Add async functions to the docs

Adds section on async / parallel processing to the trouble-
shooting guide

Adds section to the auto-scaling doc

Signed-off-by: Alex Ellis (VMware) <[email protected]>

Author: alexellis

docs/architecture/autoscaling.md

@@ -4,14 +4,16 @@ Auto-scaling in OpenFaaS allows a function to scale up or down depending on dema
 
 ## Scaling by requests per second
 
-OpenFaaS ships with a single auto-scaling rule defined in the configuration for AlertManager. AlertManager reads usage (requests per second) metrics from Prometheus in order to know when to fire an alert to the API Gateway.
+OpenFaaS ships with a single auto-scaling rule defined in the mounted configuration file for AlertManager. AlertManager reads usage (requests per second) metrics from Prometheus in order to know when to fire an alert to the API Gateway.
 
 The API Gateway handles AlertManager alerts through its `/system/alert` route.
 
 The auto-scaling provided by this method can be disabled by either deleting the AlertManager deployment or by scaling the deployment to zero replicas.
 
 The AlertManager rules ([alert.rules](https://github.com/openfaas/faas/blob/master/prometheus/alert.rules.yml)) for Swarm can be viewed here and altered as a configuration map.
 
+All calls made through the gateway whether to a synchronous function `/function/` route or via the asynchronous `/async-function` route count towards this method of auto-scaling.
+
 ### Min/max replicas
 
 The minimum (initial) and maximum replica count can be set at deployment time by adding a label to the function.

docs/deployment/troubleshooting.md

@@ -1,5 +1,25 @@
 # Troubleshooting guide
 
+## Asynchronous functions
+
+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 --Header "X-Callback-Url=https://request.bin/mybin"
+```
+
+Alternatively you can specify another asynchronous or synchronous function to run instead.
+
+* Parallelism
+
+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](https://github.com/openfaas/workshop) has more instructions on running tasks asynchronously.
+
 ## Timeouts
 
 Default timeouts are configured at the HTTP level and must be set both on the gateway and the function.