persistent-connections.xml

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<chapter xml:id="features.persistent-connections" xmlns="http://docbook.org/ns/docbook">
 <title>Persistent Database Connections</title>

 <simplesect>
  <title>What are Persistent Connections?</title>
  <simpara>
   Persistent connections are links that do not close when the
   execution of the script ends. When a persistent connection is
   requested, PHP checks whether an identical persistent
   connection (that remained open from earlier) already exists; if one does,
   it is reused, and if not, a new link is created. An 'identical'
   connection is one opened to the same host with
   the same username and password (where applicable).
  </simpara>
  <simpara>
   There is no way to request a specific connection, or to guarantee
   whether the returned connection will be an existing one or a brand new
   one (if all existing connections are in use, or the request is being
   served by a different worker, which has a separate pool of connections).
  </simpara>
  <simpara>
   PHP's persistent connections therefore cannot be used to, for example:
  </simpara>
  <simplelist>
   <member>assign a specific database session to a specific web user</member>
   <member>create a large transaction across multiple requests</member>
   <member>initiate a query on one request and collect the results on another</member>
  </simplelist>
  <simpara>
   Persistent connections do not provide <emphasis>any</emphasis>
   functionality that was not possible with non-persistent connections.
  </simpara>
 </simplesect>

 <simplesect xml:id="persistent-connections.web">
  <title>Web Requests</title>
  <simpara>
   There are two ways in which a web server can utilize PHP to generate
   web pages:
  </simpara>
  <simpara>
   The first method is to use PHP as a CGI "wrapper". When run this
   way, an instance of the PHP interpreter is created and destroyed
   for every page request (for a PHP page) to the web server.
   Because it is destroyed after every request, any resources that it
   acquires (such as a link to an SQL database server) are closed when
   it is destroyed. In this case, there is nothing to be gained from
   using persistent connections - they simply do not persist.
  </simpara>
  <simpara>
   The second, and most popular, method is to run PHP-FPM, or PHP as a module
   in a multiprocess web server (currently only Apache).
   These setups typically have one process (the parent) which
   coordinates a set of processes (its children) that actually do the
   work of serving up web pages. When a request comes in from a
   client, it is handed off to one of the children that is not already
   serving another client. This means that when the same client makes
   a second request to the server, it may be served by a different
   child process than the first time. Once a persistent connection has been
   opened, any subsequent page served by the same child process can reuse the
   already established connection to the SQL server.
  </simpara>
  <note>
   <para>
   The method in use can be checked by looking at the value of
   "Server API" in the output of <function>phpinfo</function> or the value of
   <constant>PHP_SAPI</constant>, run from a web request.
   </para>
   <para>
    If the Server API is "Apache 2 Handler" or "FPM/FastCGI", then persistent
    connections will be used across requests served by the same worker. For any
    other value, persistent connections will not persist after each request.
   </para>
  </note>
 </simplesect>

 <simplesect xml:id="persistent-connections.cli">
  <title>Command-line Processes</title>
  <simpara>
   As command-line PHP uses a new process for each script, persistent
   connections are not shared between command-line scripts, so there is no
   value in using them in transient scripts such as crons or commands.
   However, they may be useful, for example, in a long-running application
   server that serves many requests or tasks, each of which may need its
   own database connection.
  </simpara>
 </simplesect>

 <simplesect xml:id="persistent-connections.why">
  <title>Why Use Them?</title>
  <simpara>
   Persistent connections are beneficial when the overhead of creating a link
   to an SQL server is high. Whether this overhead is significant depends on
   many factors, such as the type of database, whether it resides on the same
   machine as the web server, and how loaded that machine is. When the
   connection overhead is high, persistent connections can help considerably:
   each child process connects only once for its entire lifespan, rather than
   every time it processes a page that requires a connection to the SQL
   server. This means every child that opens a persistent connection will
   maintain its own connection to the server. For example, if 20 different
   child processes each run a script that makes a persistent connection to
   the SQL server, there will be 20 separate connections to that server, one
   from each child.
  </simpara>
 </simplesect>

