This presentation is about feature included in PostgreSQL 9.3 for switchover and switchback

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Streaming Replication
Switchover/Switchback
Vibhor Kumar
Principal System Engineer.

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• PostgreSQL Streaming Replication
• Limitations in PG Version < 9.3
• Understanding Timelines
• New in PostgreSQL >=9.3
• Possible switchover/switchback
• Things to know about switchover/switchback
• recovery.conf
Agenda

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PostgreSQL Streaming Replication (SR)
• Streaming Replication (SR) provides capability to
continuously ship and apply WAL/XLOG records to any
number of standbys.
• Introduced in PostgreSQL 9.0
• Currently two types supported:
− Synchronous SR
− Asynchronous SR

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History: Streaming Replication
• 9.0: Asynchronous SR
• 9.1:
− Synchronous SR
− pg_basebackup
• 9.2:
− Cascaded Replication
− Backup from Standby
− New Sync replication mode. (remote_write, local write)
• 9.3
− Streaming only replication.
− Follow timeline changes.

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Limitations before 9.3
• Timeline streaming issue
• Intentional shutdown on master can leave Standby behind
the master before 9.1.
• If Standby is behind master:
− Promote lead to new timeline.
− Difference in Master and Standby.
− Old Master can’t be standby of new master.
− Need to take fresh backup or manual copy timeline file to old master.
• Painful to take fresh backup if database size is large.

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Understanding Timelines
• Introduced with Point-in-Time-Recovery in version 8.0
• Every time you do PITR, a new timeline is formed
• Timelines helps to differentiate WAL generated from
different PITR.
• When new WAL generated after PITR, PostgreSQL
shouldn’t overwrite old WAL.

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More about Timelines

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More about Timelines

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Timeline history file

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New in 9.3 SR
• Important Patches:
− commit 985bd7d49726c9f178558491d31a570d47340459
− Author: Fujii Masao
− Committed on Wed Jun 26 02:14:37 2013 +0900
− Make sures when we shutdown the master, walsender
should tries to send all outstandng WAL records
to Standby, and then to exit.
− Back patched till 9.1
− commit abfd192b1b5ba5216ac4b1f31dcd553106304b19
− Author: Heikki Linnakangas
− Committed on Thu Dec 13 19:00:00 2012 +0200
− Allow a streaming replication standby to follow a
timeline switch. (Only in 9.3)

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9.3: Now switchover/Switchback is possible
• Difference between Switchover and Failover. (add points)
• New commit in PostgreSQL helps:
− Clean shutdown for maintenance operation of master server
− Old master server can be new standby for New Master.
− No more fresh backup in intentional promotion.

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Things to know: switchover/switchback
• Use –m fast or –m smart shutdown on Master for clean
shutdown.
• Before promoting standby:
− Make sure all WAL sent by Master applied on elected node for New
Master.
− Use following function for when to promote:
− pg_last_xlog_receive_location
− pg_last_xlog_replay_location
− Have proper archive location accessible to old Master.
− Set proper archive_command

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recovery.conf
recovery_target_timeline = 'latest'
standby_mode = 'on’
primary_conninfo = 'user=repuser
password=repuser host=172.17.0.3 port=5432
restore_command = ‘cp /location/%f %p’

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EFM: Promote Steps
• When promote happens, EFM does following:
− Send prepare information to Master, Witness and Slave
− Agent on Master get promote signal and does following:
− Release VIP if any.
− create dummy recovery.conf, so that next startup master shouldn’t come
up.
− Message about Master failed.
− Standby signaled to be promoted by trigger.
• Explore EFM for possible switchover/switchback

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Demo

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Thanks