Live-Reload in Fission: Instant feedback on your Serverless Functions

Live reloading of functions for faster feedback

Accelerating feedback loops are an important devops principle: the sooner you find a bug, the cheaper it is to fix it.

While developing your application, you’re typically going through a cycle: write code, build, deploy into a test environment, run tests, fix, repeat. The build and deploy stages of this cycle are idle, unproductive time where you’re simply waiting. As a project grows, these stages get slower and slower. Once they’re slow enough, you end up context switching to another task, while waiting, and that makes it harder to get back into the right context and fix any bugs you find in testing.

Fission comes built-in with live-reload, a feature that drops the time from code to running tests to a few seconds. Fission is the first open source serverless function framework to do this.

Using live-reload in Fission

The fission CLI supports declarative specifications for Fission resources – such as functions, environments, triggers, etc. These resources are specified as a set of YAML files that are stored in the specs directory at the root of your Fission application. You don’t have to write these YAML files from scratch - Fission automatically generates the initial versions for you. And because configuration is declarative, you don’t have to write any imperative deployment scripts for configuration management.

As part of Declarative specifications, you can also configure Packaging – how source code is bundled up and uploaded to the cluster. Fission then uses that to consistently deploy your code on any environment - from Dev, to Test, through to Production.

The deployment of all resources in the specifications is triggered by the fission spec apply command. With this command, the fission CLI checks the state of the cluster against the desired state as configured in the specs.

To use live-reload, simply add the –watch option:

$ fission spec apply --watch

And then, edit your code in your editor as usual.

Whenever you save a file, the fission CLI notices a change on the filesystem, and automatically packages up the code and uploads it.

On the cluster, if this is a compiled language, the source is compiled. Then the function deployment is automatically updated. Within a few seconds, the new function is ready to be tested.

Since Fission automatically deploys into a test cluster as you code, you get faster feedback, enabling you to rapidly iterate through your coding and testing cycles.

This allows bugs to be found and fixed earlier in the development lifecycle. It also increases the fidelity of your tests, since you can run them in a real cluster, along with whatever other services your code depends on.

Here’s a quick demo video of live-reload in action:

Behind the scenes – how it works


[editor] -> fission spec apply --watch -> [fission storagesvc] -> [builder] -> [back to storage] -> [runtime pod]


Live reload in Fission gives you a smooth, fast development experience. It accelerates feedback loops – allowing you to test your code as you type.

Check out the Fission installation guide to get started with Fission. Join us on the Fission Slack to chat, or follow us on Twitter at @fissionio.