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<chapter id="failover">

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

<title>Query Broadcast Load Balancing</title>

<para>

Query broadcast load balancing is accomplished by having a program

intercept every query and send it to all servers. Read-only queries can

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

process it. This is unusual because most replication solutions have

each write server propagate its changes to the other servers. With

query broadcasting, each server operates independently.

Query broadcast load balancing is accomplished by having a

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

This is unique because most replication solutions have the write

server propagate its changes to the other servers. With query

broadcasting, 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>

Because each server operates independently, functions like

One limitation of this solution is that functions like

<function>random()</>, <function>CURRENT_TIMESTAMP</>, and

sequences can have different values on different servers. If

this is unacceptable, applications must query such values from

a single server and then use those values in write queries.

Also, care must also be taken that all transactions either commit

or abort on all servers Pgpool is an example of this type of

replication.

sequences can have different values on different servers. This

is because each server operates independently, and because SQL

queries are broadcast (and not actual modified rows). If this

is unacceptable, applications must query such values from a

single server and then use those values in write queries. Also,

care must be taken that all transactions either commit or abort

on all servers Pgpool is an example of this type of replication.

</para>

</sect1>

<sect1 id="clustering-for-load-balancing">

<title>Clustering For Load Balancing</title>

<para>

In clustering, each server can accept write requests, andthese

write requests are broadcastfrom the original server toall

other serversbefore each transaction commits. Heavy write

activitycan cause excessive locking, leading to poor performance.

Infact, write performance is often worse than that of a single

In clustering, each server can accept write requests, andmodified

data is transmittedfrom the original server toevery other

serverbefore 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. Clustering

is best for mostly read workloads, though its big advantage is

that any server can accept write requests--- there is no need

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

to partition workloads between read/write and read-only servers.

</para>