Kristian,
Thanks for the docs. It's a very good start. Compared to MySQL 5.6, GTID in
MariaDB has some advantages: it is enabled by default and it is easy to read.
There are a few items, though, that should need some more thought:
1) rpl_slave_state is probably ill-named. In this architecture, it is only
written by the slaves, and thus the name matches, but I wonder if the master
should write this table (or a similar one) to keep track of its progress in the
binary log. Rationale: all the info related to replication should be in tables
and easy to query.
2) rpl_slave_state does not have enough metadata. What is missing here, and
would be extremely useful, is the corresponding binlog+position for each GTID,
a timestamp of when the transaction was saved to the binary log, and the
timestamp of when it was applied. (The host name of the origin would also be
useful). Also very important (when you will implement parallel replication) is
a shard identifier.
3) Multiple domains do not integrate well with GTID. In multi-source
replication, we use "connection names", but for GTID we use "domain numbers".
They do not play well together : there is no correspondence between the
gtid_domain_id and the connection name, and a "SHOW ALL SLAVES STATUS" lists
all the GTID for every connection. For example:
mysql -e 'show all slaves status\G' |grep
'Connection_name\|Running\|Master_Host\|Master_Port\|Gtid_Pos\|Master_Log'
Connection_name: Bear
Master_Host: 127.0.0.1
Master_Port: 8203
Master_Log_File: mysql-bin.000001
Read_Master_Log_Pos: 1020
Relay_Master_Log_File: mysql-bin.000001
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Exec_Master_Log_Pos: 1020
Gtid_Pos:
100-101-21,400-104-23,200-102-8,300-103-12,500-105-20
Connection_name: Wolf
Master_Host: 127.0.0.1
Master_Port: 8204
Master_Log_File: mysql-bin.000001
Read_Master_Log_Pos: 896
Relay_Master_Log_File: mysql-bin.000001
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Exec_Master_Log_Pos: 896
Gtid_Pos:
100-101-21,400-104-23,200-102-8,300-103-12,500-105-20
Connection_name: Fox
Master_Host: 127.0.0.1
Master_Port: 8205
Master_Log_File: mysql-bin.000002
Read_Master_Log_Pos: 365
Relay_Master_Log_File: mysql-bin.000002
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Exec_Master_Log_Pos: 365
Gtid_Pos:
100-101-21,400-104-23,200-102-8,300-103-12,500-105-20
Connection_name: Deer
Master_Host: 127.0.0.1
Master_Port: 8206
Master_Log_File: mysql-bin.000001
Read_Master_Log_Pos: 1144
Relay_Master_Log_File: mysql-bin.000001
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Exec_Master_Log_Pos: 1144
Gtid_Pos:
100-101-21,400-104-23,200-102-8,300-103-12,500-105-20
Which connection is running which gtid number? Not easy to tell, as the
correspondence between domain and connections is only in the DBA's head.
4) gtid_pos is text that includes all sequences from all domains. The docs say
that I can modify it if needed, but in a case like the one above, modifying
this string can be very error prone. For example, if server_ids are assigned
using inet_aton(@IP), you could have a gtid_pos like this
'1001-170062692-230000,1002-170062593-12893445,1003-170062700-672009,1003-170062699-700002'
Instead of editing a string, and risk breaking replication in all domains, I
would expect the availability of commands like:
CHANGE GTID for domain 'Wolf' set gtid_pos=1900
That's it for now. Hope there is still room for improvements.
Cheers
Giuseppe
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