Skip to content

OpenFaaS YAML file reference

OpenFaaS has its own YAML file called a "stack file" which is used to provide configuration for functions.

This page is the reference guide to the schema and how to use each field.

Configuration is split between:

  • build-time - how to build a container from the source provided
  • deploy time - how to deploy the function to and in OpenFaaS

Generate Kubernetes resources

Did you know that the OpenFaaS YAML files can also be converted into Kubernetes resources using faas-cli generate?

If you use a GitOps tool like Argo or Flux, you can retain your stack.yml file for building functions, and testing locally, then generate a CustomResource when required with: faas-cli generate | kubectl apply -f

Functions belong together

The YAML file can hold one to many functions separated by separate entries.


$ faas-cli new --lang go fn1
$ faas-cli new --lang go fn2 --append=fn1.yml


  name: openfaas

    lang: go
    handler: ./fn1
    image: fn1:latest
    lang: go
    handler: ./fn2
    image: fn2:latest

Then rename your file to stack.yml, so you don't need to specify the -f flag when using the CLI.

mv fn1.yml stack.yml


The only valid value for provider name is openfaas.


The gateway URL can be hard-coded into the YAML file or overriden at deployment time with the --gateway flag or OPENFAAS_URL env-var.


The functions element holds a map of functions, by default all functions are acted on with CLI verbs, but you can filter them with --filter or --regex.

Function Name

The function Name is specified by a key in the functions map, i.e. fn1 in the above example. Function name must be unique within a stack.yml file.

Valid function names follow ietf rfc1035 which is also used for DNS sub-domains.

(DNS_LABEL): An alphanumeric (a-z, and 0-9) string, with a maximum length of 63 characters, with the '-' character allowed anywhere except the first or last character, suitable for use as a hostname or segment in a domain name.

Function: Language

The lang field refers to which template is going to be used to build the function. The templates are expected to be found in the ./template folder and will be pulled from GitHub if not present.

Function: Handler

The function handler field refers to a folder where the function's source code can be found, it must always be a folder and not a filename.

Function: Image

The image field refers to a Docker image reference, this could be on the Docker Hub, in your local Docker library or on another remote server.

Function: Skip build

The skip_build field controls whether the CLI will attempt to build the Docker image for the function. When true, the build step is skipped and you should see a message printed to the terminal Skipping build of: "function name".

This an optional boolean field, set to false by default.

Function: Build Options

The build_options field can be used to pass a list of additional configurations for a template.

These must be pre-defined within the template and can be used to populate the ADDITIONAL_PACKAGE field in the Dockerfile used by the template.

For instance, here's an example from the python3 template which is based upon Alpine Linux.

Note: if you want to install Python development packages, you may find that the python3-debian template is a better fit, since it comes with build tools pre-installed.

language: python3
fprocess: python3
  - name: mysql
      - mysql-client
      - mysql-dev
  - name: pillow
      - jpeg-dev
      - zlib-dev
      - freetype-dev
      - lcms2-dev
      - openjpeg-dev
      - tiff-dev
      - tk-dev
      - tcl-dev
      - harfbuzz-dev
      - fribidi-dev

Given the template defines a mysql and pillow build option, you can add either or both of them to your stack.yml file so that these preconfigured packages are installed at build time.

- ca-certificates

The packages listed will be expounded into the Dockerfile at build-time via the ADDITIONAL_PACKAGE environment variable.

FROM --platform=${TARGETPLATFORM:-linux/amd64} python:3-alpine

# Allows you to add additional packages via build-arg

# Install packages
RUN apk --no-cache add ca-certificates ${ADDITIONAL_PACKAGE}

If you don't want to or cannot update the template, then you can pass the ADDITIONAL_PACKAGE directly instead, see the next section.

Function: Build Args (build-args)

A map of build args can be passed to the container builder. These are compatible with Docker, BuildKit and Kaniko. Other containers builders may vary in their support.

