Always fearing doing queries on a lagged replica on labs? Not anymore!
While Betacommand's tool [0] was very useful, it was also very inaccurate,
as it tried to check the lag by looking at the last rows updated, which can
be a lot of time on the least popular wikis.
What I offer now is sub-second accurate lag measuring, by writing on the
production masters the current time, in microseconds, every 0.5 seconds and
making that available on all hosts (using this tool [1]). So, it is more
accurate than SHOW SLAVE STATUS, because it compares the difference with
the original master, and it will work even if replication is broken.
To read it, just do SELECT * FROM heartbeat_p.heartbeat;
And you will get:
+-------+----------------------------+------+
| shard | last_updated | lag |
+-------+----------------------------+------+
| s6 | 2015-11-25T20:20:32.000980 | 0 |
| s2 | 2015-11-25T20:20:32.001030 | 0 |
| s7 | 2015-11-25T20:20:32.001070 | 0 |
| s3 | 2015-11-25T20:20:32.001000 | 0 |
| s4 | 2015-11-25T20:20:32.000920 | 0 |
| s1 | 2015-11-25T20:20:32.000740 | 0 |
| s5 | 2015-11-25T20:20:32.000830 | 0 |
+-------+----------------------------+------+
Read the detailed documentation on: [2]
Use it, create a web page if you want to make it public! Report a ticket if
it gets too high! Report a ticket if you need more info (a record per
wiki?). But I wanted to give you the essentials, and you can build
yourselves on top of that.
Only 2 know bugs:
- There is microsecond accuracy, but it cannot be used until a bug in
MariaDB is fixed [3]
- enwiki will only report s1 lag until that server is restarted due to some
existing filters. We will schedule that at some time in the future.
[
0]<http://tools.wmflabs.org/betacommand-dev/cgi-bin/replag>
[
1]<https://www.percona.com/doc/percona-toolkit/2.2/pt-heartbeat.html>
[2]<
https://wikitech.wikimedia.org/wiki/Help:Tool_Labs/Database#Identifying_lag…
[
3]<https://mariadb.atlassian.net/browse/MDEV-9175>
--
Jaime Crespo
<http://wikimedia.org>