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<chapter id="failover">

<title>Failover, Replication, Load Balancing,andClustering Options</title>

<chapter id="high-availability">

<title>High AvailabilityandLoad Balancing</title>

<indexterm><primary>high availability</></>

<indexterm><primary>failover</></>

<indexterm><primary>replication</></>

<indexterm><primary>load balancing</></>

<indexterm><primary>clustering</></>

<indexterm><primary>data partitioning</></>

<para>

Database servers can work together to allow a second server to

quickly take over if the primary server fails (failover), 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/writeservers 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 serverhas to be propagated to

all servers so that future read requests to those servers return

consistent results.

quickly take overquicklyif the primary server fails (high

availability), or toallow several computers to serve the same

data (load balancing).Ideally, database servers could work

together seamlessly. Webservers serving static web pages can

be combined quite easily bymerely load-balancing web requests

to multiple machines. Infact, read-only database servers can

be combined relatively easilytoo. Unfortunately, most database

servers have a read/writemix of requests, and read/write servers

are much harder to combine.This is because though read-only

data needs to be placed on each serveronly once, a write to any

server has to be propagated toall servers so that future read

requests to those servers returnconsistent 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.

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>

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