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Asynchronous Functions

OpenFaaS enables long-running tasks or function invocations to run in the background through the use of NATS Streaming. This decouples the HTTP transaction between the caller and the function.

  • The HTTP request is serialized to NATS Streaming through the gateway as a "producer".
  • The queue-worker acts as a subscriber and deserializes the HTTP request and uses it to invoke the function directly

The asynchronous workflow can have a longer, separate timeout compared with synchronous timeout on the gateway.


You're working with a partner's webhook. They send you data, but if you don't send a HTTP 200 OK within 1 second, they assume failure and retry the message. This works well if your function can complete in less than a second on every invocation, but if there's any risk that it can't, you need another solution. So you will give your partner a URL to the asynchronous function URL instead and it will reply within several milliseconds whilst still processing the data.

How it works

Any function can be invoked asynchronously by changing the route on the gateway from /function/<name> to /async-function/<name>. A 202 Accepted message will be issued in response to asynchronous calls.

If you would like to receive a value from an asynchronous call you should pass a HTTP header with the URL to be used for the call-back.

$ faas invoke figlet -H "X-Callback-Url: https://request.bin/mybin"

It will pass back the X-Call-Id you had when you sent the initial request.

You can use netcat to check the Call Id during invocation:

$ curl -d "Hello" "X-Callback-Url: http://<your-ip>:8888"
$ nc -l 8888
HTTP/1.1 200 OK
Content-Length: 174
Content-Type: application/x-www-form-urlencoded
Date: Tue, 04 Dec 2018 09:24:55 GMT
X-Call-Id: eb8283f5-1679-48e0-afec-194544b054aa
X-Duration-Seconds: 0.002885
X-Start-Time: 1543915495384346700

 _   _      _ _       
| | | | ___| | | ___  
| |_| |/ _ \ | |/ _ \ 
|  _  |  __/ | | (_) |
|_| |_|\___|_|_|\___/ 

Alternatively you can specify another asynchronous or synchronous function to run instead.


By default there is one queue-worker replica deployed which is set up to run a single task of up to 30 seconds in duration with one task in parallel. You can increase the parallelism by scaling the queue-worker up - i.e. 5 replicas for 5 parallel tasks.

You can tune the values for the number of tasks each queue worker may run in parallel as well as the maximum duration of any asynchronous task that worker processes. Edit the Kubernetes helm chart, YAML or Swarm docker-compose.yml files.

The OpenFaaS workshop has more instructions on running tasks asynchronously.

  • Verbose Output

The Queue Worker component enables asynchronous processing of function requests. The default verbosity level hides the message content, but this can be viewed by setting write_debug to true when deploying.

Callback request headers

The following additional request headers will be set when invoking the call back URL:

Header Description
X-Call-Id The original function call's tracing UUID
X-Duration-Seconds Time taken in seconds to execute the original function call
X-Function-Status HTTP status code returned by the original function call