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MySQL 9.4 Reference Manual

29.12.3.5 The socket_instances Table

Thesocket_instances table provides a real-time snapshot of the active connections to the MySQL server. The table contains one row per TCP/IP or Unix socket file connection. Information available in this table provides a real-time snapshot of the active connections to the server. (Additional information is available in socket summary tables, including network activity such as socket operations and number of bytes transmitted and received; seeSection 29.12.20.9, “Socket Summary Tables”).

mysql> SELECT * FROM performance_schema.socket_instances\G*************************** 1. row ***************************           EVENT_NAME: wait/io/socket/sql/server_unix_socketOBJECT_INSTANCE_BEGIN: 4316619408            THREAD_ID: 1            SOCKET_ID: 16                   IP:                 PORT: 0                STATE: ACTIVE*************************** 2. row ***************************           EVENT_NAME: wait/io/socket/sql/client_connectionOBJECT_INSTANCE_BEGIN: 4316644608            THREAD_ID: 21            SOCKET_ID: 39                   IP: 127.0.0.1                 PORT: 55233                STATE: ACTIVE*************************** 3. row ***************************           EVENT_NAME: wait/io/socket/sql/server_tcpip_socketOBJECT_INSTANCE_BEGIN: 4316699040            THREAD_ID: 1            SOCKET_ID: 14                   IP: 0.0.0.0                 PORT: 50603                STATE: ACTIVE

Socket instruments have names of the formwait/io/socket/sql/socket_type and are used like this:

The server has a listening socket for each network protocol that it supports. The instruments associated with listening sockets for TCP/IP or Unix socket file connections have asocket_type value ofserver_tcpip_socket orserver_unix_socket, respectively.
When a listening socket detects a connection, the server transfers the connection to a new socket managed by a separate thread. The instrument for the new connection thread has asocket_type value ofclient_connection.
When a connection terminates, the row insocket_instances corresponding to it is deleted.

Thesocket_instances table has these columns:

EVENT_NAME
The name of thewait/io/socket/* instrument that produced the event. This is aNAME value from thesetup_instruments table. Instrument names may have multiple parts and form a hierarchy, as discussed inSection 29.6, “Performance Schema Instrument Naming Conventions”.
OBJECT_INSTANCE_BEGIN
This column uniquely identifies the socket. The value is the address of an object in memory.
THREAD_ID
The internal thread identifier assigned by the server. Each socket is managed by a single thread, so each socket can be mapped to a thread which can be mapped to a server process.
SOCKET_ID
The internal file handle assigned to the socket.
IP
The client IP address. The value may be either an IPv4 or IPv6 address, or blank to indicate a Unix socket file connection.
PORT
The TCP/IP port number, in the range from 0 to 65535.
STATE
The socket status, eitherIDLE orACTIVE. Wait times for active sockets are tracked using the corresponding socket instrument. Wait times for idle sockets are tracked using theidle instrument.
A socket is idle if it is waiting for a request from the client. When a socket becomes idle, the event row insocket_instances that is tracking the socket switches from a status ofACTIVE toIDLE. TheEVENT_NAME value remainswait/io/socket/*, but timing for the instrument is suspended. Instead, an event is generated in theevents_waits_current table with anEVENT_NAME value ofidle.
When the next request is received, theidle event terminates, the socket instance switches fromIDLE toACTIVE, and timing of the socket instrument resumes.

Thesocket_instances table has these indexes:

Primary key on (OBJECT_INSTANCE_BEGIN)
Index on (THREAD_ID)
Index on (SOCKET_ID)
Index on (IP,PORT)

TRUNCATE TABLE is not permitted for thesocket_instances table.

TheIP:PORT column combination value identifies the connection. This combination value is used in theOBJECT_NAME column of theevents_waits_xxx tables, to identify the connection from which socket events come:

For the Unix domain listener socket (server_unix_socket), the port is 0, and the IP is''.
For client connections via the Unix domain listener (client_connection), the port is 0, and the IP is''.
For the TCP/IP server listener socket (server_tcpip_socket), the port is always the master port (for example, 3306), and the IP is always0.0.0.0.
For client connections via the TCP/IP listener (client_connection), the port is whatever the server assigns, but never 0. The IP is the IP of the originating host (127.0.0.1 or::1 for the local host)

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