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help / color / mirror / Atom feedFrom: Fujii Masao <[email protected]>
To: Shinya Kato <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: pg_stat_replication.*_lag sometimes shows NULL during active replication
Date: Fri, 20 Mar 2026 02:13:25 +0900
Message-ID: <CAHGQGwGLUXmjC1+A1fzg-ynP1pdKC-0yfmLYcnnu4YJSEDnuQw@mail.gmail.com> (raw)
In-Reply-To: <CAOzEurRV117N2neo1N_O+xWPv=-R7qou+i7k-h79JjTP9sO1Fg@mail.gmail.com>
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On Thu, Mar 19, 2026 at 10:58 PM Shinya Kato <[email protected]> wrote:
>
> On Tue, Mar 17, 2026 at 11:00 AM Fujii Masao <[email protected]> wrote:
> >
> > On Mon, Mar 16, 2026 at 9:26 AM Shinya Kato <[email protected]> wrote:
> > > Thank you for the v4 patch. I think this approach is better than mine.
> > > I tested the patch and confirmed that the issue no longer reproduces
> > > with physical replication. However, with logical replication, the lag
> > > columns in pg_stat_replication still show NULL periodically at
> > > wal_receiver_status_interval, since send_feedback() in worker.c can
> > > still send duplicate positions.
> >
> > I was thinking that if a feedback message triggered by
> > wal_receiver_status_interval has the same LSNs as the previous message,
> > it's expected for the lag columns to become NULL. But you see it differently,
> > don't you? Sorry, I failed to understand your point...
>
> Sorry for the confusion. I ran a script inserting one row every 0.5
> seconds under logical replication and confirmed that NULL still
> appears in the lag columns even while replication is actively running.
> I was initially mistaken that this was tied to
> wal_receiver_status_interval timing — that turned out to be unrelated.
>
> I haven't had time to investigate further, but my current impression
> is that the existing approach may not be sufficient for logical
> replication.
Thanks for the clarification! I understand your point now.
I think the issue occurs when the positions in the first message point to
the same LSN (e.g., 0/030D5230), and the second message reports the same but
larger LSN (e.g., 0/030D52E0).
I've updated the patch to address this. It removes fullyAppliedLastTime,
tracks the positions from the previous reply, and clears the lag values only
when the positions remain unchanged across two consecutive messages.
Patch attached. Could you test and review this updated patch?
Regards,
--
Fujii Masao
Attachments:
[application/octet-stream] v5-0001-Avoid-sending-duplicate-WAL-locations-in-standby-.patch (10.8K, 2-v5-0001-Avoid-sending-duplicate-WAL-locations-in-standby-.patch)
download | inline diff:
From f8732fcf673478aef5907817ca3384d9c48dceda Mon Sep 17 00:00:00 2001
From: Fujii Masao <[email protected]>
Date: Fri, 20 Mar 2026 00:34:06 +0900
Subject: [PATCH v5] Avoid sending duplicate WAL locations in standby status
replies
Previously, when the startup process applied WAL and requested walreceiver
to send an apply notification to the primary, walreceiver sent a status reply
unconditionally, even if the WAL locations had not advanced since
the previous update.
As a result, the standby could send two consecutive status reply messages
with identical WAL locations even though wal_receiver_status_interval had
not yet elapsed. This could unexpectedly reset the reported replication lag,
making it difficult for users to monitor lag. The second message was also
unnecessary because it reported no progress.
This commit updates walreceiver to send a reply only when the apply location
has advanced since the last status update, even when the startup process
requests a notification.
---
src/backend/replication/walreceiver.c | 70 ++++++++++++++++-----------
src/backend/replication/walsender.c | 35 ++++++++------
src/include/replication/walreceiver.h | 4 +-
3 files changed, 62 insertions(+), 47 deletions(-)
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index fabe3c73034..f5d5379edc7 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -143,7 +143,7 @@ static void XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr,
TimeLineID tli);
static void XLogWalRcvFlush(bool dying, TimeLineID tli);
static void XLogWalRcvClose(XLogRecPtr recptr, TimeLineID tli);
-static void XLogWalRcvSendReply(bool force, bool requestReply);
+static void XLogWalRcvSendReply(bool force, bool requestReply, bool replyApply);
static void XLogWalRcvSendHSFeedback(bool immed);
static void ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime);
static void WalRcvComputeNextWakeup(WalRcvWakeupReason reason, TimestampTz now);
@@ -417,7 +417,7 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
WalRcvComputeNextWakeup(i, now);
/* Send initial reply/feedback messages. */
- XLogWalRcvSendReply(true, false);
+ XLogWalRcvSendReply(true, false, false);
XLogWalRcvSendHSFeedback(true);
/* Loop until end-of-streaming or error */
@@ -493,7 +493,7 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
}
/* Let the primary know that we received some data. */
- XLogWalRcvSendReply(false, false);
+ XLogWalRcvSendReply(false, false, false);
/*
* If we've written some records, flush them to disk and
@@ -539,7 +539,7 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
ResetLatch(MyLatch);
CHECK_FOR_INTERRUPTS();
- if (walrcv->force_reply)
+ if (walrcv->reply_apply)
{
/*
* The recovery process has asked us to send apply
@@ -547,9 +547,9 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
* false in shared memory before sending the reply, so
* we don't miss a new request for a reply.
