Watchdog

The FaaS watchdog is designed to marshal a HTTP request between your public HTTP URL and a individual function.

Creating a function:

• 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.
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
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".