Re: Unmonitored Master
Posted by: Mark Leith
Date: December 22, 2011 03:47AM

Hey Oliver,

MEM tries to do topology discovery in a couple of different ways:

1) It first tries to read the master.info file from the slave server, and use the credentials within it to connect to the master server, and select from the mysql.inventory table that every MEM agent creates. This requires that the agent is started as the same user as MySQL (so that it can read the master.info in the data directory), and that slave user has SELECT privileges against the mysql.inventory table as well.

2) If the above does not work, we fall back to IP matching - we read the Master_host / Master_port from SHOW SLAVE STATUS on the slave, if Master_host is not an IP address we do an IP lookup based on the host name, and report that back as the "Master_IP" to the MEM server. Then, every single agent reports about all of the network interfaces on each machine, so we use all of this to generate the topology based on where the IPs are pointing.

The first solution is fairly bullet proof, there's not much that stops it from getting the right topologies, given the right permissions to read everything.

The second solution has problems when using things like virtual IPs - the slave may connect to a virtual IP through to the master, but the end point IP address on the master that we know about is only as reported by the agent installed upon that master - it doesn't know about the virtual IP at the network layer before it.

In summary - I'd make sure your agent is started as the user MySQL is started as (the "mysql" user generally), and that the slave user on the master has SELECT privileges on mysql.inventory. That will give the most reliable topology discovery.

Regards,

Mark

Best regards,

Mark

--
Mark Leith
Senior Software Development Manager
MySQL Enterprise Tools @ Oracle

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