monitoring MySQL slave status - mysql

Question

I'm using Nagios with the mysql_check_health pugin to monitor my MySQL databases. I need to be able to return a numeric value to my plugin from an sql query to tell me if the replicated database is up and running.
so here is what I have...
SHOW GLOBAL STATUS like 'slave_running'
will return:
Variable_name Value
Slave_running OFF/ON
I need to retrun a numeric value from a simple query for the plugin, anyone have any ideas... my thought was to return 3-LENGTH(Slave_running) that would == 1 for ON 0 for off but having trouble using the return values in that way.

Answer 1

The global status variable will be accessible in the information_schema.GLOBAL_STATUS table, from which you can query just the value. That makes it easy to conditionally convert it to a 0 or 1 based on the ON/OFF state.
For example:
> SELECT VARIABLE_VALUE
FROM information_schema.GLOBAL_STATUS
WHERE VARIABLE_NAME = 'slave_running';
+----------------+
| VARIABLE_VALUE |
+----------------+
| ON |
+----------------+
So to convert that into a zero or one, there are a few possibilities. MySQL will treat booleans as 0 or 1, so you can just compare it = 'ON'
Wrapping the above in a subquery (since it returns one value) and comparing to 'ON':
> SELECT (
'ON' = (SELECT VARIABLE_VALUE
FROM information_schema.GLOBAL_STATUS
WHERE VARIABLE_NAME = 'slave_running')
) AS state;
+-------+
| state |
+-------+
| 1 |
+-------+
Or a similar expression formatted as a CASE:
> SELECT CASE WHEN (
SELECT VARIABLE_VALUE
FROM information_schema.GLOBAL_STATUS
WHERE VARIABLE_NAME = 'slave_running') = 'ON' THEN 1
ELSE 0 END AS state;
+-------+
| state |
+-------+
| 1 |
+-------+
In both of the above, I aliased the result as 'state', but you could use any column alias name to read output, replacing AS state accordingly.
What's already out there?
I couldn't help but wonder if there was already a Nagios plugin built for this purpose, and found this as a possibility.

Answer 2

I would strongly consider using:
SHOW SLAVE STATUS
As your informational query. This gives you a few more key fields to monitor on.
From this, you might consider alarming on the following fields:
Slave_IO_Running: Yes / No (tells you if binary log feed from master is working)
Slave_SQL_Running: Yes / No (tells you if slave's SQL execution thread is runing)
Seconds_Behind_Master: INT value (you should set appropriate low value here to alarm off of)
The Slave_running global status is OK for determining overall state (that value is only 'On' when both IO thread and SQL thread on slave are running), but may not give you what you want in terms of more granular monitoring. For example, an interruption in the IO thread may be considered a higher severity event than the SQL thread breaking (and may have totally different recovery scenarios). The Seconds_Behind_Master may also be key to monitor, as you might have both IO and SQL threads happily running, while not realizing that the slave can't keep up for some reason.
If you need to convert to INT value results for slave status values, you could do something like:
SELECT
(CASE WHEN a.Slave_IO_Running = 'Yes' THEN 1 ELSE 0 END)
AS Slave_IO_Running,
(CASE WHEN a.Slave_SQL_Running = 'Yes' THEN 1 ELSE 0 END)
AS Slave_SQL_Running,
a.Seconds_Behind_Master AS Seconds_Behind_Master
FROM (SHOW SLAVE STATUS) AS a