*/
- walrcv->force_reply = false;
+ walrcv->reply_apply = false;
pg_memory_barrier();
- XLogWalRcvSendReply(true, false);
+ XLogWalRcvSendReply(false, false, true);
}
}
if (rc & WL_TIMEOUT)
@@ -595,7 +595,7 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
wakeup[WALRCV_WAKEUP_PING] = TIMESTAMP_INFINITY;
}
- XLogWalRcvSendReply(requestReply, requestReply);
+ XLogWalRcvSendReply(requestReply, requestReply, false);
XLogWalRcvSendHSFeedback(false);
}
}
@@ -886,7 +886,7 @@ XLogWalRcvProcessMsg(unsigned char type, char *buf, Size len, TimeLineID tli)
/* If the primary requested a reply, send one immediately */
if (replyRequested)
- XLogWalRcvSendReply(true, false);
+ XLogWalRcvSendReply(true, false, false);
break;
}
default:
@@ -1053,7 +1053,7 @@ XLogWalRcvFlush(bool dying, TimeLineID tli)
/* Also let the primary know that we made some progress */
if (!dying)
{
- XLogWalRcvSendReply(false, false);
+ XLogWalRcvSendReply(false, false, false);
XLogWalRcvSendHSFeedback(false);
}
}
@@ -1107,24 +1107,33 @@ XLogWalRcvClose(XLogRecPtr recptr, TimeLineID tli)
}
/*
- * Send reply message to primary, indicating our current WAL locations, oldest
- * xmin and the current time.
+ * Send reply message to primary, indicating our current WAL locations and
+ * time.
*
- * If 'force' is not set, the message is only sent if enough time has
- * passed since last status update to reach wal_receiver_status_interval.
- * If wal_receiver_status_interval is disabled altogether and 'force' is
- * false, this is a no-op.
+ * The message is sent if 'force' is set, if enough time has passed since the
+ * last update to reach wal_receiver_status_interval, or if WAL locations have
+ * advanced since the previous status update. If wal_receiver_status_interval
+ * is disabled and 'force' is false, this function does nothing. Set 'force' to
+ * send the message unconditionally.
+ *
+ * Set 'replyApply' when the apply location is expected to have advanced from the
+ * previous update (for example, when the startup process requests an apply
+ * notification to be sent to the primary). In that case, the write, flush, and
+ * apply locations are compared to determine whether WAL has advanced.
+ * Otherwise the apply location is assumed unchanged and is not checked,
+ * so only the write and flush locations are considered.
*
* If 'requestReply' is true, requests the server to reply immediately upon
* receiving this message. This is used for heartbeats, when approaching
* wal_receiver_timeout.
*/
static void
-XLogWalRcvSendReply(bool force, bool requestReply)
+XLogWalRcvSendReply(bool force, bool requestReply, bool replyApply)
{
static XLogRecPtr writePtr = InvalidXLogRecPtr;
static XLogRecPtr flushPtr = InvalidXLogRecPtr;
- XLogRecPtr applyPtr;
+ static XLogRecPtr applyPtr = InvalidXLogRecPtr;
+ XLogRecPtr latestApplyPtr = InvalidXLogRecPtr;
TimestampTz now;
/*
@@ -1140,17 +1149,19 @@ XLogWalRcvSendReply(bool force, bool requestReply)
/*
* We can compare the write and flush positions to the last message we
* sent without taking any lock, but the apply position requires a spin
- * lock, so we don't check that unless something else has changed or 10
- * seconds have passed. This means that the apply WAL location will
- * appear, from the primary's point of view, to lag slightly, but since
- * this is only for reporting purposes and only on idle systems, that's
- * probably OK.
+ * lock, so we don't check that unless it is expected to advance since the
+ * previsou update, i.e., when 'replyApply' is true.
