Difference between pages "Relationship between max pool, num init children, and max connections" and "Documentation"

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Gurjeet Singh (last updated: 22 August 2012)
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== Official documentation ==
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* '''pgpool-II'''
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** [http://www.pgpool.net/docs/latest/pgpool-en.html pgpool-II manual (English)]
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** [http://www.pgpool.net/docs/latest/pgpool-ja.html pgpool-II manual (Japanese)]
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** [http://www.pgpool.net/docs/latest/pgpool-zh_cn.html pgpool-II manual(Simplified Chinese)]
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** [http://www.pgpool.net/docs/latest/pgpool-fr.html pgpool-II manual (French)]
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** [http://www.pgpool.net/docs/latest/tutorial-en.html pgpool-II tutorial (English)]
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** [http://www.pgpool.net/docs/latest/tutorial-ja.html pgpool-II tutorial (Japanese)]
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** [http://www.pgpool.net/docs/latest/tutorial-zh_cn.html pgpool-II tutorial (Simplified Chinese)]
  
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max_pool parameter configures how many connections to cache _per
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* '''pgpoolAdmin'''
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child_. So if num_init_children is configured to 100, and max_pool is
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** [http://www.pgpool.net/pgpoolAdmin/doc/index_en.html pgpoolAdmin manual (English)]
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configured to 3, then pgpool can potentially open 300 (=3*100) connections
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** [http://www.pgpool.net/pgpoolAdmin/doc/index_ja.html pgpoolAdmin manual (Japanese)]
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to the backend database.
 
  
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A child process opens a new backend connection only if the requested
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== User contributed documentation ==
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[user,database] pair is not already in the cache. So if the application
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* '''Documentation'''
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uses only one user to connect to only one database, say [pguser1,pgdb1],
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** [http://www.pgpool.net/pgpool-web/contrib_docs/pgpool-II_for_beginners.pdf Gerd Koenig's "pgpool-II for beginners"] (English)
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then each child will continue to reuse the first connection and will never
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** [http://lets.postgresql.jp/documents/technical/pgpool/1 What is pgpool-II] (Japanese)
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open a second connection, so in effect pgpool will open no more than 100
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** [http://lets.postgresql.jp/documents/technical/pgpool/2 Creating a replication system using pgpool-II] (Japanese)
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backend connections even though max_pool is set to 3.
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** [http://www.sraoss.co.jp/event_seminar/2010/20100702-03char10.pdf Making master/slave systems work better with pgpool-II] (English, PDF)
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** [[Relationship_between_max_pool,_num_init_children,_and_max_connections|Relationship between max_pool, num_init_children, and max_connections]](English, 2012/8/25)
  
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But if the application uses more than one pair of [user,database]
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* '''Tutorials of Streaming Replication with pgpool-II'''
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connection parameters, then each child will cache the connection it might
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** Simple Streaming replication setting with pgpool-II
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already have for another pair, and open a new backend connection for the
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*** For pgpool-II 3.1 and PostgreSQL 9.1: [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting/index.html English (2012/01/31) ]  [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting/index-ja.html Japanese (2012/6/1) ]
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requested pair.
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*** For pgpool-II 3.0 and PostgreSQL 9.0: [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting_3.0/index.html English] [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting_3.0/index-ja.html Japanese]
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** multiple server version
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*** For pgpool-II 3.1 and PostgreSQL 9.1: [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting2/index.html English (2012/01/31)] [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting2_3.0/index-ja.html Japanese (2012/6/1)]
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*** For pgpool-II 3.0 and PostgreSQL 9.0: [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting2/index.html English] [http://www.pgpool.net/pgpool-web/contrib_docs/simple_sr_setting2_3.0/index-ja.html Japanese]
  
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For eg., if the application now uses these 4 pairs: [user1,db1]
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* '''Tutorials of pgpool-II 3.2'''
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[user1,db2] [user2,db1] [user2,db2] to connect to pgpool, then each child
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** On memory query cache: [http://www.pgpool.net/pgpool-web/contrib_docs/memqcache/en.html English (2012/07/20)] [http://www.pgpool.net/pgpool-web/contrib_docs/memqcache/ja.html Japanese (2012/07/20)]
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process can cache up to 3 connections for the first 3 different pairs it
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** Watchdog:
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receives connection requests for. But as soon as it receives a request for
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*** Basic: [http://www.pgpool.net/pgpool-web/contrib_docs/watchdog/en.html English (2012/07/20)] [http://www.pgpool.net/pgpool-web/contrib_docs/watchdog/ja.html Japanese (2012/07/20)]
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the 4th pair that it does not yet have a connection for, then it will
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*** master-slave mode: [http://www.pgpool.net/pgpool-web/contrib_docs/watchdog_master_slave/en.html English (2012/10/22)] [http://www.pgpool.net/pgpool-web/contrib_docs/watchdog_master_slave/ja.html Japanese (2012/10/15)]
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disconnect the oldest connection in the cache and open a new connection for
 
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the 4th pair.
 
  
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As we already know that there's no guarantee as to which child process
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== Developer's documentation ==
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will handle an incoming connection request, max_pool tries to improve the
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* [http://www.pgpool.net/pgpool-web/pgpoolAdmin-add-directives.txt How to add configurfation directives to pgpoolAdmin]
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performance a little bit by caching connections of different pairs, in the
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* [http://www.pgpool.net/pgpool-web/contrib_docs/ClusterMeeting2012.pdf 3rd Postgres Cluster Hackers Summit, pgCon 2012 "pgpool-II Development Status Updates"]
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hopes that an incoming connection request might match one of the
 
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connections cached by the child process. But this also causes an explosion
 
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in the number of connections that pgpool would request from the database.
 
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So, in order to guarantee that the application connection requests are
 
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never rejected, and that the connection requests wait until a database
 
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connection is available, the following condition should be met:
 
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max_pool*num_init_children <= (max_connections - superuser_reserved_connections)
 
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If the application uses superuser connections (which is not
 
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recommended), then the condition is reduced to:
 
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max_pool*num_init_children <= max_connections
 
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Setting max_pool to 1 will guarantee that the number of database
 
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connections opened by pgpool child processes never exceeds the
 
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num_init_children value. If for performance reasons, as explained above,
 
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you do wish to set max_pool to more than 1, then max_connections will also
 
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have to be increased accordingly so that application connection requests do
 
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not get denied.
 

Revision as of 01:11, 13 December 2012

Official documentation

User contributed documentation

Developer's documentation

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