YAML Specs

Source Code Organization and Your Development Workflow

You’ve made a Hello World function in your favorite language, and you’ve run it on your Fission deployment. What’s next?

How should you organize source code when you have lots of functions? How should you automate deployment into the cluster? What about version control? How do you test before deploying?

The answers to these questions start from a common first step: how do you specify an application?


Instead of invoking the Fission CLI commands, you can specify your functions in a set of YAML files. This is better than scripting the fission CLI, which is meant as a user interface, not a programming interface.

You’ll usually want to track these YAML files in version control along with your source code. Fission provides CLI tools for generating these specification files, validating them, and “applying” them to a Fission installation.

What does it mean to apply a specification? It means putting specification to effect: figuring out the things that need to be changed on the cluster, and updating them to make them the same as the specification.

Applying a Fission spec goes through these steps:

  • Resources (functions, triggers, etc) that are in the specification but don’t exist on the cluster are created. Local source files are packaged and uploaded.
  • Resources that are both in the specs and on the cluster are compared. If they’re different, the ones on the cluster are changed to match the spec.
  • Resources present only on the cluster and not in the spec are destroyed. (This deletion is limited to resources that were created by a previous apply; this makes sure that Fission doesn’t delete unrelated resources. See below for how this calculation works.)

Note that running apply more than once is equivalent to running it once: in other words, it’s idempotent.

Usage Summary

Start using Fission’s declarative application specifications in 3 steps:

  1. Initialize a directory of specs: fission spec init
  2. Generate some YAMLs: fission function create --spec ...
  3. Apply them to a cluster: fission spec apply --wait

You can also deploy continuously with fission spec apply --watch.

We’ll see examples of all these commands in the tutorial below.


This tutorial assumes you’ve already set up Fission, and tested a simple hello world function to make sure everything’s working. To learn how to do that, head over to the installation guide.

We’ll make a small calculator app with one python environment and two functions, all of which will be declaratively specified using YAML files. This is a somewhat contrived example, but it is just meant as an illustration.

Make an empty directory

$ mkdir spec-tutorial
$ cd spec-tutorial

Initialize the specs directory

$ fission spec init

This creates a specs/ directory. You’ll see a fission-config.yaml in there. This file has a unique ID (deployment ID) in it; everything created on the cluster from these specs will be annotated with that deployment ID.

Note that the deployment ID is generated automatically whenever you initialized the specs directory. In some cases you may want to do initialization for multiple times. In order to update to the same set of resources, you can specify the deployment ID by adding --deployid.

$ fission spec init --deployid xxxx-yyyy-zzzz

Setup a Python environment

$ fission env create --spec --name python --image fission/python-env --builder fission/python-builder

This command creates a YAML file under specs called specs/env-python.yaml.

Code two functions

We will create two functions in python along with an empty requirements.txt file so that builder is able to build the code. We will put the functions in their own directory with the requirements.txt file.

├── eval
│   ├── eval.py
│   └── requirements.txt
├── form
│   ├── form.py
│   └── requirements.txt
└── specs

First function simply returns a simple web form, here are the contents of the file form.py:

def main():
    return """
           <form action="/eval" method="GET">
             Number 1 : <input name="num_1"/>
             Number 2: <input name="num_2"/>
             Operator: <input name="operator"/>
             <input type="submit" value="submit">

The form accepts a simple arithmetic expression. When it is submitted, it makes a request to the second function, which calculates the expression entered.

The second function eval.py is pretty simple too:

from flask import request

def main():
    num_1 = int(request.args.get('num_1'))
    num_2 = int(request.args.get('num_2'))
    operator = request.args.get('operator')

    if operator == '+':
        result = num_1 + num_2
    elif operator == '-':
        result = num_1 - num_2

    return "%s %s %s = %s" % (num_1, operator, num_2, result)

Create specs for these functions

Let’s create a specification for each of these functions. This specifies the function name, where the code lives, and associates the function with the python environment:

$ fission function create --spec --name calc-form --env python --src "form/*" --entrypoint form.main
$ fission function create --spec --name calc-eval --env python --src "eval/*" --entrypoint eval.main

You can see the generated YAML files in specs/function-calc-form.yaml and specs/function-calc-eval.yaml.

