26.12.9 Performance Schema Connection Attribute Tables

Connection attributes are key-value pairs that application programs can pass to the server at connect time. Applications based on the C API define the attribute set using the mysql_options() and mysql_options4() functions. Other MySQL Connectors may provide their own attribute-definition methods.

These tables expose attribute information:

session_account_connect_attrs: Connection attributes for the current session, and other sessions associated with the session account
session_connect_attrs: Connection attributes for all sessions

Attribute names that begin with an underscore (_) are reserved for internal use and should not be created by application programs. This convention permits new attributes to be introduced by MySQL without colliding with application attributes, and enables application programs to define their own attributes that do not collide with internal attributes.

The set of connection attributes visible on a given connection varies depending on factors such as your platform, MySQL Connector used to establish the connection, or client program.

The libmysqlclient client library (provided in MySQL and MySQL Connector/C distributions) sets these attributes:

_client_name: The client name (libmysql for the client library).
_client_version: The client library version.
_os: The operating system (for example, Linux, Win64).
_pid: The client process ID.
_platform: The machine platform (for example, x86_64).
_thread: The client thread ID (Windows only).

Other MySQL Connectors may define their own connection attributes.

MySQL Connector/C++ 8.0.16 and higher defines these attributes for applications that use X DevAPI or X DevAPI for C:

_client_license: The connector license (for example GPL-2.0).
_client_name: The connector name (mysql-connector-cpp).
_client_version: The connector version.
_os: The operating system (for example, Linux, Win64).
_pid: The client process ID.
_platform: The machine platform (for example, x86_64).
_source_host: The host name of the machine on which the client is running.
_thread: The client thread ID (Windows only).

MySQL Connector/J defines these attributes:

_client_name: The client name
_client_version: The client library version
_os: The operating system (for example, Linux, Win64)
_client_license: The connector license type
_platform: The machine platform (for example, x86_64)
_runtime_vendor: The Java runtime environment (JRE) vendor
_runtime_version: The Java runtime environment (JRE) version

MySQL Connector/NET defines these attributes:

_client_version: The client library version.
_os: The operating system (for example, Linux, Win64).
_pid: The client process ID.
_platform: The machine platform (for example, x86_64).
_program_name: The client name.
_thread: The client thread ID (Windows only).

PHP defines attributes that depend on how it was compiled:

Compiled using libmysqlclient: The standard libmysqlclient attributes, described previously.
Compiled using mysqlnd: Only the _client_name attribute, with a value of mysqlnd.

Many MySQL client programs set a program_name attribute with a value equal to the client name. For example, mysqladmin and mysqldump set program_name to mysqladmin and mysqldump, respectively. MySQL Shell sets program_name to mysqlsh.

Some MySQL client programs define additional attributes:

mysql (as of MySQL 8.0.17):
- os_user: The name of the operating system user running the program. Available on Unix and Unix-like systems and Windows.
- os_sudouser: The value of the SUDO_USER environment variable. Available on Unix and Unix-like systems.
mysql connection attributes for which the value is empty are not sent.
mysqlbinlog:
- _client_role: binary_log_listener
Replication slave connections:
- program_name: mysqld
- _client_role: binary_log_listener
- _client_replication_channel_name: The channel name.
FEDERATED storage engine connections:
- program_name: mysqld
- _client_role: federated_storage

There are limits on the amount of connection attribute data transmitted from client to server:

A fixed limit imposed by the client prior to connect time.
A fixed limit imposed by the server at connect time.
A configurable limit imposed by the Performance Schema at connect time.

For connections initiated using the C API, the libmysqlclient library imposes a limit of 64KB on the aggregate size of connection attribute data on the client side: Calls to mysql_options() that cause this limit to be exceeded produce a CR_INVALID_PARAMETER_NO error. Other MySQL Connectors may impose their own client-side limits on how much connection attribute data can be transmitted to the server.

On the server side, these size checks on connection attribute data occur:

The server imposes a limit of 64KB on the aggregate size of connection attribute data it will accept. If a client attempts to send more than 64KB of attribute data, the server rejects the connection. Otherwise, the server considers the attribute buffer valid and tracks the size of the longest such buffer in the Performance_schema_session_connect_attrs_longest_seen status variable.
For accepted connections, the Performance Schema checks aggregate attribute size against the value of the performance_schema_session_connect_attrs_size system variable. If attribute size exceeds this value, these actions take place:
- The Performance Schema truncates the attribute data and increments the Performance_schema_session_connect_attrs_lost status variable, which indicates the number of connections for which attribute truncation occurred.
- The Performance Schema writes a message to the error log if the log_error_verbosity system variable is greater than 1:
```
Connection attributes of length N were truncated
(N bytes lost)
for connection N, user user_name@host_name
(as user_name), auth: {yes|no}
```
  The information in the warning message is intended to help DBAs identify clients for which attribute truncation occurred.
- A _truncated attribute is added to the session attributes with a value indicating how many bytes were lost, if the attribute buffer has sufficient space. This enables the Performance Schema to expose per-connection truncation information in the connection attribute tables. This information can be examined without having to check the error log.

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Document heeft de 26/06/2006 gemaakt, de laatste keer de 26/10/2018 gewijzigd
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