Read replication lag as three numbers
pg_stat_replication reports write, flush and replay lag separately. A healthy standby showed 346, 356 and 427 microseconds, and which of the three grows during an incident tells you whether the problem is the network, the standby's disk, or replay itself.
Problem
What you're actually looking at
The symptom as it shows up on a real server.
Replication lag is usually reduced to a single number, which is why it so rarely leads anywhere. PostgreSQL exposes three, corresponding to three different journeys a WAL record makes: arriving at the standby, being made durable there, and being applied so queries can see it. Each has a different cause and a different fix, and collapsing them hides the answer.
Meridian's dashboard shows the replica lagging. Nobody can say whether the WAL has not arrived, has not been flushed, or has arrived and cannot be applied.
Simple terms
A change on the primary makes three separate journeys to reach a replica: it has to arrive, it has to be written down safely, and it has to be applied so queries can see it. PostgreSQL times all three. If only the first is slow, the network or the sender is the problem. If the second is slow, the replica's disk is. If the third is slow while the first two are fine, the data is already there and something on the replica is blocking it from being applied. One combined number would hide all of that.
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Related runbooks
Continue the same incident path
Sibling procedures that cover the adjacent setup, recovery, or prevention step.
More in this category
Other WAL & replication runbooks
Neighbouring incidents that share the same diagnostic surface.
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How this connects to the rest of the library
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