An example of a build argument may be for enabling Go modules, or a HTTP_PROXY as per below:

    handler: ./with_go_modules
    lang: go
      GO111MODULE: on

These can also be passed via the CLI using faas-cli build --build-arg key=value or faas-cli up --build-arg key=value

Function: Environmental variables

You can set configuration via environmental variables either in-line within the YAML file or in a separate external file. Do not store confidential or private data in environmental variables. See: secrets.

  • Define environment in-line within the file:

Imagine you needed to define a http_proxy variable to operate within a corporate network:

    lang: python
    handler: ./sample/url-ping
    image: alexellis2/faas-urlping
      no_proxy: http://gateway/
  • environment_file - defined in zero to many external files
    - file1.yml
    - file2.yml

If you specify a variable such as "rss_feed_url" in more than one environment_file file then the last file in the list will take priority.

Environment file format:

  rss_feed_url: key1
  include_images: key2

Note: external files take priority over in-line environmental variables. This allows you to specify a default and then have overrides within an external file.

Function: Secure secrets

OpenFaaS functions can make use of secure secrets using the secret store from Kubernetes or faasd. This is the recommended way to store secure access keys, tokens and other private data.

Create the secret with your orchestration tool i.e. kubectl or docker secret create then list the secret name as part of an array of secrets.

  - s3_access_key
  - s3_secret_key

Function: Read-Only Root Filesystem

The readonly_root_filesystem indicates that the function file system will be set to read-only except for the temporary folder /tmp. This prevents the function from writing to or modifying the filesystem (e.g. system files). This is used to provide tighter security for your functions. You can set this value as a boolean:

readonly_root_filesystem: true

This an optional boolean field, set to false by default.

Function: Constraints

Constraints are passed directly to the underlying container orchestrator. They allow you to pin a function to certain host or type of host.

Here is an example of picking only hosts with a Linux OS in Docker Swarm:

     - "node.platform.os == linux"

Or only using nodes running with Windows:

     - "node.platform.os == windows"

To assign a function to a given NodePool or Node:

First, update stack.yml:

version: 1.0
  name: openfaas
    lang: go
    handler: ./low
    image: alexellis2/low:latest
    - "lowmem=true"

And run faas-cli up or faas-cli deploy.

You'll see an error like the following. So we need to add the lowmem=true label to one or more nodes.

kubectl get event -n  openfaas-fn -w
LAST SEEN   TYPE      REASON              KIND         MESSAGE
15m         Warning   FailedScheduling    Pod          0/3 nodes are available: 3 node(s) didn't match node selector.

Apply a label to the nodes or nodepool:

kubectl label node/primary-ofc-3irgb --overwrite lowmem=true

Check the label was applied:

kubectl get nodes -l lowmem=true
NAME                STATUS   ROLES    AGE    VERSION
primary-ofc-3irgb   Ready    <none>   136d   v1.19.3

Now deploy the code again, or wait for the scheduler to move the pod to the matching node.

kubectl get pod -n openfaas-fn -o wide
NAME                 READY   STATUS    IP             NODE
low-5fbb9fd6-t69q4   1/1     Running   primary-ofc-3irgb

Function: Labels

Labels can be applied through a map which is passed directly to the container scheduler. Labels are also available from the OpenFaaS REST API for querying or grouping functions.

Example of using a label to group by user or apply a canary label:

     canary: true
     Git-Owner: alexellis

Important note: When used with a Kubernetes provider, labels support a restricted character set and length. "Valid label values must be 63 characters or less and must be empty or begin and end with an alphanumeric character ([a-z0-9A-Z]) with dashes (-), underscores (_), dots (.), and alphanumerics between."

See Syntax and character set for more information

Function: Annotations

Annotations are a collection of meta-data which is stored with the function by the provider. Annotations are also available from the OpenFaaS REST API for querying.

Example of setting a "topic" for the Kafka event connector:

     topic: "kafka.payments-received"
     expire-date: "Wed Aug  8 07:40:18 BST 2018"

Example of setting a custom HTTP health check path and initial check delay:

   annotations: "/healthz" "30s"

Function: Memory/CPU limits

Applying memory and CPU limits can be done through the limits and requests fields. It is advisable to always set a limit for your functions to prevent them consuming too many resources in your system.

