Improve replication lag interpolation after idle period

After sitting idle and fully replayed for a while and then encountering
a new burst of WAL activity, we interpolate between an ancient sample and the
not-yet-reached one for the new traffic. That produced a corner case report
of lag after receiving first new reply from standby, which might sometimes
be a large spike.

Correct this by resetting last_read time and handle that new case.

Author: Thomas Munro

diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 82f7b5937342af49da542d824ab2cc6540acce55..f845180873e144362957f1bde389351d2e874576 100644 (file)
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -3443,6 +3443,16 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
            (LagTracker.read_heads[head] + 1) % LAG_TRACKER_BUFFER_SIZE;
    }
 
+   /*
+    * If the lag tracker is empty, that means the standby has processed
+    * everything we've ever sent so we should now clear 'last_read'.  If we
+    * didn't do that, we'd risk using a stale and irrelevant sample for
+    * interpolation at the beginning of the next burst of WAL after a period
+    * of idleness.
+    */
+   if (LagTracker.read_heads[head] == LagTracker.write_head)
+       LagTracker.last_read[head].time = 0;
+
    if (time > now)
    {
        /* If the clock somehow went backwards, treat as not found. */
@@ -3459,9 +3469,14 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
         * eventually start moving again and cross one of our samples before
         * we can show the lag increasing.
         */
-       if (LagTracker.read_heads[head] != LagTracker.write_head &&
-           LagTracker.last_read[head].time != 0)
+       if (LagTracker.read_heads[head] == LagTracker.write_head)
        {
+           /* There are no future samples, so we can't interpolate. */
+           return -1;
+       }
+       else if (LagTracker.last_read[head].time != 0)
+       {
+           /* We can interpolate between last_read and the next sample. */
            double      fraction;
            WalTimeSample prev = LagTracker.last_read[head];
            WalTimeSample next = LagTracker.buffer[LagTracker.read_heads[head]];
@@ -3494,8 +3509,14 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
        }
        else
        {
-           /* Couldn't interpolate due to lack of data. */
-           return -1;
+           /*
+            * We have only a future sample, implying that we were entirely
+            * caught up but and now there is a new burst of WAL and the
+            * standby hasn't processed the first sample yet.  Until the
+            * standby reaches the future sample the best we can do is report
+            * the hypothetical lag if that sample were to be replayed now.
+            */
+           time = LagTracker.buffer[LagTracker.read_heads[head]].time;
        }
    }