Moving again with Otto: Step Libraries

I have finally started to come back to Otto, an experimental playground for some of my thoughts on what an improved CI/CD tool might look like. After setting the project aside for a number of months and letting ideas marinate, I wanted to share some of my preliminary thoughts on managing the trade-offs of extensibility. From my time in the Jenkins project, I can vouch for the merits of a robust extensibility model. For Otto however, I wanted to implement something that I would call “safer” or “more scalable”, from the original goals of Otto:

Extensibility must not come at the expense of system integrity. Systems which allow for administrator, or user-injected code at runtime cannot avoid system reliability and security problems. Extensibility is an important characteristic to support, but secondary to system integrity.

Usage cannot grow across an organization without user-defined extension. The operators of the system will not be able to provide for every eventual requirement from users. Some mechanism for extending or consolidating aspects of a continuous delivery process must exist.

Starting with Jenkins and Jenkins Pipeline as a frame of reference. I do this not only because I am intimately familiar with how it works, but also because Jenkins Pipeline is the most successful and widely adopted pipeline modeling language. Key to its success are “steps.” There are a number of default steps provided by the system and new plugins introduced on the controller provide new steps for users. The “execution environment” for steps in Jenkins Pipeline is however incredibly confusing. If I were to interview a Jenkins developer or administrator, I would give them a sample Jenkinsfile and ask them to explain to me what is executing where as the pipeline progress. In essence, steps can execute code on both the controller and the agents, hopefully with users never knowing about the quirks of the runtime dance between the two.

For Otto’s pipeline language, I wanted steps to have a perfectly clear execution environment: agent only. Along with this are a number of other requirements that I have in mind:

  • Language-independent: I want steps to be implemented in whatever language a developer sees fit. Therefore the tooling needs remain flexible enough to distribute and execute Python-based steps as well as native compiled steps.
  • Statically verifiable: A step invocation in a pipeline should be verifiable without actually executing the step. That is to say, it should be known before execution whether parameters and types are correct.
  • Lowest necessary privilege: Steps shouldn’t be able to “know” anything about the system, credentials, configuration, etc, without an administrator or user being aware. If a step needs to access a shared configuration variable, it must self-declare that requirement. Steps should never be allowed to simply poke around in global variables or configuration of the environment.

The approach I’m settling on with “step libraries” is that each step is a package (.tar.gz) containing a manifest file and whatever other assets it requires to execute. The manifest file contains the description of the parameters, the entrypoint, and configuration values the step may require.

At runtime, the step’s entrypoint will always be invoked with a single invocation file that contains all the information necessary to execute the step correct. For this I debated a couple different approaches: setting environment variables, piping JSON data into the process, or even having the processes request a JSON payload of data from a central server. I ultimately decided on the invocation file approach since that requires the least system knowledge for the step to actually be executed by an agent.

The role of the agent in this process remains fairly simple, regardless of which steps are being executed:

  • Consider the steps which it should execute. (e.g. echo, sh, junit)
  • Retrieve the appropriate step library artifacts, originally this is going to be from a centralized store but I can easily imagine an agent retrieving “remote step libraries” in a distant future.
  • Unpack the step libraries
  • Validate that the step libraries support the parameters specified by the user’s pipeline.
  • Iterate through the steps and execute the entrypoint.

In this commit I managed to get something dumb and primitive working with this model. Excusing the STEPS_DIR hack to avoid needing to reach out to fetch steps, the basic test pipeline referenced in the commit contains the essence of how I believe step libraries can provide a powerful and safe extensibility model for Otto.

There are still a number of open questions I need to answer:

  • How will credentials be accessed by a step in a secure manner?
  • How will I balance the trade-off of “bring your own step libraries” with “don’t leak credentials.” Right now I’m thinking about “trusted” versus “untrusted” step libraries, and everything user-defined would be untrusted unless added to an “allow” list by an administrator.
  • For more complex step parameters, like files, how well will the invocation file format hold up?
  • How should steps affect the flow control of a pipeline? Conventionally a non-zero exit of a step will halt the pipeline in Jenkins, but is there a more granular flow control system that can be extended to steps which are defined in a step library?

Despite sparingly little free time, I am enjoying getting back into this part of Otto. I had let myself fall into a tar pit of distributed systems problems and stalled any progress with Otto. Bringing the focus back to the pipeline model and extensibility has allowed me re-focus on some of the challenges unique to the CI/CD space.

If you’re curious to learn more, you’re welcome to join #otto on the Freenode IRC network, or follow along on GitHub