The watchdog provides an unmanaged and generic interface between the outside world and your function. Its job is to marshal a HTTP request accepted on the API Gateway and to invoke your chosen appliaction. The watchdog is a tiny Golang webserver - see the diagram below for how this process works.

Above: a tiny web-server or shim that forks your desired process for every incoming HTTP request

Every function needs to embed this binary and use it as its ENTRYPOINT or CMD, in effect it is the init process for your container. Once your process is forked the watchdog passses in the HTTP request via stdin and reads a HTTP response via stdout. This means your process does not need to know anything about the web or HTTP.

Create a new function the easy way

Create a function via the CLI

The easiest way to create a function is to use a template and the FaaS CLI. The CLI allows you to abstract all Docker knowledge away, you just have to write a handler file in one of the supported programming languages.

Package your function

Here's how to package your function if you don't want to use the CLI or have existing binaries or images:

  • Use an existing or a new Docker image as base image FROM
  • Add the fwatchdog binary from the Releases page via curl or ADD https://
  • Set an fprocess environmental variable with the function you want to run for each request
  • Expose port 8080
  • Set the CMD to fwatchdog

Example Dockerfile for an echo function:

FROM alpine:3.5

ADD /usr/bin
RUN chmod +x /usr/bin/fwatchdog

# Define your UNIX binary here
ENV fprocess="/bin/cat"

CMD ["fwatchdog"]

Implementing a Docker Healthcheck

A Docker Healthcheck is not required but is best practice. It will make sure that the watchdog is ready to accept a request before forwarding requests via the API Gateway. If the function or watchdog runs into an unrecoverable issue Swarm will also be able to restart the container.

Here is an example of the echo function implementing a healthcheck with a 5-second checking interval.

FROM functions/alpine

ENV fprocess="cat /etc/hostname"

HEALTHCHECK --interval=5s CMD [ -e /tmp/.lock ] || exit 1

The watchdog process creates a .lock file in /tmp/ on starting its internal Golang HTTP server. [ -e file_name ] is shell to check if a file exists. With Windows Containers this is an invalid path so you may want to set the suppress_lock environmental variable.

Read my Docker Swarm tutorial on Healthchecks:

Environmental Overrides:

The watchdog can be configured through environmental variables. You must always specifiy an fprocess variable.

Option Usage
fprocess The process to invoke for each function call. This must be a UNIX binary and accept input via STDIN and output via STDOUT.
cgi_headers HTTP headers from request are made available through environmental variables - Http_X_Served_By etc. See section: Handling headers for more detail. Enabled by default.
marshal_requests Instead of re-directing the raw HTTP body into your fprocess, it will first be marshalled into JSON. Use this if you need to work with HTTP headers and do not want to use environmental variables via the cgi_headers flag.
content_type Force a specific Content-Type response for all responses.
write_timeout HTTP timeout for writing a response body from your function (in seconds)
read_timeout HTTP timeout for reading the payload from the client caller (in seconds)
suppress_lock The watchdog will attempt to write a lockfile to /tmp/ for swarm healthchecks - set this to true to disable behaviour.
exec_timeout Hard timeout for process exec'd for each incoming request (in seconds). Disabled if set to 0.
write_debug Write all output, error messages, and additional information to the logs. Default is false.

Advanced / Tuning

Working with HTTP Headers

Headers and other request information are injected into environmental variables in the following format:

The X-Forwarded-By header becomes available as Http_X_Forwarded_By

  • Http_Method - GET/POST etc
  • Http_Query - QueryString value
  • Http_ContentLength - gives the total content-length of the incoming HTTP request received by the watchdog.

This behaviour is enabled by the cgi_headers environmental variable which is enabled by default.

Here's an example of a POST request with an additional header and a query-string.

$ cgi_headers=true fprocess=env ./watchdog &
2017/06/23 17:02:58 Writing lock-file to: /tmp/.lock

$ curl -X POST "localhost:8080?q=serverless&page=1" \
    -H X-Forwarded-By:

This is what you'd see if you had set your fprocess to env on a Linux system:


You can also use the GET verb:

$ curl "localhost:8080?action=quote&qty=1&productId=105"

The output from the watchdog would be:


You can now use HTTP state from within your application to make decisions.

HTTP Methods

The HTTP methods supported for the watchdog are:

With a body: * POST, PUT, DELETE, UPDATE

Without a body: * GET


The API Gateway currently supports the POST route for functions.

Content-Type of Request/Response

By default the watchdog will match the response of your function to the "Content-Type" of the client.

  • If your client sends a JSON post with a Content-Type of application/json this will be matched automatically in the response.
  • If your client sends a JSON post with a Content-Type of text/plain this will be matched automatically in the response too

To override the Content-Type of all your responses set the content_type environmental variable.

I don't want to use the watchdog

This is an unsupported use-case for the OpenFaaS project however if your container exposes HTTP POST on port 8080 then the OpenFaaS API gateway and other tooling will manage your container.

Tuning Auto-scaling

Auto-scaling starts at 1 replica and steps up in blocks of 5:

  • 1->5
  • 5->10
  • 10->15
  • 15->20

You can override the upper limit of auto-scaling by setting the following label on your container:

com.faas.max_replicas: "10"

If you want to disable scaling, set the com.faas.max_replicas value to "1".

Contribute to the Watchdog

To contribute to the OpenFaaS Watchdog component checkout the "./" scripts and acompanying Dockerfiles.