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  1. MariaDB MaxScale
  2. MXS-3615

Clarify session_track_system_variables requirement for readwritesplit's causal_reads parameter

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      Description

      The documentation for the causal_reads parameter for readwritesplit has the following note:

      Note: This feature requires MariaDB 10.2.16 or newer to function. In addition to this, the session_track_system_variables parameter must be set to last_gtid.

      https://mariadb.com/kb/en/mariadb-maxscale-25-readwritesplit/#causal_reads

      This seems to imply that you should do something like this:

      SET GLOBAL session_track_system_variables='last_gtid';
      

      and:

      [mariadb]
      session_track_system_variables=last_gtid
      

      However, in 10.3 and later, this system variable has a non-empty default value that some users may want to keep: autocommit,character_set_client,character_set_connection,character_set_results,time_zone.

      Therefore, it may be more appropriate for users to set it like this:

      SET GLOBAL session_track_system_variables=CONCAT(@@global.session_track_system_variables, ',last_gtid');
      

      and:

      [mariadb]
      session_track_system_variables=autocommit,character_set_client,character_set_connection,character_set_results,time_zone,last_gtid
      

      Maybe it would be more appropriate for the documentation to say something like this?:

      Note: This feature requires MariaDB 10.2.16 or newer to function. In addition to this, the session_track_system_variables parameter must include last_gtid in its list of tracked system variables.

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            Assignee:
            markus makela markus makela
            Reporter:
            GeoffMontee Geoff Montee
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