Postgres 9.4 has added a new feature allowing to delay WAL replay on standbys or even to delay recovery by a given amount of time. It has been introduced by this commit.
commit 36da3cfb457b77a55582f68208d815f11ac1399e Author: Simon Riggs Date: Thu Dec 12 10:53:20 2013 +0000 Allow time delayed standbys and recovery Set min_recovery_apply_delay to force a delay in recovery apply for commit and restore point WAL records. Other records are replayed immediately. Delay is measured between WAL record time and local standby time. Robert Haas, Fabrizio de Royes Mello and Simon Riggs Detailed review by Mitsumasa Kondo
Even if this does not replace the backup and WAL archive you should have in case of disaster, Postgres did not cover up to now the possibility to have a slave replaying WALs behind a master with a certain delay, WAL are replayed once they are available. This can actually save time from some stupid DROP TABLE that would force to do a PITR to recover the data to the point previous to data deletion. In this case the delay allowed between the master node and its standby(s) is the one you give yourself to repair the mistake you might have done.
This is controlled with a new recovery parameter called recovery_min_apply_delay. Note that it is a minimum amount of time, as WAL will be replayed when at least the commit time on master has reached the threshold time specified by this parameter. As the calculation is based on the commit time of transaction that occurred on master and the local clock of standby server, you should as well be aware that if the clocks if the master and slave server are not synchronized correctly this delay would be not exact. This has as well as consequence that the delay is not cumulative in cascading replication, so all the slaves will have the same delay.
Different timezones on master and slave nodes might lead to incorrect calculation as well, so it is important to have system settings consistent with this parameter, or you might find yourself with a slave that replays WAL files before (or later) it should.
This parameter is represented as an int32 and its default unit is ms, so the maximum delay time allowed is roughly 2 billion milliseconds, or 25 days.
Now let’s see how this works with 2 nodes listening ports 5432 and 5433 running on a local machine and the following simple script:
#!/bin/bash psql -c "CREATE TABLE aa AS SELECT 1 AS a" > /dev/null 2>&1 echo "Start time: " `date` while [ /bin/true ]; do TUPLE=`psql -A -t -p 5433 -c "select * from aa" 2> /dev/null` if [ $TUPLE -a $TUPLE == 1 ]; then echo "Finish time: " `date` exit 0 fi sleep 1 done
It simply consists in creating a new 1-column table with one single value and it checks every second if the tuple is present on slave. In this case psql -A and -t to make the results respectively unaligned and tuple-only.
Now, when the delay is set to 10s, running this script gives the following result:
$ ./script.bash Start time: Wed Dec 25 01:26:35 JST 2013 Finish time: Wed Dec 25 01:26:45 JST 2013
Of course that worked.