replication/output_plugin.h
@@ -243,6 +243,7 @@ typedef struct OutputPluginCallbacks
/* Functions in replication/logical/logical.c */
extern void OutputPluginPrepareWrite(struct LogicalDecodingContext *ctx, bool last_write);
extern void OutputPluginWrite(struct LogicalDecodingContext *ctx, bool last_write);
-extern void OutputPluginUpdateProgress(struct LogicalDecodingContext *ctx, bool skipped_xact);
+extern void OutputPluginUpdateProgress(struct LogicalDecodingContext *ctx, bool skipped_xact,
+ bool end_xact);
#endif /* OUTPUT_PLUGIN_H */
--
2.18.4
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Subject: RE: Logical replication timeout problem
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