 <simplesect xml:id="persistent-connections.drawbacks.conn-limits">
  <title>Potential Drawbacks: Connection Limits</title>
  <simpara>
   Note, however, that this can have drawbacks when using a
   database with connection limits that are exceeded by persistent
   child connections. If the database has a limit of 16 simultaneous
   connections, and during a busy server session 17 child
   processes attempt to connect, one of them will fail. If there are
   bugs in the scripts that prevent connections from shutting
   down (such as infinite loops), a database with only 16 connections
   may be rapidly swamped.
  </simpara>
  <simpara>
   Persistent connections will usually increase the number of connections open
   at any given time, because idle workers still hold on to the connections they
   opened for previous requests. If a large number of workers are spun up to
   handle a spike in traffic, the connections they opened will remain until
   the worker is terminated or the database server closes the connection.
  </simpara>
  <simpara>
   Ensure that the maximum number of connections allowed by the database server
   is greater than the maximum number of web request workers (plus any other
   usage such as crons or administrative connections).
  </simpara>
  <simpara>
   Check the database documentation for information on handling abandoned or
   idle connections (timeouts). Long timeouts may significantly increase the
   number of persistent connections open at any one time.
  </simpara>
 </simplesect>

 <simplesect xml:id="persistent-connections.drawbacks.state">
  <title>Potential Drawbacks: Maintaining Connection State</title>
  <simpara>
   Some database extensions perform automatic cleanup when the connection is
   reused; others leave this task at the discretion of the application developer.
   Depending on the chosen database extension and the application design, manual
   cleanup may be needed before the script exits. Changes that may leave
   connections in an unexpected state include:
  </simpara>
  <simplelist>
   <member>Selected / default database</member>
   <member>Table locks</member>
   <member>Uncommitted transactions</member>
   <member>Temporary tables</member>
   <member>Connection specific settings or features such as profiling</member>
  </simplelist>
  <simpara>
   Table locks and transactions that are not cleaned up or closed may cause
   other queries to be blocked indefinitely and/or cause subsequent reuse of
   the connection to cause unexpected changes.
  </simpara>
  <simpara>
   Having the wrong database selected will cause subsequent reuse of the
   connection to be unable to execute queries as expected (or execute them on
   the wrong database if schemas are similar enough).
  </simpara>
  <simpara>
   If temporary tables are not cleaned up, subsequent requests will not be able
   to recreate the same table.
  </simpara>
  <simpara>
   Cleanup can be implemented using class destructors or
   <function>register_shutdown_function</function>. Dedicated connection
   pooling proxies that include this as part of their functionality may
   also be considered.
  </simpara>
 </simplesect>

 <simplesect xml:id="persistent-connections.final-words">
  <title>Final Words</title>
  <simpara>
   Given their behavior and potential drawbacks described above, persistent
   connections should not be used without careful consideration. They should
   not be used without implementing additional changes to the application and
   careful configuration of the database server and web server and/or PHP-FPM.
  </simpara>
  <simpara>
   Consider alternative solutions such as investigating and fixing the causes of
   connection creation overheads (for example, disabling reverse DNS lookups on
   the database server), or dedicated connection pooling proxies.
  </simpara>
  <simpara>
   For high volume web APIs, consider using alternative runtimes or long-running
   application servers.
  </simpara>
 </simplesect>

 <simplesect role="seealso" xml:id="persistent-connections.seealso">
  &reftitle.seealso;
  <simplelist>
   <member><function>ibase_pconnect</function></member>
   <member><function>oci_pconnect</function></member>
   <member><function>odbc_pconnect</function></member>
   <member><function>pfsockopen</function></member>
   <member><function>pg_connect</function></member>
   <member><link linkend="mysqli.persistconns">MySQLi and Persistent Connections</link></member>
   <member><link linkend="pdo.connections">PDO Connection Management</link></member>
  </simplelist>
 </simplesect>
</chapter>

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