Archive for August, 2011

Using Jenkins as a Cron Server

Monday, August 22nd, 2011

There are a number of problems with cron which cause lots of grief for system administrators with big ones being manageability, cron-spam and auditability. To fix some of these issues I have lately started using Jenkins. Jenkins is an open source Continuous Integration server it has lots of features that make it a great cron replacement for a number of uses. These are some of the problems it solves for me


Jenkins can be configured to retain logs of all jobs that it has run. You can set it up to keep last 10 runs or you can set it up to keep only last 2 weeks of logs. This is incredibly useful since sometimes jobs can fail silently so it's useful to have the output instead of sending it to /dev/null.

Centralized management

I have my most important jobs centralized. I can export all Jenkins jobs as XML and check it into a repository. If I need to execute jobs on remote hosts I simply have Jenkins ssh and execute command remotely. Alternatively you can use Jenkins slaves.

Cron Spam

Cron spam is a common problem with solutions such as this, this and this. To avoid this condition I only have Jenkins alert me when a particular job fails ie. a job exits with return code other than 0.  In addition you can use the awesome Jenkins Text Finder plugin which allows you to specify words or regular expressions to look for in console output. They can be used to mark a "job" unstable. For example in text finder config I checked

X Also search the console output

and specified

Regular expression ([Ee]rror*).*

This has saved our bacon since we used the script which "swallows" up the errors codes from the mysqldump command and exits normally. Text Finder caught this

mysqldump: Error 2020: Got packet bigger than 'max_allowed_packet' bytes when dumping table `users` at row: 234

Happily we caught this one on time.

Job dependency

Often you will have job dependencies ie. main backup job where you first dump a database locally then upload it somewhere off-site or to the cloud. The way we have done this in the past is to leave a sufficiently large window between the first job and consecutive job to be sure first job has finished. This says nothing about what to do if the first job fails. Likely the second one will too. With Jenkins I no longer have to do that. I can simply tell Jenkins to trigger "backup to the cloud" once local DB backup concludes successfully.

Test immediately

While you are adding a job it's useful to test whether job runs properly. With cron you often had to wait until the job executed at e.g. 3 am in the morning to discover that PATH wasn't set properly or there was some other problem with your environment. With Jenkins I can click Build Now and job will run immediately.

Easy setup

Setting up jobs is easy. I have engineers set up their own job by copying an existing job and modifying it to do what they need to do. I don't remember last time someone asked me how to do it :-).

What I don't use Jenkins for

I don't use Jenkins to run jobs that collect metrics or anything that has to run too often.


Testing your web pages with fantomtest

Tuesday, August 2nd, 2011

Coming from web operations background my web site/page monitoring had largely focused at looking at metrics such as average request duration, 90th percentile request duration etc. These are all great metrics however through Velocity Conferences I have come to appreciate that there is a lot more to web performance than simply knowing how long it takes to load HTML in a web page. As a result I have been looking for ways to try to get better metrics by utilizing real browsers instead of Perl/Ruby/Python scripts. For some time I have been playing with Selenium RC to give me an easy way to test and time my web application. Unfortunately I found it heavy and slow. At last Velocity conference I was fortunate enough to see a demo of PhantomJS. PhantomJS is a semi-headless webkit browser with Javascript support. What I really appreciated about it is that it is light weight, fast and very easy to instrument using Javascript. In addition it includes a number of useful examples such as netsniff.js which output a HTTP Archive (HAR) of requests to a certain web page. From a HAR file you can builds among other things waterfall charts. There are a number of services you can use to have your site tested for free e.g. Limitation is that they can't test your intranet infrastructure since that is usually behind a firewall or it doesn't allow you to test remote sites that are connected to your intranet via a VPN.

That is why I'm introducing fantomTest. A simple web application that allows you to generate waterfall graphs using PhantomJS. It will also take a screenshot of a rendered page. Here is what that looks like

What's interesting in this particular case is that Google is not utilizing web performance recommendations by using a HTTP redirect from to

Anyways to get fantomTest go to