<!-- doc/src/sgml/high-availability.sgml -->

<chapter id="high-availability">
 <title>High Availability, Load Balancing, and Replication</title>

 <indexterm><primary>high availability</primary></indexterm>
 <indexterm><primary>failover</primary></indexterm>
 <indexterm><primary>replication</primary></indexterm>
 <indexterm><primary>load balancing</primary></indexterm>
 <indexterm><primary>clustering</primary></indexterm>
 <indexterm><primary>data partitioning</primary></indexterm>

 <para>
  Database servers can work together to allow a second server to
  take over quickly if the primary server fails (high
  availability), or to allow several computers to serve the same
  data (load balancing).  Ideally, database servers could work
  together seamlessly.  Web servers serving static web pages can
  be combined quite easily by merely load-balancing web requests
  to multiple machines.  In fact, read-only database servers can
  be combined relatively easily too.  Unfortunately, most database
  servers have a read/write mix of requests, and read/write servers
  are much harder to combine.  This is because though read-only
  data needs to be placed on each server only once, a write to any
  server has to be propagated to all servers so that future read
  requests to those servers return consistent results.
 </para>

 <para>
  This synchronization problem is the fundamental difficulty for
  servers working together.  Because there is no single solution
  that eliminates the impact of the sync problem for all use cases,
  there are multiple solutions.  Each solution addresses this
  problem in a different way, and minimizes its impact for a specific
  workload.
 </para>

 <para>
  Some solutions deal with synchronization by allowing only one
  server to modify the data.  Servers that can modify data are
  called read/write, <firstterm>master</firstterm> or <firstterm>primary</firstterm> servers.
  Servers that track changes in the primary are called <firstterm>standby</firstterm>
  or <firstterm>secondary</firstterm> servers. A standby server that cannot be connected
  to until it is promoted to a primary server is called a <firstterm>warm
  standby</firstterm> server, and one that can accept connections and serves read-only
  queries is called a <firstterm>hot standby</firstterm> server.
 </para>

 <para>
  Some solutions are synchronous,
  meaning that a data-modifying transaction is not considered
  committed until all servers have committed the transaction.  This
  guarantees that a failover will not lose any data and that all
  load-balanced servers will return consistent results no matter
  which server is queried. In contrast, asynchronous solutions allow some
  delay between the time of a commit and its propagation to the other servers,
  opening the possibility that some transactions might be lost in
  the switch to a backup server, and that load balanced servers
  might return slightly stale results.  Asynchronous communication
  is used when synchronous would be too slow.
 </para>

 <para>
  Solutions can also be categorized by their granularity.  Some solutions
  can deal only with an entire database server, while others allow control
  at the per-table or per-database level.
 </para>

 <para>
  Performance must be considered in any choice.  There is usually a
  trade-off between functionality and
  performance.  For example, a fully synchronous solution over a slow
  network might cut performance by more than half, while an asynchronous
  one might have a minimal performance impact.
 </para>

 <para>
  The remainder of this section outlines various failover, replication,
  and load balancing solutions.
 </para>

 <sect1 id="different-replication-solutions">
 <title>Comparison of Different Solutions</title>

 <variablelist>

  <varlistentry>
   <term>Shared Disk Failover</term>
   <listitem>

    <para>
     Shared disk failover avoids synchronization overhead by having only one
     copy of the database.  It uses a single