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There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.

If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file. See \line mysqld_multi.

Assume an existing server is configured for the default port number and socket file. Then configure the new server with a configure command something like this:

shell> ./configure --with-tcp-port=port_number \
--with-unix-socket-path=file_name \
--prefix=/usr/local/mysql-3.22.9

Here port_number and file_name should be different from the default port number and socket file pathname, and the --prefix value should specify an installation directory different from the one under which the existing MySQL installation is located.

You can check the socket used by any currently executing MySQL server with this command:

shell> mysqladmin -h hostname --port=port_number variables

Note that if you specify "localhost" as a hostname, mysqladmin will default to using Unix sockets instead of TCP/IP.

If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.

You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them at runtime as options to safe_mysqld:

shell> /path/to/safe_mysqld --socket=file_name --port=port_number

mysqld_multi can also take safe_mysqld (or mysqld) as an argument and pass the options from a configuration file to safe_mysqld and further to mysqld.

If you run the new server on the same database directory as another server with logging enabled, you should also specify the name of the log files to safe_mysqld with --log, --log-update, or --log-slow-queries. Otherwise, both servers may be trying to write to the same log file.

Warning: normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!

If you want to use another database directory for the second server, you can use the --datadir=path option to safe_mysqld.

Note also that starting several MySQL servers (mysqlds) in different machines and letting them access one data directory over NFS is generally a bad idea! The problem is that the NFS will become the bottleneck with the speed. It is not meant for such use. And last but not least, you would still have to come up with a solution how to make sure that two or more mysqlds are not interfering with each other. At the moment there is no platform that would 100% reliable do the file locking (lockd daemon usually) in every situation. Yet there would be one more possible risk with NFS; it would make the work even more complicated for lockd daemon to handle. So make it easy for your self and forget about the idea. The working solution is to have one computer with an operating system that efficiently handles threads and have several CPUs in it.

When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:

· Start the client with --host 'hostname' --port=port_number to connect with TCP/IP, or [--host localhost] --socket=file_name to connect via a Unix socket.

· In your C or Perl programs, you can give the port or socket arguments when connecting to the MySQL server.

· If your are using the Perl DBD::mysql module you can read the options from the MySQL option files. See Option files.

$dsn = "DBI:mysql:test;mysql_read_default_group=client;
mysql_read_default_file=/usr/local/mysql/data/my.cnf"
$dbh = DBI->connect($dsn, $user, $password);

· Set the MYSQL_UNIX_PORT and MYSQL_TCP_PORT environment variables to point to the Unix socket and TCP/IP port before you start your clients. If you normally use a specific socket or port, you should place commands to set these environment variables in your .login file. See Environment variables.

· Specify the default socket and TCP/IP port in the .my.cnf file in your home directory. See Option files.