Watchdog

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

Every FaaS function should embed this binary and uses it as its entrypoint. It is in effect a tiny web-server or shim that will fork your desired process for every HTTP request.

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:

Advanced / tuning

Handling headers

You can get hold of the HTTP headers by enabling the cgi_headers environmental variable.

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 "localhost:8080?q=serverless&page=1" -X POST -H X-Forwarded-By:http://my.vpn.com

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

Http_User-Agent=curl/7.43.0
Http_Accept=*/*
Http_X-Forwarded-By=http://my.vpn.com
Http_Method=POST
Http_Query=q=serverless&page=1

You can also use the GET verb:

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

The output from the watchdog would be:

Http_User-Agent=curl/7.43.0
Http_Accept=*/*
Http_Method=GET
Http_Query=action=quote&qty=1&productId=105

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

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".