Properly indent SGML file.
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Bruce Momjian
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<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.18 2007/11/08 19:16:30 momjian Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.19 2007/11/08 19:18:23 momjian Exp $ -->
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<chapter id="high-availability">
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<chapter id="high-availability">
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<title>High Availability, Load Balancing, and Replication</title>
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<title>High Availability, Load Balancing, and Replication</title>
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<variablelist>
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<variablelist>
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<varlistentry>
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<varlistentry>
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<term>Shared Disk Failover</term>
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<term>Shared Disk Failover</term>
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<listitem>
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<listitem>
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<para>
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<para>
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Shared disk failover avoids synchronization overhead by having only one
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Shared disk failover avoids synchronization overhead by having only one
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copy of the database. It uses a single disk array that is shared by
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copy of the database. It uses a single disk array that is shared by
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multiple servers. If the main database server fails, the standby server
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multiple servers. If the main database server fails, the standby server
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is able to mount and start the database as though it was recovering from
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is able to mount and start the database as though it was recovering from
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a database crash. This allows rapid failover with no data loss.
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a database crash. This allows rapid failover with no data loss.
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</para>
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</para>
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<para>
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<para>
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Shared hardware functionality is common in network storage devices.
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Shared hardware functionality is common in network storage devices.
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Using a network file system is also possible, though care must be
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Using a network file system is also possible, though care must be
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taken that the file system has full POSIX behavior (see <xref
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taken that the file system has full POSIX behavior (see <xref
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linkend="creating-cluster-nfs">). One significant limitation of this
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linkend="creating-cluster-nfs">). One significant limitation of this
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method is that if the shared disk array fails or becomes corrupt, the
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method is that if the shared disk array fails or becomes corrupt, the
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primary and standby servers are both nonfunctional. Another issue is
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primary and standby servers are both nonfunctional. Another issue is
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that the standby server should never access the shared storage while
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that the standby server should never access the shared storage while
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the primary server is running.
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the primary server is running.
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</para>
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</para>
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>File System Replication</term>
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<term>File System Replication</term>
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<listitem>
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<listitem>
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<para>
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<para>
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A modified version of shared hardware functionality is file system
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A modified version of shared hardware functionality is file system
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replication, where all changes to a file system are mirrored to a file
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replication, where all changes to a file system are mirrored to a file
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system residing on another computer. The only restriction is that
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system residing on another computer. The only restriction is that
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the mirroring must be done in a way that ensures the standby server
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the mirroring must be done in a way that ensures the standby server
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has a consistent copy of the file system — specifically, writes
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has a consistent copy of the file system — specifically, writes
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to the standby must be done in the same order as those on the master.
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to the standby must be done in the same order as those on the master.
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DRBD is a popular file system replication solution for Linux.
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DRBD is a popular file system replication solution for Linux.
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</para>
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</para>
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<!--
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<!--
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https://forge.continuent.org/pipermail/sequoia/2006-November/004070.html
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https://forge.continuent.org/pipermail/sequoia/2006-November/004070.html
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@ -128,150 +128,150 @@ only committed once to disk and there is a distributed locking
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protocol to make nodes agree on a serializable transactional order.
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protocol to make nodes agree on a serializable transactional order.
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-->
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-->
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>Warm Standby Using Point-In-Time Recovery (<acronym>PITR</>)</term>
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<term>Warm Standby Using Point-In-Time Recovery (<acronym>PITR</>)</term>
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<listitem>
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<listitem>
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<para>
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<para>
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A warm standby server (see <xref linkend="warm-standby">) can
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A warm standby server (see <xref linkend="warm-standby">) can
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be kept current by reading a stream of write-ahead log (WAL)
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be kept current by reading a stream of write-ahead log (WAL)
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records. If the main server fails, the warm standby contains
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records. If the main server fails, the warm standby contains
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almost all of the data of the main server, and can be quickly
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almost all of the data of the main server, and can be quickly
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made the new master database server. This is asynchronous and
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made the new master database server. This is asynchronous and
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can only be done for the entire database server.
