Use an existing or a new Docker image
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:

FROM alpine:3.5

ADD https://github.com/alexellis/faas/releases/download/v0.5-alpha/fwatchdog /usr/bin
RUN chmod +x /usr/bin/fwatchdog

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

CMD ["fwatchdog"]

Implementing the a healthcheck

Docker swarm will keep your function out of the DNS-RR / IPVS pool if the task (container) is not healthy.

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.

Swarm tutorial on Healthchecks:

Test-drive Docker Healthcheck in 10 minutes

Environmental overrides:

A number of environmental overrides can be added for additional flexibility and options:

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.
`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
`read_timeout`	HTTP timeout for reading the payload from the client caller
`suppress_lock`	The watchdog will attempt to write a lockfile to /tmp/ for swarm healthchecks - set this to true to disable behaviour.

Advanced / tuning

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.

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".