Create HTTP trigger specs

$ fission route create --spec --method GET --url /form --function calc-form
$ fission route create --spec --method GET --url /eval --function calc-eval

This creates YAML files specifying that GET requests on /form and /eval invoke the functions calc-form and calc-eval respectively.

Validate your specs

Spec validation does some basic checks: it makes sure there are no duplicate functions with the same name, and that references between various resources are correct.

$ fission spec validate

You should see no errors.

Apply: deploy your functions to Fission

You can simply use apply to deploy the environment, functions and HTTP triggers to the cluster. This command will wait for builds of both functions to complete before exiting:

$ fission spec apply --wait
1 environment created: python
2 packages created: python-1543660299-o4e9, python-1543660287-byam
2 functions created: calc-eval, calc-form
2 HTTPTriggers created: bac55924-03a8-42e1-81b9-8079a8885f3a, f16c8459-3c23-46ad-901f-9312f38cec2a
--- Build SUCCEEDED ---
--- Build SUCCEEDED ---

If the build fails, you can rebuild the package using rebuild command:

--- Build FAILED: ---
Build timeout due to environment builder not ready
$ fission package rebuild --name python-1543660299-o4e9

Test a function

You can check the function is working with fission fn test but since this function returns a HTML, it is best to open in browser.

$ fission function test --name calc-form

Open the URL of the Fission router service suffixed by the name of route at which form function is exposed. For more details on getting the address of Fission router please check the link.


You can enter two number and operator and see the results. Currently this function only supports addition and subtraction.

(If you don’t know the address of the Fission router, you can find it with kubectl: kubectl -n fission get service router.)

Modify the function and re-deploy it

Let’s try modifying a function: let’s change the calc-eval function to support multiplication, too.


    elif operator == '*':
        result = num_1 * num_2


You can add the above lines to eval.py. To deploy your changes, simply apply the specs again:

$ fission spec apply --wait

This should output something like:

1 archive updated: calc-eval-xyz
1 package updated: calc-eval-xyz
1 function updated: calc-eval

Your new updated function is deployed! Test it out by entering a * for the operator in the form!

Add dependencies to the function

Let’s say you’d like to add a pip dependency in requirements.txt to your function, and include some libraries in it, so you can import them in your functions. Add a library to the requirements.txt and modify the ArchiveUploadSpec inside specs/function-<name>.yaml. Once again, deploying is the same:

$ fission spec apply --wait

This command figures out that one function has changed, uploads the source to the cluster, and waits until the Fission builder on the cluster finishes rebuilding this updated source code.

A bit about how this works

Kubernetes manages its state as a set of resources. Deployments, Pod, Services are examples of resources. They represent a target state, and Kubernetes then does the work to ensure this target state is met.

Kubernetes resources can be extended, using Custom Resources. Fission runs on top of Kubernetes and sets up your functions, environments and triggers as Custom Resources. You can see even these custom resources from kubectl: try kubectl get customeresourcedefinitions or kubectl get function.fission.io

Your specs directory is, basically, set of resources plus a bit of configuration. Each YAML file contains one or more resources. They are separated by a “—” separator. The resources are functions, environments, triggers.

There’s a special resource there, ArchiveUploadSpec. This is in fact not a resource, just looks like one in the YAML files. It is used to specify and name a set of files that will be uploaded to the cluster. fission spec apply uses these ArchiveUploadSpecs to create archives locally and upload them. The specs reference these archives using archive:// URLs. These aren’t “real” URLs; they are replaced by http URLs by the fission spec implementation after the archives are uploaded to the cluster. On the cluster, Archives are tracked with checksums; the Fission CLI only uploads archives when their checksum has changed.

Improve Portability of Spec (1.7.0+)

Sometimes you may want to release spec files only without the function source code or the compiled binary. To improve the portability, you can specify a URL that points to the target archive by following the step described in here.

Custom Resources References

You can refer latest definitions for Fission Custom Resources at doc.crds.dev/github.com/fission/fission

More Examples

For more spec examples, please visit fission/examples.

Last modified April 21, 2022: Updated Links for examples (#173) (9de3552)