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can only be done for the entire database server.
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</para>
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</para>
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>Master-Slave Replication</term>
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<term>Master-Slave Replication</term>
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<listitem>
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<listitem>
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<para>
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<para>
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A master-slave replication setup sends all data modification
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A master-slave replication setup sends all data modification
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queries to the master server. The master server asynchronously
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queries to the master server. The master server asynchronously
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sends data changes to the slave server. The slave can answer
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sends data changes to the slave server. The slave can answer
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read-only queries while the master server is running. The
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read-only queries while the master server is running. The
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slave server is ideal for data warehouse queries.
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slave server is ideal for data warehouse queries.
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</para>
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</para>
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<para>
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<para>
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Slony-I is an example of this type of replication, with per-table
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Slony-I is an example of this type of replication, with per-table
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granularity, and support for multiple slaves. Because it
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granularity, and support for multiple slaves. Because it
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updates the slave server asynchronously (in batches), there is
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updates the slave server asynchronously (in batches), there is
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possible data loss during fail over.
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possible data loss during fail over.
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</para>
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</para>
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>Statement-Based Replication Middleware</term>
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<term>Statement-Based Replication Middleware</term>
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<listitem>
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<listitem>
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<para>
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<para>
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With statement-based replication middleware, a program intercepts
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With statement-based replication middleware, a program intercepts
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every SQL query and sends it to one or all servers. Each server
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every SQL query and sends it to one or all servers. Each server
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operates independently. Read-write queries are sent to all servers,
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operates independently. Read-write queries are sent to all servers,
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while read-only queries can be sent to just one server, allowing
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while read-only queries can be sent to just one server, allowing
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the read workload to be distributed.
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the read workload to be distributed.
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</para>
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</para>
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<para>
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<para>
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If queries are simply broadcast unmodified, functions like
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If queries are simply broadcast unmodified, functions like
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<function>random()</>, <function>CURRENT_TIMESTAMP</>, and
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<function>random()</>, <function>CURRENT_TIMESTAMP</>, and
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sequences would have different values on different servers.
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sequences would have different values on different servers.
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This is because each server operates independently, and because
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This is because each server operates independently, and because
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SQL queries are broadcast (and not actual modified rows). If
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SQL queries are broadcast (and not actual modified rows). If
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this is unacceptable, either the middleware or the application
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this is unacceptable, either the middleware or the application
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must query such values from a single server and then use those
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must query such values from a single server and then use those
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values in write queries. Also, care must be taken that all
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values in write queries. Also, care must be taken that all
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transactions either commit or abort on all servers, perhaps
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transactions either commit or abort on all servers, perhaps
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using two-phase commit (<xref linkend="sql-prepare-transaction"
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using two-phase commit (<xref linkend="sql-prepare-transaction"
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endterm="sql-prepare-transaction-title"> and <xref
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endterm="sql-prepare-transaction-title"> and <xref
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linkend="sql-commit-prepared" endterm="sql-commit-prepared-title">.
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linkend="sql-commit-prepared" endterm="sql-commit-prepared-title">.
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Pgpool and Sequoia are an example of this type of replication.
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Pgpool and Sequoia are an example of this type of replication.
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</para>
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</para>
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>Asynchronous Multi-Master Replication</term>
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<term>Asynchronous Multi-Master Replication</term>
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<listitem>
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<listitem>
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<para>
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<para>
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For servers that are not regularly connected, like laptops or
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For servers that are not regularly connected, like laptops or
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remote servers, keeping data consistent among servers is a
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remote servers, keeping data consistent among servers is a
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challenge. Using asynchronous multi-master replication, each
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challenge. Using asynchronous multi-master replication, each
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server works independently, and periodically communicates with
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server works independently, and periodically communicates with
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the other servers to identify conflicting transactions. The
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the other servers to identify conflicting transactions. The
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conflicts can be resolved by users or conflict resolution rules.
