Create chaining_functions.md

guide/chaining_functions.md

+# Chaining OpenFaaS functions
+
+We will discuss client-side piping, server-side piping and the "function director" pattern.
+
+## Client-side piping
+
+The easiest way to chain functions is to do it on the client-side via your application code or a `curl`.
+
+Here is an example:
+
+We pipe a string or file into the markdown function, then pipe it into a Slack function
+
+```
+$ curl -d "# test" localhost:8080/function/markdown | \
+  curl localhost:8080/function/slack --data-binary -
+```
+
+You could also do this via code, or through the `faas-cli`:
+
+```
+$ echo "test" | faas-cli invoke --name markdown | \
+faas-cli invoke --name slack
+```
+
+## Server-side access via gateway
+
+On the server side you can access any other function by calling it on the gateway over HTTP.
+
+### Function A calls B
+
+Let's say we have two functions:
+* geolocatecity - gives a city name for a lat/lon combo in JSON format
+* findiss - finds the location of the International Space Station then pretty-prints the city name by using the `geolocatecity` function
+
+findiss Python 2.7 handler:
+
+```
+import requests
+
+def get_space_station_location():
+    return {"lat": 0.51112, "lon": -0.1234}
+
+def handler(st):
+    location = get_space_station_location()
+    r = requests.post("http://gateway:8080/function/geolocatecity", location)
+
+    print("The ISS is over the following city: " + r.content)
+```
+
+
+### Function Director pattern
+
+In the Function Director pattern - we create a "wrapper function" which pipes the result of function call A into function call B then returns the value to the caller. This saves on bandwidth and latency vs. client-side piping:
+
+Take our previous example:
+
+```
+$ curl -d "# test" localhost:8080/function/markdown | \
+  curl localhost:8080/function/slack --data-binary -
+```
+
+markdown2slack Python 2.7 handler:
+
+```
+import requests
+
+def handler(req):
+    
+    markdown = requests.post("http://gateway:8080/function/markdown", req)
+    slack_result = requests.post("http://gateway:8080/function/slack", markdown.content)
+
+    print("Slack result: " + str(slack_result.status_code))
+```