mysql-plugin-flush_rewriter

A MySQL Plugin to rewrite all FLUSH statements as FLUSH LOCAL to avoid those commands to be sent to replicas.

Sending FLUSH commands to replicas can be an issue, but for old DBAs like me, the LOCAL keyword is not something we often use.

See https://lefred.be/content/as-a-dba-develop-a-critical-mind-when-receiving-advice/

 MySQL > install plugin flush_rewriter SONAME 'flush_rewriter.so';
 Query OK, 0 rows affected (0.0099 sec)
 
 MySQL > SELECT PLUGIN_NAME, PLUGIN_STATUS, PLUGIN_TYPE 
         FROM INFORMATION_SCHEMA.PLUGINS WHERE PLUGIN_NAME LIKE 'flush%';
+----------------+---------------+-------------+
| PLUGIN_NAME    | PLUGIN_STATUS | PLUGIN_TYPE |
+----------------+---------------+-------------+
| flush_rewriter | ACTIVE        | AUDIT       |
+----------------+---------------+-------------+
1 row in set (0.0016 sec)

 MySQL > show global variables like 'flush_rewriter%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| flush_rewriter_force_local | OFF   |
+----------------------------+-------+
1 row in set (0.0041 sec)

 MySQL > flush status;
Query OK, 0 rows affected (0.0081 sec)

 MySQL > set global flush_rewriter_force_local=1;
Query OK, 0 rows affected (0.0004 sec)

 MySQL > show global variables like 'flush_rewriter%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| flush_rewriter_force_local | ON    |
+----------------------------+-------+
1 row in set (0.0041 sec)

 MySQL > flush status;
Query OK, 0 rows affected, 1 warning (0.0007 sec)
Note (code 1105): Query 'flush status' rewritten to 'FLUSH LOCAL status' by a query rewrite plugin

 MySQL > flush NO_WRITE_TO_BINLOG status;
Query OK, 0 rows affected (0.0005 sec)

 MySQL > flush /*fred*/ status;
Query OK, 0 rows affected, 1 warning (0.0003 sec)
Note (code 1105): Query 'flush /*fred*/ status' rewritten to 'FLUSH LOCAL /*fred*/ status' by a query rewrite plugin

Limitations

The plugin doesn't rewrite correctly the command when the statement starts with a comment:

 MySQL > /* fred */ flush status;
Query OK, 0 rows affected (0.0018 sec)

Instrumentation

The plugin's memory consumption is intrumented in Performance_Schema:

 MySQL > SELECT * FROM performance_schema.memory_summary_global_by_event_name 
         where event_name like '%flush_rewriter'\G
*************************** 1. row ***************************
                  EVENT_NAME: memory/rewriter/flush_rewriter
                 COUNT_ALLOC: 2
                  COUNT_FREE: 2
   SUM_NUMBER_OF_BYTES_ALLOC: 111
    SUM_NUMBER_OF_BYTES_FREE: 111
              LOW_COUNT_USED: 0
          CURRENT_COUNT_USED: 0
             HIGH_COUNT_USED: 1
    LOW_NUMBER_OF_BYTES_USED: 0
CURRENT_NUMBER_OF_BYTES_USED: 0
   HIGH_NUMBER_OF_BYTES_USED: 60
1 row in set (0.0064 sec)