Important note: The value for memory for Kubernetes needs to be in the format "Mi".

Here we constrain the url-ping function to only use 40Mb of RAM at a maximum.

    lang: python
    handler: ./sample/url-ping
    image: alexellis/faas-url-ping:0.2
      memory: 40Mi
      memory: 20Mi

Here we constrain a function to use only 100m which is equivalent to 1/10 of an Intel Hyperthread core.

    lang: python
    handler: ./sample/url-ping
    image: alexellis/faas-url-ping:0.2
      cpu: 100m
      cpu: 100m

The meanings and formats of limits and requests may vary depending on whether you are using Kubernetes or Docker Swarm. In general:

  • Requests ensures the stated host resource is available for the container to use
  • Limits specify the maximum amount of host resources that a container can consume

Read more for: Kubernetes.


The configuration section allows you to define additional configuration that is global to the entire stack, currently this mostly impacts function build time options.


The templates list allows you to define the information required to pull the templates for your functions. This list of templates will automatically be pulled when you build your functions. When configured correctly, this allows you to completely build your functions with just faas-cli build. Without this section, you must manually faas-cli template pull <source> before you use faas-cli build.

version: 1.0
  name: openfaas
    lang: python3-http-debian
    handler: ./create-batch
    image: welteki2/create-batch:0.0.1

    - name: python3-http


The copy list allows you to define additional project paths that will be copied into your function's handler folder.

version: 1.0
  name: openfaas
    lang: python3
    handler: ./hello
    image: openfaas/hello:0.1.0

    - ./common
    - ./data
    - ./models

Given the above configuration, the build folder for a python hello function would look like

└── hello
    ├── Dockerfile
    ├── function
       ├── models
          └── ...
       ├── data
          └── ...
       ├── common
          └── ...
       └── requirements.txt
    ├── requirements.txt
    └── template.yml

The CLI build command also has a related flag --copy-extra. When this flag is used, the paths specified by the flag will be merged into the list from the YAML. This means it will extend, not replace, the values specified in the file.

Note: These paths must be subpaths of the project and not equal to the entire project. For example, you can not reference ../ or $HOME/.ssh, any path outside of the current directory will be skipped.

YAML - environment variable substitution

The YAML stack format supports the use of envsubst-style templates. This means that you can have a single file with multiple configuration options such as for different user accounts, versions or environments.

Here is an example use-case, in your project there is an official and a development Docker Hub username/account. For the CI server images are always pushed to exampleco, but in development you may want to push to your own account such as alexellis2.

    lang: python
    handler: ./sample/url-ping
    image: ${DOCKER_USER:-exampleco}/faas-url-ping:0.2

Use the default (exampleco):

$ faas-cli build
$ DOCKER_USER="" faas-cli build

Override with "alexellis2" through an environment variable:

$ DOCKER_USER="alexellis2" faas-cli build

YAML - template stack configuration

The configuration field stand alone and not part of the function field, add it to the top level of the YAML file.

version: 1.0
  name: openfaas


    - name: perl-alpine
    - name: rust

Pull templates listed under the configuration.templates field:

$ faas-cli template pull stack

By default if only name is provided the template will be pulled from the template store.

The templates will be automatically pulled during build time.

YAML - pinning versions of templates

Templates may change over time, including breaking changes.

If you want additional stability, or have run into an issue with a newer version of a template, you can pin it.

You can pin a template to a specific release tag or branch by adding # plus the name required to the URL for the source field.

For example:

    - name: golang-middleware

Then run:

$ faas-cli template pull stack

Pulling template: golang-middleware from configuration file: stack.yml
Fetch templates from repository: at 0.7.0
2022/09/21 16:18:56 Attempting to expand templates from
2022/09/21 16:18:58 Fetched 2 template(s) : [golang-http golang-middleware] from

As an alternative, you can fork any template and customise it or change it to suit your needs, and use that URL instead, even if it has the same name as the original template.

For example:

    - name: golang-middleware