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I probably should have said WAL archiving, but this is the act of
sending the WAL segments to a standby server. Technically, using
synchronous streaming setup, this is not really required, but it's
still a good insurance policy, especially if for whatever reason,
the standby loses its connection to the primary. Also, synchronous
streaming replication uses a walreceiver process to send
transactions, one at a time, to the standby servers. This is not the
act of shipping a WAL segment to that standby, but if you're still
sending them to that standby, then when connectivity is restored,
the standby can effectively catch back up to the primary. This is
one of the major improvements in V9.4.x where the primary keeps
track of the segments and you no longer have to guess how many to
keep on your primary in the event of connectivity loss. In 9.3.x and
below, you had tell PostgreSQL how long to hang onto WAL segments
and guess correctly that this would be longer than any outage. On 4/8/2015 11:35 AM, Wei Shan wrote:
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