Allows ProxySQL to automatically move around writers and readers in a replication topology

[renecannao]

Why

ProxySQL is, by design, not aware replication.
Although, sometime replication topology is so simple (1 master, N slaves) that ProxySQL should be able to correctly switch traffic without being reconfigured externally, but detecting the change of variable read_only

What

• define a new table (mysql_replication_hostgroups) that defines writer_hostgroup (servers with read_only=0) and reader_hostgroup (servers with read_only=1)
• Monitor module should verify variable read_only for all servers in writer_hostgroup and reader_hostgroup
• if read_only is changed for any host, that host is migrated to the right hostgroup
• LOAD MYSQL SERVERS and SAVE MYSQL SERVERS will also operate on new table mysql_replication_hostgroups

[renecannao]

Another field should be added in table mysql_replication_hostgroups to define if writers are exclusive writers or if they can be used as readers as well.

During the implementation I figured this feature is not exhaustive as described above. In fact, should also be defined:

• if writers can also be readers : this could be done with an extra columns that specifies a default weight for writers in reader_hostgroup
• if the switchover should be immediate or if a grace time period should be present in which the old writers stay in OFFLINE_SOFT before being removed : this could be done with an additional column that specifies the grace time period

[renecannao]

We need to define also:

• new logging table : monitor.mysql_server_read_only_log
• new global variables that defines how frequently the check needs to be performed