*/
- if (!force
- && writePtr == LogstreamResult.Write
- && flushPtr == LogstreamResult.Flush
- && now < wakeup[WALRCV_WAKEUP_REPLY])
- return;
+ if (!force && now < wakeup[WALRCV_WAKEUP_REPLY])
+ {
+ if (replyApply)
+ latestApplyPtr = GetXLogReplayRecPtr(NULL);
+
+ if (writePtr == LogstreamResult.Write
+ && flushPtr == LogstreamResult.Flush
+ && (!replyApply || applyPtr == latestApplyPtr))
+ return;
+ }
/* Make sure we wake up when it's time to send another reply. */
WalRcvComputeNextWakeup(WALRCV_WAKEUP_REPLY, now);
@@ -1158,7 +1169,8 @@ XLogWalRcvSendReply(bool force, bool requestReply)
/* Construct a new message */
writePtr = LogstreamResult.Write;
flushPtr = LogstreamResult.Flush;
- applyPtr = GetXLogReplayRecPtr(NULL);
+ applyPtr = (latestApplyPtr == InvalidXLogRecPtr) ?
+ GetXLogReplayRecPtr(NULL) : latestApplyPtr;
resetStringInfo(&reply_message);
pq_sendbyte(&reply_message, PqReplMsg_StandbyStatusUpdate);
@@ -1382,7 +1394,7 @@ WalRcvForceReply(void)
{
ProcNumber procno;
- WalRcv->force_reply = true;
+ WalRcv->reply_apply = true;
/* fetching the proc number is probably atomic, but don't rely on it */
SpinLockAcquire(&WalRcv->mutex);
procno = WalRcv->procno;
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 08253103cb3..66507e9c2dd 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -2472,7 +2472,9 @@ ProcessStandbyReplyMessage(void)
TimestampTz now;
TimestampTz replyTime;
- static bool fullyAppliedLastTime = false;
+ static XLogRecPtr prevWritePtr = InvalidXLogRecPtr;
+ static XLogRecPtr prevFlushPtr = InvalidXLogRecPtr;
+ static XLogRecPtr prevApplyPtr = InvalidXLogRecPtr;
/* the caller already consumed the msgtype byte */
writePtr = pq_getmsgint64(&reply_message);
@@ -2505,22 +2507,23 @@ ProcessStandbyReplyMessage(void)
applyLag = LagTrackerRead(SYNC_REP_WAIT_APPLY, applyPtr, now);
/*
- * If the standby reports that it has fully replayed the WAL in two
- * consecutive reply messages, then the second such message must result
- * from wal_receiver_status_interval expiring on the standby. This is a
- * convenient time to forget the lag times measured when it last
- * wrote/flushed/applied a WAL record, to avoid displaying stale lag data
- * until more WAL traffic arrives.
+ * If the standby reports that it has fully replayed the WAL, and the
+ * write/flush/apply positions remain unchanged across two consecutive
+ * reply messages, forget the lag times measured when it last
+ * wrote/flushed/applied a WAL record.
+ *
+ * The second message with unchanged positions typically results from
+ * wal_receiver_status_interval expiring on the standby, so lag values are
+ * usually cleared after that interval when there is no activity. This
+ * avoids displaying stale lag data until more WAL traffic arrives.
*/
- clearLagTimes = false;
- if (applyPtr == sentPtr)
- {
- if (fullyAppliedLastTime)
- clearLagTimes = true;
- fullyAppliedLastTime = true;
- }
- else
- fullyAppliedLastTime = false;
+ clearLagTimes = (applyPtr == sentPtr && flushPtr == sentPtr &&
+ writePtr == prevWritePtr && flushPtr == prevFlushPtr &&
+ applyPtr == prevApplyPtr);
+
+ prevWritePtr = writePtr;
+ prevFlushPtr = flushPtr;
+ prevApplyPtr = applyPtr;
/* Send a reply if the standby requested one. */
if (replyRequested)
diff --git a/src/include/replication/walreceiver.h b/src/include/replication/walreceiver.h
index 9b9bd916314..024ebcf4f37 100644
--- a/src/include/replication/walreceiver.h
+++ b/src/include/replication/walreceiver.h
@@ -156,11 +156,11 @@ typedef struct
pg_atomic_uint64 writtenUpto;
/*
- * force walreceiver reply? This doesn't need to be locked; memory
+ * request walreceiver reply? This doesn't need to be locked; memory
* barriers for ordering are sufficient. But we do need atomic fetch and
* store semantics, so use sig_atomic_t.
*/
- sig_atomic_t force_reply; /* used as a bool */
+ sig_atomic_t reply_apply; /* used as a bool */
} WalRcvData;
extern PGDLLIMPORT WalRcvData *WalRcv;
--
2.51.2
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To: [email protected]
Cc: [email protected], [email protected], [email protected]
Subject: Re: pg_stat_replication.*_lag sometimes shows NULL during active replication
In-Reply-To: <CAHGQGwGLUXmjC1+A1fzg-ynP1pdKC-0yfmLYcnnu4YJSEDnuQw@mail.gmail.com>
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