The OpenFaaS watchdog with mode-abstractions for both HTTP & STDIO pipes
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alexellis Add X-Duration-Seconds for HTTP mode
- adds X-Duration-Seconds header to the HTTP mode only to bring
parity with the Classic Watchdog. Tested with error and success
HTTP codes.

Signed-off-by: Alex Ellis <>
Latest commit b899713 Nov 10, 2018


The of-watchdog is a new version of the OpenFaaS watchdog which provides the original STDIO mode from the Classic Watchdog along with a new HTTP mode.

See also: Classic Watchdog


  • Cleaner abstractions for maintenance
  • Keep function process warm for lower latency / caching / persistent connections
  • Explore streaming for large files (beyond disk/RAM capacity)

Watchdog modes:

History/context: the original watchdog supported mode the Serializing fork mode only and Afterburn was available for testing via a pull request.

When the of-watchdog is complete this version will support four modes as listed below. We may consolidate or remove some of these modes before going to 1.0 so please consider modes 2-4 experimental.

1. HTTP (mode=http)

1.1 Status

The HTTP mode is set to become the default mode for future OpenFaaS templates.

The following templates have been available for testing:

Template HTTP framework Repo
Node.js 8 Express.js
Node.js 10 (LTS) Express.js
Python 3 & 2.7 Flask
Golang Go HTTP (stdlib)
Golang (http.HandlerFunc)
Ruby Sinatra
Java 8 Sun HTTP / Maven

Unofficial: .NET Core / C# and Kestrel

1.2 Description

The HTTP mode is similar to AfterBurn.

A process is forked when the watchdog starts, we then forward any request incoming to the watchdog to a HTTP port within the container.


  • Fastest option - high concurrency and throughput

  • Does not require new/custom client libraries like afterburn but makes use of a long-running daemon such as Express.js for Node or Flask for Python

Example usage for testing:

  • Forward to an NGinx container:
$ go build ; mode=http port=8081 fprocess="docker run -p 80:80 --name nginx -t nginx" upstream_url= ./of-watchdog
  • Forward to a Node.js / Express.js hello-world app:
$ go build ; mode=http port=8081 fprocess="node expressjs-hello-world.js" upstream_url= ./of-watchdog


  • Questionable as to whether this is actually "serverless"

  • Daemons such as express/flask/sinatra could be hard to configure or potentially unpredictable when used in this way

  • One more HTTP hop in the chain between the client and the function

2. Serializing fork (mode=serializing)

2.1 Status

This mode is designed to replicate the behaviour of the original watchdog for backwards compatibility.

2.2 Description

Forks one process per request. Multi-threaded. Ideal for retro-fitting a CGI application handler i.e. for Flask.

Limited to processing files sized as per available memory.

Reads entire request into memory from the HTTP request. At this point we serialize or modify if required. That is then written into the stdin pipe.

  • Stdout pipe is read into memory and then serialized or modified if necessary before being written back to the HTTP response.

  • HTTP headers can be set even after executing the function.

  • A static Content-type can be set ahead of time.

  • Exec timeout: supported.

3. Streaming fork (mode=streaming) - default.

Forks a process per request and can deal with more data than is available memory capacity - i.e. 512mb VM can process multiple GB of video.

HTTP headers cannot be sent after function starts executing due to input/output being hooked-up directly to response for streaming efficiencies. Response code is always 200 unless there is an issue forking the process. An error mid-flight will have to be picked up on the client. Multi-threaded.

  • Input is sent back to client as soon as it's printed to stdout by the executing process.

  • A static Content-type can be set ahead of time.

  • Exec timeout: supported.

4. Afterburn (mode=afterburn)

4.1 Status

Afterburn should be considered for deprecation in favour of the HTTP mode.

Several sample templates are available under the OpenFaaS incubator organisation.

4.2 Details

Uses a single process for all requests, if that request dies the container dies.

Vastly accelerated processing speed but requires a client library for each language - HTTP over stdin/stdout. Single-threaded with a mutex.

  • Limited to processing files sized as per available memory.

  • HTTP headers can be set even after executing the function.

  • A dynamic Content-type can be set from the client library.

  • Exec timeout: not supported.


Environmental variables:

Option Implemented Usage
function_process Yes The process to invoke for each function call function process (alias - fprocess). This must be a UNIX binary and accept input via STDIN and output via STDOUT.
read_timeout Yes HTTP timeout for reading the payload from the client caller (in seconds)
write_timeout Yes HTTP timeout for writing a response body from your function (in seconds)
exec_timeout Yes Exec timeout for process exec'd for each incoming request (in seconds). Disabled if set to 0.
port Yes Specify an alternative TCP port for testing
write_debug No Write all output, error messages, and additional information to the logs. Default is false.
content_type Yes Force a specific Content-Type response for all responses - only in forking/serializing modes.
suppress_lock Yes The watchdog will attempt to write a lockfile to /tmp/ for swarm healthchecks - set this to true to disable behaviour.
upstream_url Yes http mode only - where to forward requests i.e.

Note: the .lock file is implemented for health-checking, but cannot be disabled yet. You must create this file in /tmp/.