These are completely different things, but they are often required and/or provided
together. It is important to understand the difference between them in order to properly
analyze the requirements of an application. It is possible to provide load balancing
without high availability??”for example, consider a group of servers presented to
the Internet via round-robin DNS. The load is distributed roughly equally over
the group of servers, but the system is certainly not highly available! If one server
goes down, DNS will still faithfully distribute requests to it, and every one in N
requests will go unanswered.
Conversely, high availability can be provided without load balancing. High availability
necessitates the use of redundant components, but nothing says that those components
must be online and in use. A common configuration is the hot spare: a
duplicate server that stays powered up but offline, continually monitoring its online
twin, ready to take over if necessary. This can actually be more economical than trying
to balance requests between the two servers and keep them in sync.
In this section, we review the primary load balancing and high availability solutions
for common database management systems.
Load Balancing and High Availability | 121
MySQL
Replication
MySQL has built-in support for master-slave replication. The master logs all transactions
to a binlog (binary log).
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