WL#6797: Method for clearing session state
Affects: Server-5.7 — Status: Complete — Priority: Medium
In load-balanced and high-availability situations, it is necessary to be able to clear the session state completely. This should behave like using COM_CHANGE_USER to the same user as before, except that it should not require re-authentication. User Documentation ================== http://dev.mysql.com/doc/refman/5.7/en/mysql-reset-connection.html http://dev.mysql.com/doc/refman/5.7/en/mysql-commands.html (for the resetconnection command) http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-3.html (changelog entry)
Problem Description: ==================== Existing COM_CHANGE_USER command effectively does a re-login without closing/opening the connection. Important side effect of this command includes cleaup of the session context (temporary tables, session variables, etc.) in the MySQL server. Thread pool library uses COM_CHANGE_USER to clean up this session context. Thus cleanup of the session context requires additional authentication to be done as part of COM_CHANGE_USER causing more network roundtrips between server and client. New implementation w.r.t this WL: ================================= With this WL we introduce a new API called mysql_reset_connection which will initiate clean up of the session context from both client and server side. This new API will decouple cleanup of session context from COM_CHANGE_USER. Thus if a thread pool library wants to cleanup the session context, will invoke this API without needing to call COM_CHANGE_USER. Also a new mysql command line command (resetconnection) will be introduced. ** Server-side changes ** ========================= New server command ================== A new server command (COM_RESET_CONNECTION) will be implemented to do clean up of the session context. Backward/Cross Compatibility ============================ Backward/Cross compatibility is guaranteed by COM_RESET_CONNECTION by default. For instance : 1) Old client <-> New server Will never send COM_RESET_CONNECTION command. 2) New client <-> old server In this case the server will return with as error. Server side clean up ==================== Sever side cleanup includes following things: delete cached transactions, release transaction locks. close cached tables, temporary tabels. release global read lock. clean stored procedure/function cache. clean error state. All the above cleanup is done as part of existing THD::change_user() function which will be renamed to THD::cleanup_connection(). ** Client-side changes(libmysql) ** =================================== New libmysql API ================ A new function will be introduced which will send COM_RESET_CONNECTION packet to the server. This packet has a 4 byte header followed by 1 byte command. This command will do the clean up of session context. int mysql_reset_connection(MYSQL *mysql); Parameters ---------- mysql [IN] Pointer to the connection handle. Return Values ------------- 0 - Success 1 - Error New mysql Command ================= A new mysql command (resetconnection) will be introduced which will clear the current session context from both client and server side. static int com_resetconnection(String *str,char*); Parameters ---------- both the parameters will be unused. Return Values ------------- 0 - Success 1 - Error Client side clean up ==================== For client side cleanup we detach all open statements from current connection by making a call to mysql_detach_stmt_list().
Adding a flag set to reset the individual components of the session state was considered, but ruled out because these would be handled better with the SQL level RESET command.
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