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<chapter id="high-availability">

<title>High Availability and Load Balancing</title>

</varlistentry>

<varlistentry>

<term>Multi-MasterClustering</term>

<term>Multi-MasterReplication</term>

<listitem>

<para>

Inclustering, each server can accept write requests, and

modified data is transmitted from the original server to every

other server before each transaction commits. Heavy write

activity can cause excessive locking, leading to poor performance.

In fact, write performance is often worse than that of a single

server. Read requests can be sent to any server. Some

implementations use cluster-wide shared memory or shared disk

to reduce the communication overhead. Clustering is best for

mostly read workloads, though its big advantage is that any

server can accept write requests &mdash; there is no need to

partition workloads between master and slave servers, and

because the data changes are sent from one server to another,

Inmulti-master replication, each server can accept write

requests, andmodified data is transmitted from the original

server to everyother server before each transaction commits.

Heavy writeactivity can cause excessive locking, leading to

poor performance.In fact, write performance is often worse

than that of a singleserver. Read requests can be sent to

any server. Someimplementations use cluster-wide shared memory

or shared diskto reduce the communication overhead. Clustering

is best formostly read workloads, though its big advantage is

that anyserver can accept write requests &mdash; there is no

need topartition workloads between master and slave servers,

andbecause the data changes are sent from one server to another,

there is no problem with non-deterministic functions like

<function>random()</>.

</para>

</varlistentry>

<varlistentry>

<term>Clustering For Parallel Query Execution</term>

<term>Multi-Server Parallel Query Execution</term>

<listitem>

<para>