Mark Needham

Thoughts on Software Development

Fabric: Tailing log files on multiple machines

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We wanted to tail one of the log files simultaneously on 12 servers this afternoon to try and see if a particular event was being logged and rather than opening 12 SSH sessions decided to get Fabric to help us out.

My initial attempt to do this was the following:

fab -H host1,host2,host3 -- tail -f /var/www/awesome/current/log/production.log

It works but the problem is that by default Fabric runs the specified command one machine after the other so we’ve actually managed to block Fabric with the tail command on ‘host1′.

The output of host1′s log file will be printed to the terminal but nothing from the other two hosts.

Nathan showed me how to get around this problem by making use of Fabric’s parallel execution which we can enable with the ‘-P’ option:

fab -P --linewise -H host1,host2,host3 -- tail -f /var/www/awesome/current/log/production.log

We also used the ‘likewise’ flag to ensure that data between the different tail processes didn’t get mixed up although this wasn’t necessary because Fabric defaults to likewise if you’re using parallel execution mode anyway.

On a side-note, Paul Ingles wrote up the approach taken to make data from log files more accessible using a Kafka driven event pipeline but in this case we haven’t got round to wiring this data up yet so Fabric it is for now.

Written by Mark Needham

January 15th, 2013 at 12:20 am

Posted in DevOps

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