Learning Jsonnet

Mon, Oct 12, 2020 4-minute read

Jsonnet is a powerful data templating language. It extends JSON with variables, conditionals, functions, imports and more. As an engineer who never touched the technology before, I often struggled to understand it. In this post, I share my experience learning Jsonnet and my thoughts behind developing a starter.

With my departure from Indeed, coworkers asked that I provide monitoring for my open source project. As I searched around looking for examples to follow, I quickly discovered Jsonnet. While it appeared to be a powerful tool, I couldn’t find any good starters for it. Many existing projects out there were great references, but lacked set up and direction. To keep things simple, I started with the kubernetes-mixin project structure.

Fighting through the trough

When I learn something new, I often go through the Hype Cycle. At first, there are high expectations for what I’ll be able to do with a given tool. After gaining some familiarity, I dig in. Then, the trough of disillusionment hits. Here, I spend most of my time learning (often hands on). While here, I try not to expect anything to work the first time. Only that I’m making progress, both iteratively and visibly. Eventually, I reach the plateau of productivity. At this point, I’m familiar enough with the semantics and sharp edges to be productive.

With Jsonnet, I found the trough to be longer than I initially expected. Like many languages, issues with files were identified by line and character numbers. Where I ran into more problems was at the command-line tool / language interface. There were several moments where the output of the jsonnet CLI stumped me. At one point, I pushed up my changes and gave it a break. After some needed rest, I was able to track down the issue and was back on track. (Granted, I was attempting to build four Grafana dashboards my first time through.)

References were key to making it through the trough. Without them, I would still be working through the pull request. They can provide great examples, reference points, and even ideas on what you should monitor.

Composition

deps.cloud is an open source project of mine that helps track, monitor, and connect project dependencies across languages and toolchains. I built it as a successor to a previous system I implemented at Indeed. Unlike it’s predecessor, deps.cloud supports many different languages. It implements a common set of gRPC microservices that can be swapped out or reconfigured. For example, https://api.deps.cloud is backed by a read-only version of the tracker process (for now). As the operator of that service, I might want a dedicated dashboard or set of alerts for that deployment.

Remember, iterative and visible progress. When I started the dashboards, I used a similar layout to the kubernetes-mixin project. Once I had something working and checked in, I started to look at how I could improve it. For one, I found their format didn’t support the composition model I was hoping for. It used a few tools like [jsonnet-bundler] which allows for package management. With a few modifications, I was able to get a more modular structure in place.

jsonnet-monitoring-starter

I created the jsonnet-monitoring-starter repository to help others get started with monitoring tools in a declarative way. Each service in your system is given space for alerts, dashboards, recording rules, and default configuration.

└── service-template
    ├── config.libsonnet
    ├── mixin.libsonnet
    ├── alerts
    │   ├── alert-template.libsonnet
    │   └── alerts.libsonnet
    ├── dashboards
    │   ├── dashboard-template.libsonnet
    │   └── dashboards.libsonnet
    └── rules
        ├── rule-template.libsonnet
        └── rules.libsonnet

mixin.libsonnet allows services to be composed into higher level abstractions. This is done using the import directive in jsonnet. For an example of how this is done, see any mixin.libsonnet file in the repository. Finally, the starter provides some boilerplate for generating output files.

└── pkg
    ├── alerts.jsonnet
    ├── dashboards.jsonnet
    └── rules.jsonnet

To see this approach in action, stop on over at depscloud/deploy and take a look at the monitoring setup there.