hot-functions.md

Hot functions

Fn is built on top of container technologies. For each incoming workload, it spins a new container, feed it with the payload and sends the answer back to the caller. You can expect an average start time of 300ms per container. You may refer to this blog post to understand the details better.

In the case you need faster start times for your function, you may use a hot container instead.

Hot functions are started once and kept alive while there is incoming workload. Thus, it means that once you decide to use a hot function, you must be able to tell the moment it should reading from standard input to start writing to standard output.

Currently, Fn implements a HTTP-like protocol to operate hot containers, but instead of communication through a TCP/IP port, it uses standard input/output.

Implementing a hot function

Let's look at a simple hot function. The examples directory contains a simple example of a hot function written in go. Here's func.go from that example.

The basic cycle comprises three steps: read standard input up to a previously known point, process the work, the write the output to stdout with some information about when functions daemon should stop reading from stdout.

In the case at hand, we serve a loop, whose first part is plugging stdin to a HTTP request parser:

r := bufio.NewReader(os.Stdin)
req, err := http.ReadRequest(r)

// ...
} else {
	l, _ := strconv.Atoi(req.Header.Get("Content-Length"))
	p := make([]byte, l)
	r.Read(p)
}

Note how Content-Length is used to help determinate how far standard input must be read.

The next step in the cycle is to do some processing:

	//...
	var buf bytes.Buffer
	fmt.Fprintf(&buf, "Hello %s\n", p)
	for k, vs := range req.Header {
		fmt.Fprintf(&buf, "ENV: %s %#v\n", k, vs)
	}
	//...

And finally, we return the result with a Content-Length header, so Fn would know when to stop reading the gotten response.

res := http.Response{
	Proto:      "HTTP/1.1",
	ProtoMajor: 1,
	ProtoMinor: 1,
	StatusCode: 200,
	Status:     "OK",
}
res.Body = ioutil.NopCloser(&buf)
res.ContentLength = int64(buf.Len())
res.Write(os.Stdout)

Rinse and repeat for each incoming workload.

Enabling a hot function

In your func.yaml, add "format: http". That's it.

format (mandatory) either "default" or "http". If "http", then it is a hot function.

idle_timeout (optional) - idle timeout (in seconds) before function termination, default 30 seconds.