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<chapter id="failover">

<title>Failover, Replication, Load Balancing, and Clustering Options</title>

</varlistentry>

<varlistentry>

<term>Data Partitioning</term>

<listitem>

<para>

Data partitioning splits tables into data sets. Each set can

be modified by only one server. For example, data can be

partitioned by offices, e.g. London and Paris. While London

and Paris servers have all data records, only London can modify

London records, and Paris can only modify Paris records. This

is similar to the "Master/Slave Replication" item above, except

that instead of having a read/write server and a read-only

server, each server has a read/write data set and a read-only

data set.

</para>

<para>

Such partitioning provides both failover and load balancing. Failover

is achieved because the data resides on both servers, and this is an

ideal way to enable failover if the servers share a slow communication

channel. Load balancing is possible because read requests can go to any

of the servers, and write requests are split among the servers. Of

course, the communication to keep all the servers up-to-date adds

overhead, so ideally the write load should be low, or localized as in

the London/Paris example above.

</para>

<para>

Data partitioning is usually handled by application code, though rules

and triggers can be used to keep the read-only data sets current. Slony-I

can also be used in such a setup. While Slony-I replicates only entire

tables, London and Paris can be placed in separate tables, and

inheritance can be used to access both tables using a single table name.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Multi-Master Replication Using Query Broadcasting</term>

<term>Query Broadcasting</term>

<listitem>

<para>

One way to do multi-master replication is by having a program

intercept every SQL query and send it to all servers. Each

server operates independently. Read-only queries can be sent

to a single server because there is no need for all servers to

process it.

In query broadcasting, a program intercepts every SQL query

and sends it to all servers. Each server operates independently.

Read-only queries can be sent to a single server because there

is no need for all servers to process it.

</para>

<para>

</listitem>

</varlistentry>

<varlistentry>

<term>Data Partitioning</term>

<listitem>

<para>

Data partitioning splits tables into data sets. Each set can

be modified by only one server. For example, data can be

partitioned by offices, e.g. London and Paris, with a server

in each office. If queries combining London and Paris data

are necessary, an application can query both servers, or

master/slave replication can be used to keep a read-only copy

of the other office's data on each server.

</para>

</listitem>

</varlistentry>

<varlistentry>

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

<listitem>