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Table of Contents [+/-]
Important
For more information on MySQL Load Balancer, including how to be
included in the beta programme, contact
<enterprise-beta@mysql.com>.
The MySQL Load Balancer is an application that communicates with one or more MySQL servers and provides connectivity to those servers for multiple clients. The MySQL Load Balancer is logically placed between the clients and the MySQL server; instead of clients connecting directly to each MySQL server, all clients connect to the MySQL Load Balancer, and the MySQL Load Balancer forwards the connection on to one of the MySQL servers.
The initial release of the MySQL Load Balancer provides read-only load balancing over a number of MySQL servers. Initially, you populate the MySQL Load Balancer configuration with the list of available MySQL servers to use when distributing work. The MySQL Load Balancer automatically and evenly distributes connections from clients to each server. Distribution is handled by a simple count for the number connections distributed to each server - new connections are automatically sent to the server with the lowest count.
When used in combination with a replication setup, the MySQL Load Balancer also monitors the replication status. The master and slaves within the replication setup are monitored and additional decisions about the routing of incoming connections to MySQL servers are made based on the replication status:
If MySQL Load Balancer identifies that the slave is lagging behind the master for it's replication threads, then the slave is automatically taken out of the list of available servers. Work will therefore be distributed to other MySQL servers within the slave replication group.
If the replication thread on a slave is identified as no longer running, the slave is also automatically removed from the list of available servers.
If either situation changes, such as the replication delay decreases to an acceptable level, or the replication thread on the failed slave is restarted and the replication process catches up, then the slave will be brought back in to the list of available MySQL servers.
The MySQL Load Balancer is based on the MySQL Proxy, and consists of two modules which work together to achieve its goal:
The proxy, which uses Lua scripts to customize the handling of connections and query execution. The `proxy` connects to several backend MySQL instances to which it can send queries.
The monitor plugin connects to each of the backends the proxy knows about and executes queries on each one in regular intervals. The results of those queries are used to determine the state of each backend.
For more information on MySQL Proxy, see Chapter 27, MySQL Proxy.
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