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<chapter id="high-availability">

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

</para>

<para>

One limitation of this solution is that functions like

If queries are simply broadcast unmodified, functions like

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

sequencescan 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, perhaps using two-phase commit (<xref

linkend="sql-prepare-transaction"

sequenceswould have different values on different servers.

Thisis because each server operates independently, and because

SQLqueries are broadcast (and not actual modified rows). If

thisis unacceptable,either the middleware or the application

must query such values from asingle server and then use those

values in write queries.Also, care must be taken that all

transactions either commitor abort on all servers, perhaps

using two-phase commit (<xreflinkend="sql-prepare-transaction"

endterm="sql-prepare-transaction-title"> and <xref

linkend="sql-commit-prepared" endterm="sql-commit-prepared-title">.

Pgpool is an example of this type of replication.