What's Uplink 06 May 2019

Making changes safely to an application like Jenkins is incredibly tricky. Jenkins is distributed to hundreds of thousands of independently owned and operated servers and is used in a myriad of ways. Our changes with the best intentions, can still result in confounding bugs and errors for users with different configurations, or different combinations of plugins. Over on the Jenkins project blog, Daniel wrote about the first use of “telemetry” by Jenkins core, a project on which we collaborated. I ended up building the backend service for receiving this telemetry, Uplink, and I hope it paves the way for making smarter changes across Jenkins core in the future.

Many developers within the Jenkins community know about the Jenkins Enhancement Proposal, but few will know each proposal by number except for JEP-200. While JEP-200 described an important security design improvement, its rollout to the Jenkins userbase left a large number of problems in its wake. We simply didn’t know enough about how plugins were using some features and how they might break.

The SECURITY ticket which inspired this telemetry-oriented approach was another one of those tickets everybody seems to know only by its number. The problem was, as Daniel describes in his blog post:

Jenkins uses the Stapler web framework for HTTP request handling. Stapler’s basic premise is that it uses reflective access to code elements matching its naming conventions. [..] As these naming conventions closely match common code patterns in Java, accessing crafted URLs could invoke methods never intended to be invoked this way. [..] We identified a number of URLs that could be abused to access otherwise inaccessible jobs, or even invoke internal methods in the web application server to invalidate all sessions.

Some ideas for fixing the problem had been discussed over many months, but we kept getting stuck on one big detail: how can we possibly make a major change without causing a major breakage, like we had inadvertently done with JEP-200. The exact chronology I do not recall, but I believe that I eventually convinced Daniel that we could cheaply and easily stand up a telemetry service. One which would allow Jenkins instances to give the security team the answers they needed to move forward with a fix. In doing so, I volunteered to write and deploy the backend service (Uplink) which would receive the data.

The entirety of the telemetry system is described in JEP-214, but I want to highlight some specific characteristics of the system.

Privacy is an important trait to both Daniel and I, and we designed this telemetry system with that in mind. In Jenkins core, telemetry is captured and sent as part of “trials.” These trials must have a pre-defined start and end date, which ensures that the Jenkins project stops collecting data after a certain point in time. The trials are supposed to answer questions which drive decision making, not provide behavioral or user-analytics. Instances which opt-out of reporting anonymous usage statistics, naturally will not report telemetry either.

Inside of the payloads sent to Uplink, we send an instance ID. The instance ID allows us to de-duplicate, or observe how data changes within a given trial. For additional privacy, we enhanced this system to generate a different instance ID for each trial, preventing the ability to correlate behavior of a single instance across different trials. I will also note that this enhancement was added after telemetry had been initially released, but we went back after the fact and transformed the instance IDs already in the database to apply our privacy principles evenly.

On the server-side we have designed Uplink to make it easy to delete data. None our trials are old enough yet to warrant it, but since instances stop sending data at a certain point, we can truncate tables once we’re finished implementing the fix which inspired the trial. Uplink also restricts access to telemetry data to a very select group of people. Since this data could be valuable for a developer working to improve Jenkins, Uplink allows for trial-based access control. Should a developer create a new scripted-noncps-pipeline-usage trial, we would be able to grant that developer access to only those events in the Uplink dashboard.

As an aside, I should mention that Uplink is the second Feathers JS service which I have developed. The backend for Jenkins Evergreen being the first. The framework has been enjoyable to use, and I would recommend it for building simple little Node-based microservices.

For just about any piece of software, without information about how the software runs in a real user’s environment, it can be tricky to deliver changes safely. Many people are familiar with this requirement for web applications they own and control, but those distributing software don’t tend to apply the same principles of monitoring and continuous feedback. Adding feedback mechanisms to software running on a user’s machine can be done, but must be done in a way that strives to protect their privacy and allows them to disable it entirely.

With the Jenkins Telemetry system, and the Uplink backend service, I am particularly proud of how we have balanced these concerns while enabling developers to build a better Jenkins!