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<!entity failover SYSTEM "failover.sgml">

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<chapter id="failover">

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

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/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.

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.

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&failover;