How Jenkins usage statistics work 23 May 2019
jenkins cdf

For years the Jenkins project has published anonymous usage statistics to Despite its warts, the system has ultimately proven useful for determining which plugins are most frequently installed, big coarse-grained changes in growth, and providing various marketing departments with the validation they so desperately crave. Like many of the tucked away corners of the Jenkins project, being an infrastructure maintainer affords me an understanding of how the system works, and sometimes doesn’t. As I promised to the CDF Technical Oversight Committee many weeks ago, in this post I will attempt to describe how this system works.

Buckle up, it’s about to get messy.

The party starts in Jenkins core, with which is responsible for collecting and sending statistics, assuming the administrator has not opted out. This data includes items like what the current installed Jenkins core version is, how many executors have been configured, the plugins installed, and a few other things. This information we don’t consider anonymized by default, and take efforts to encrypt the data before it is sent to a backend server. When this was first developed, daemons like Jenkins were not expected to contact outside services for any reason and as such it was much more common for corporate firewalls to block outgoing traffic from Jenkins instances. The clever workaround put into place was to generate the usage statistics payload, and then to periodically add it to a user’s web page. The presumption being that while the server might not be able to access, it was unlikely that a user’s web browser would restrict the HTTP request. For reasons that escape me, this was all done via an HTTP GET request, which also means the encrypted statistics payload must be base-64 encoded in order to pass properly in the query string of the request.

As an aside, this “make the user’s browser do the work” pattern was first developed for the Update Center which distributes Jenkins plugins to users. We found early on that Jenkins instances would not be able to update due to overzealous firewalls. And so for many years, the Jenkins administrator’s browser session would download the update-center.json into their browser, and then POST it back to Jenkins. Now Jenkins defaults to hitting the Update Center directly, which is good because that update-center.json has blossomed to over 1MB in size.

Where were we again?

Oh right, encrypting statistics, base-64 encoding them, and stuffing them into a web page for an HTTP GET request to send them along to the backend server.

Much of Jenkins was developed in the era before BIG DATA was commonplace. We don’t use some sort of high performance data collector to receive all of this statistics information, instead we use tried-and true Apache. “Huh, so there’s a service being reverse proxied by Apache?” you might be thinking. No. We use Apache. The HTTP GET requests flow into Apache, which logs the HTTP GET request to its access logs whose query string contains the base-64 encoded and encrypted payload. Ta-da! Data collected.

At this point we have access logs sitting on a server somewhere with usage statistics, which have been encrypted, and unless your big data toolbox is only grep, they’re largely unusable at this point.

On some interval, some machine somewhere in Kohsuke’s basement downloads these access logs and runs usage-log-decrypter on them. This process will decrypt the data, fuzz custom plugin names and a number of other values to suitably anonymize the data, before uploading those logs back to another Jenkins project server. Unfortunately usage statistics processing is one of those community processes which still has a very thorny single-point-of-failure sitting in Kohsuke’s basement, but it’s not important enough for anybody to spend much time working on it.

The journey for these data points isn’t over yet however!

The data processing which turns anonymized usage logs into blue SVGs and CSV files was originally developed for funsies by a contributor who wanted to play with the data some weekend many years ago. Like lots of other things in open source projects, these weekend hobby projects have a tendency to survive despite all odds. That code has evolved in the infra-statistics repository. On a monthly basis, the scripting in that repo will load the anonymized JSON usage statistics into a local MongoDB data store, then run a bunch of different queries against it in order to produce the files for which are then committed to the gh-pages branch and pushed, serving the site’s static content via GitHub Pages. As you might guess, loading bunches of data into an in-memory data store has its limitations, namely with the memory. Last year, I believe, we exceeded the amount of memory available for the Jenkins agents which traditionally ran the monthly job. In the meantime, Andrew has been running the statistics processing manually on a local workstation.

To recap, usage statistics from every Jenkins instance which does not opt-out, are generated in a JSON object, which is then encrypted, then base-64 encoded, it is thereafter smuggled along an HTTP GET request to an Apache server which logs the request to the access log, from which it is downloaded, fuzzed, reconstituted and uploaded back to a Jenkins server, then on a monthly cadence it is downloaded, loaded into MongoDB, queried, and then finally formatted into various CSV, JSON, and SVG files which are uploaded to GitHub Pages.

Setting aside the obviously degraded state this aging system is currently in, the right fix involves tearing the entire thing down.

I don’t think a new system is terribly difficult to design and build, but similar to my work with on Uplink, it requires Jenkins core and backend service changes to be made in concert, something we’re not very adept at doing in the project.

The original context in which I was asked about this system was as a reference for the common problem affecting the other projects within the CDF: how do we measure success and adoption.

I think this is a great problem to solve, and worth serious investment. I have many thoughts on how to revisit the topic with what we now know from our experiences in the Jenkins project. A better approach which delivers usable data more rapidly and with greater focus on user privacy, but that’s a topic for another day, and perhaps another blog post.