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conflicts can be resolved by users or conflict resolution rules.
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</para>
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</para>
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>Synchronous Multi-Master Replication</term>
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<term>Synchronous Multi-Master Replication</term>
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<listitem>
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<listitem>
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<para>
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<para>
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In synchronous multi-master replication, each server can accept
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In synchronous multi-master replication, each server can accept
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write requests, and modified data is transmitted from the
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write requests, and modified data is transmitted from the
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original server to every other server before each transaction
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original server to every other server before each transaction
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commits. Heavy write activity can cause excessive locking,
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commits. Heavy write activity can cause excessive locking,
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leading to poor performance. In fact, write performance is
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leading to poor performance. In fact, write performance is
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often worse than that of a single server. Read requests can
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often worse than that of a single server. Read requests can
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be sent to any server. Some implementations use shared disk
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be sent to any server. Some implementations use shared disk
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to reduce the communication overhead. Synchronous multi-master
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to reduce the communication overhead. Synchronous multi-master
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replication is best for mostly read workloads, though its big
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replication is best for mostly read workloads, though its big
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advantage is that any server can accept write requests —
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advantage is that any server can accept write requests —
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there is no need to partition workloads between master and
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there is no need to partition workloads between master and
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slave servers, and because the data changes are sent from one
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slave servers, and because the data changes are sent from one
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server to another, there is no problem with non-deterministic
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server to another, there is no problem with non-deterministic
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functions like <function>random()</>.
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functions like <function>random()</>.
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</para>
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</para>
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<para>
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<para>
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<productname>PostgreSQL</> does not offer this type of replication,
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<productname>PostgreSQL</> does not offer this type of replication,
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though <productname>PostgreSQL</> two-phase commit (<xref
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though <productname>PostgreSQL</> two-phase commit (<xref
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linkend="sql-prepare-transaction"
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linkend="sql-prepare-transaction"
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endterm="sql-prepare-transaction-title"> and <xref
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endterm="sql-prepare-transaction-title"> and <xref
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linkend="sql-commit-prepared" endterm="sql-commit-prepared-title">)
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linkend="sql-commit-prepared" endterm="sql-commit-prepared-title">)
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can be used to implement this in application code or middleware.
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can be used to implement this in application code or middleware.
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</para>
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</para>
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>Data Partitioning</term>
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<term>Data Partitioning</term>
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<listitem>
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<listitem>
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<para>
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<para>
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Data partitioning splits tables into data sets. Each set can
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Data partitioning splits tables into data sets. Each set can
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be modified by only one server. For example, data can be
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be modified by only one server. For example, data can be
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partitioned by offices, e.g. London and Paris, with a server
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partitioned by offices, e.g. London and Paris, with a server
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in each office. If queries combining London and Paris data
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in each office. If queries combining London and Paris data
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are necessary, an application can query both servers, or
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are necessary, an application can query both servers, or
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master/slave replication can be used to keep a read-only copy
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master/slave replication can be used to keep a read-only copy
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of the other office's data on each server.
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of the other office's data on each server.
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</para>
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</para>
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</listitem>
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</listitem>
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</varlistentry>
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</varlistentry>
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<varlistentry>
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<varlistentry>
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<term>Commercial Solutions</term>
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<term>Commercial Solutions</term>
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<listitem>
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<listitem>
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<para>
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<para>
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Because <productname>PostgreSQL</> is open source and easily
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Because <productname>PostgreSQL</> is open source and easily
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extended, a number of companies have taken <productname>PostgreSQL</>
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extended, a number of companies have taken <productname>PostgreSQL</>
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and created commercial closed-source solutions with unique
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and created commercial closed-source solutions with unique
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failover, replication, and load balancing capabilities.
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failover, replication, and load balancing capabilities.
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</para>
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</variablelist>
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