Unix Socket

A Unix Socket (or Unix domain socket) is a communication endpoint that allows two processes on the same operating system to exchange data without going through the TCP/IP network stack. In MySQL, it is the default connection method when connecting to localhost.

How it works #

When a MySQL client connects specifying -h localhost, the client does not use TCP. It uses the Unix socket file (typically /var/run/mysqld/mysqld.sock) to communicate directly with the MySQL server process. This communication happens entirely within the kernel, with no network overhead, and is faster than a TCP connection even on the same host.

What it’s for #

In multi-instance environments, each MySQL instance has its own socket file (e.g. mysqld.sock, mysqld-app2.sock). Specifying the correct socket with --socket=/path/to/socket is the only reliable way to connect to the intended instance. Without specifying the socket, the client uses the default one — which almost always points to the primary instance.

When to use it #

Unix sockets are used for all local connections to MySQL. In environments with multiple instances, it is essential to explicitly specify the socket for each connection. For remote connections (from another host), TCP with -h <ip> -P <port> is used instead.