An introduction to pgpool-II Version 3 - PostgreSQL Sessions
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An introduction to pgpool-II Version 3 - PostgreSQL Sessions
Pgpool-II does query dispatching and failover Write query
Streaming replication
Read/write queries pgpool-II
Standby Read query
Copyright(c)2011 Tatsuo Ishii
8
Pgpool-II V3 helps PostgreSQL ●
New “stream” mode dedicated for SR/HS
●
Automatic query dispatching
●
●
●
Allow to load balance SELECTs in an explicit transaction Allow to specify functions which are writing to DB(they need to be executed on the primary) Adding standby servers without stopping pgpool-II Copyright(c)2011 Tatsuo Ishii
9
Load balance SELECTs in an explicit transaction transaction BEGIN;
Transaction starts
SELECT
Load balance ok
SELECT
Load balance ok
UPDATE
Write to database
SELECT : :
Load balance is not ok anymore because the table might be changed
Copyright(c)2011 Tatsuo Ishii
10
Replication delay detection: how it works Primary PostgreSQL
Standby PostgreSQL
SELECT pg_current_ xlog_location()
SELECT pg_last_xlog_ receive_location()
call the functions during each “health check period” second and calculate the difference. if it exceeds the delay threshold, sends read queries to the primary sever only. worker process Copyright(c)2011 Tatsuo Ishii
11
Automatic promoting standby health checking
pgpool parent
X
ssh host touch trigger file failover_command
Primary goes down
kill and restart all pgpool child
Standby server promotes to primary Copyright(c)2011 Tatsuo Ishii
12
Logging replication delay ●
log_standby_delay ● ●
●
'none': no logging delay 'if_over_threshold': log only when delay exceeds delay_threshold 'always': always log delay
pid 13223: Replication of node:1 is behind 1228800 bytes from the primary server (node:0) pid 13223: Replication of node:1 is behind 3325952 bytes from the primary server (node:0) pid 13223: Replication of node:1 is behind 974848 bytes from the primary server (node:0) pid 13223: Replication of node:1 is behind 2990080 bytes from the primary server (node:0) pid 13223: Replication of node:1 is behind 901120 bytes from the primary server (node:0) pid 13223: Replication of node:1 is behind 2433024 bytes from the primary server (node:0)
Copyright(c)2011 Tatsuo Ishii
13
New show commands ●
show pool_nodes ●
●
show pool_processes ●
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show pgpool child process information
show pool_pools ●
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show DB node information
show connection pool information
show pool_version ●
show pgpool-II version Copyright(c)2011 Tatsuo Ishii
14
Plans for pgpool-II 3.1 ● ●
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Import PostgreSQL 9.0 SQL parser Allow to switch back/force primary node and standby node without restarting pgpool-II Allow to use regular expression in black/white_function_list
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Allow to use syslog
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Better handling for more than 1 standby
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French translation Copyright(c)2011 Tatsuo Ishii
15
Switch back/force failover Pgpool-II 3.0 Node 0
Node 1
Primary Down
Pgpool-II 3.1 Node 0
Node 1
Standby
Primary
Standby
Primary
Down
Primary
Standby
Primary
Primary
Down
Primary
Standby
Primary
Node 2
Standby
Cannot make node 0 as primary. Because primary needs to be “younger” than standby node. Copyright(c)2011 Tatsuo Ishii
Switch back/force failover! 16
More than 1 standby servers. What's the problem anyway?
X
X
Primary Standby A Standby B
Primary Which standby should be promoted? Current rule is “the least position in the config file”. This is not too intelligent.
Standby A New primary
Copyright(c)2011 Tatsuo Ishii
X
The old standby cannot connect to new primary. So there's no standby server which can backup the system. Moreover, if the client connects to the old standby, it will read outdated data.
