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Watchdog

The watchdog provides an interface between the outside world and your function. It starts a new process for each request and uses STDIO for communication.

Classic watchdog architecture

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.

Next-gen: of-watchdog

Are you looking for more control over your HTTP responses, "hot functions", persistent connection pools or to cache a machine-learning model in memory? Then check out the http mode of the new of-watchdog.

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.

Delve deeper

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 (function process) 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:3.13

ADD https://github.com/openfaas/faas/releases/download/0.18.10/fwatchdog /usr/bin
RUN chmod +x /usr/bin/fwatchdog

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

CMD ["fwatchdog"]

Tip: You can optimize Docker to cache getting the watchdog by using curl, instead of ADD. To do so, replace the related lines with:

RUN apk --no-cache add curl \
    && curl -sL https://github.com/openfaas/faas/releases/download/0.9.14/fwatchdog > /usr/bin/fwatchdog \
    && chmod +x /usr/bin/fwatchdog

Implementing a health-check

At any point in time, if you detect that your function has become unhealthy and needs to restart, then you can delete the /tmp/.lock file which invalidates the check and causes Swarm to re-schedule the function.

  • Kubernetes

For Kubernetes the health check is added through automation without you needing to alter the Dockerfile.

  • Swarm

A Docker Swarm Healthcheck is required and 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 health check 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:

Environment variable overrides:

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

Option Usage
fprocess The process to invoke for each function call (function process). 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_request 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)
healthcheck_interval Interval (in seconds) for HTTP healthcheck by container orchestrator i.e. kubelet. Used for graceful shutdowns.
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
combine_output True by default - combines stdout/stderr in function response, when set to false stderr is written to the container logs and stdout is used for function response
max_inflight Limit the maximum number of requests in flight
jwt_auth For OpenFaaS for Enterprises customers only. When set to true, the watchdog will require a JWT token to be passed as a Bearer token in the Authorization header. This token can only be obtained through the OpenFaaS gateway using a token exchange using the http://gateway.openfaas:8080 address as the authority.
jwt_auth_debug Print out debug messages from the JWT authentication process (OpenFaaS for Enterprises only).
jwt_auth_local When set to true, the watchdog will attempt to validate the JWT token using a port-forwarded or local gateway running at http://127.0.0.1:8080 instead of attempting to reach it via an in-cluster service name (OpenFaaS for Enterprises only).

Metrics

Name Description Type
http_requests_total Total number of requests Counter
http_request_duration_seconds Duration of requests Histogram
http_requests_in_flight Number of requests in-flight Gauge

Advanced / tuning

(New) of-watchdog and HTTP mode

  • of-watchdog

Forking a new process per request has advantages such as process isolation, portability and simplicity. Any process can be made into a function without any additional code. The of-watchdog and its "HTTP" mode is an optimization which maintains one single process between all requests.

A new version of the watchdog is being tested over at openfaas-incubator/of-watchdog.

This re-write is mainly structural for on-going maintenance. It will be a drop-in replacement for the existing watchdog and also has binary releases available.

Graceful shutdowns

The watchdog is capable of working with health-checks to provide a graceful shutdown.

When a SIGTERM signal is detected within the watchdog process a Go routine will remove the /tmp/.lock file and mark the HTTP health-check as unhealthy and return HTTP 503. The code will then wait for the duration specified in write_timeout. During this window the container-orchestrator's health-check must run and complete.

Now the orchestrator will mark this replica as unhealthy and remove it from the pool of valid HTTP endpoints.

Now we will stop accepting new connections and wait for the value defined in write_timeout before finally allowing the process to exit.

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 and Http_Content_Length - gives the total content-length of the incoming HTTP request received by the watchdog, see note below
  • Http_Transfer_Encoding - only set when provided, if set to chunked the Content-Length will be -1 to show that it does not apply

This behaviour is enabled by the cgi_headers environmental variable which is enabled (true) 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 "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

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.