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The Solution to Too Many Servers is More Servers

Or: how to use Tilt reactive extensions and how to write your own

Setting up a dev environment isn’t about setting up a single tool in a carefully manicured garden.

We have messy systems that need to interoperate!

One way to help systems interoperate is to build piles of configuration. That’s how teams end up with large YAML templates to express the ways server X’s configuration depends on server A, B, and C’s configuration.

Another way is to react in realtime and have servers, or controllers, that manage the configuration for us.

Both strategies have their pros and cons. In this blog post, we’re going to explore Tilt extensions that use each strategy, and how they work in practice.

Plugging Tools Together With More Config

Lots of teams want to extend Tilt to support more types of dev environments. We gave them the ability to add new functions to their Tiltfile.

Here’s a good example of a Tilt extension: git_resource. From the README:

Author: Bob Jackman

Deploy a dockerfile from a remote repository – or specify the path to a local checkout for local development.

Install a Remote OR Local Repository

load('ext://git_resource', 'git_resource')

git_resource('myResourceName', '')

# -- OR --

git_resource('myResourceName', '/path/to/local/checkout')

This will clone/pull your repo, build your dockerfile, and deploy your image into the cluster all in one fell swoop. This function is syntactic sugar and would be identical to sequentially making calls to git_checkout() and deploy_from_dir()

When you start Tilt:

  1. load() checks if you’ve downloaded the code for the git_resource extension.
  2. If the code isn’t loaded, the function blocks while it downloads.
  3. load() executes the extension code, then imports the git_resource function.
  4. The git_resource() function checks if you’ve downloaded the given repository.
  5. If the repository isn’t cloned, the function blocks while it does a git clone.
  6. The git_resource() function finds the Dockerfile (for any image builds) and Kubernetes YAML (for any deploys) in the repository.

We love the git_resource extension! We see teams use this a lot for multi-repo projects (where each service is in its own repo). They can load configs from multiple repositories and duct tape them together.

All that duct tape happens at Tilt startup. And it happens synchronously, so it’s easy to reason about. If any part of it fails, Tilt won’t start.

But this also has downsides! As the extension ecosystem grew, we started to see them:

  • If we have a lot of synchronous extensions, starting Tilt can become slow. Reloading the configuration can become slow. Optimizing may be a pain.
  • We can’t parallelize load() or git_resource() while they’re downloading. We could try to change the API to make them async. But then we would need better primitives for expressing dependencies and pipelines of async operations.
  • People started asking for the ability to pass more configuration parameters to load(), so we could change the execution of the extensions themselves. Or they wanted a way to hook into the “end” of the Tiltfile, so they could inspect all the resources that were defined.

When this starts to happen, it’s a sign that we need a better tool: reactive controllers.

Plugging Tools Together With Reactive Controllers

With a reactive controller extension, there are no functions to load. There are no arguments to pass.

You can load them from the Tiltfile. You can load them from the CLI.

They’re intended for auto-managing config, rather than for piles of configuration.

Here’s what it looks like.

Installing Extensions

In your Tiltfile, we write:

v1alpha1.extension_repo(name='default', url='')

This registers a new extension repo named default. In the background, Tilt downloads the repo.

The url will work for most major git hosts. If you’re developing an extension, you can use a file:// URL. Here’s the declaration I use when I’m developing extensions locally:

v1alpha1.extension_repo(name='default', url='file:///home/nick/src/tilt-extensions')

Then, we load the extension we want with the line:

v1alpha1.extension(name='cancel', repo_name='default', repo_path='cancel')

An extension needs 3 parameters:

  • A name, which is what it will show up as in the Tilt UI.
  • A repo_name, which refers to the name of the repo we installed with extension_repo.
  • A repo_path, which is the directory that the extension lives at in the repo.

Once the extension repo is finished downloading, Tilt will load the extension configuration. This is an extension Tiltfile that loads independently of the rest of your main Tiltfile.

Managing Extensions

If you have a running Tilt session, you can use the CLI to inspect the extensions you have installed and their current status.

$ tilt get extensionrepo
default   2021-08-25T00:30:29Z
$ tilt get extension
cancel   2021-08-25T00:30:29Z

We can even install and uninstall extensions from the command-line with tilt delete and tilt apply. Or write extensions that manage other extensions! 😈

Configs vs. Controllers

Last week, I wrote about the cancel extension in more detail. cancel registers a server that watches Tilt resources, and adds cancel buttons to their UI dashboard if they need it.

If we were trying to implement cancel with config management tools, we would write a new template function. Maybe we’d call it local_resource_and_cancel_button. Then anything in our project that wanted a cancel button would need to call our new function.

But with a cancel server, there are no templates and no dependency management. We don’t need to pass around button configuration. We simply have our cancel server watch for new resources, respond, and auto-configure the buttons!

In a future blog post, I’m going to write about how we use this new philosophy to handle a more complex use-case that a lot of teams struggle with: auto-configuring and auto-restarting kubefwd. Until next week!


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