The pool of VIPs is distributed
via a DNS round-robin list, so the application will pick a VIPmore or
less at random.
PostgreSQL
There are several load-balancing and high-availability options for PostgreSQL.
Because there is no single company behind PostgreSQL, the options are provided by
different organizations and companies. Each product typically embodies a different
replication or clustering paradigm. Some of the options are described in this section.
High availability: Warm standby
Warm standby is a simple way to achieve high availability under PostgreSQL. It
takes some configuration, but the configuration is documented well. Warm standby
uses the write-ahead log (WAL) that PostgreSQL logs activity to. Changes are written
in the WAL prior to being committed, so the database state can be reconstructed
even if a transaction is interrupted catastrophically. Log shipping is the process of
sending the WAL as files from the master to a slave.
Under a warm standby setup, a server is on standby, in restore mode. It is continuously
restoring from the primary server, using a restore command that waits for
WALs to become available and applies them as soon as they do. If the primary server
dies, a monitoring system (which must be provided by the user) designates the
standby as the new primary server.
Master-slave replication: Slony-I
Slony-I is a master-slave replication system similar to the replication mechanisms
included with MySQL.
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