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Context:

Let's say that, while using Streaming Replication/Hot Standby on a Postgres 9.1 cluster, a standby node goes down. It stays down for an day, during which time a lot of DML occurs on the master. The standby's recovery.conf doesn't contain a 'restore_command' entry (for restoring from WAL journal files), but does contain a 'primary_conninfo' string (for Streaming Replication).

Question:

If I start the standby again after a day of changes on the master. Will it "catch up" (eventually come into a state which mirrors the master) using only Streaming Replication? Or do I have to enable WAL file archiving and let it apply files archived during the outage to ensure currency?

I've checked the WAL archiving/streaming replication doc here, and it says that you don't have to enable both WAL archiving and Streaming Replication, but it is unclear whether or not catch-up will happen without WAL file archiving being enabled.

Thanks!

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3 Answers 3

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Yes, it will catch up, using streaming only, if (and only if), the number of WAL segments generated since the last update on the standby is less than the value of wal_keep_segments in postgresql.conf. This is covered in this section of the documentation: Replication

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    This answer is correct, but it highlights the problem. If you do ever pass wal_keep_segments, your replication is dead. Setting up file-based replication is not optional if you want a system that will survive a long disconnection from the master and catch back up.
    – Greg Smith
    Sep 15, 2012 at 6:48
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at standby node,you can set restore_command on recovery.conf and then copy masters pg_xlog (which missing on standby) files to folder which restore_command points. You can easily find which xlog files missing by starting startup node and typing 

ps aux | grep postgres

you will see there "waiting for 000000020000005200000025" or something like this, that tells you which pg_xlog you should start copying from master to standby's restore_command path.

if you enable wal_archiving, it will start archive from the moment you set-up.

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  • I understand that using file-based WAL archiving and a giving the standby a restore_command to load WAL files, I can ensure that it catches up. That's not my question, though; I want to know whether or not the standby will catch up if I use only streaming replication (no WAL file shipping, just the replication stream as specified in 'primary_conninfo').
    – Zac B
    Jan 13, 2012 at 14:48
  • no. postgres doesnt do that. you should copy log files if there is a delay on your replication.
    – sftsz
    Jan 16, 2012 at 11:51
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No, I set up an instance of streaming replication and it fell out of sync somehow, I was unable to get it working again until I did a manual rsync of the WAL archives.

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