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Basics of writing a containerised function

Once you've obtained the templates and made the necessary edits, you are ready to write and insert your own function. Regardless of the language and dependencies, all functions need to use the same basic approach.
  • They exist in a stateless container - it has no memory of previous inputs, downloads, results, etc. Imagine it runs fresh, from scratch, every time. There are implications here for required data, etc. But the benefits are that you have a predictable, reproducible and scalable way to respond to requests.
  • They will receive and produce JSON, a human-readable text format for describing data.

STDIN, STDOUT and JSON

The containerised functions have a very simple way of interacting with the outside world. They assume everything coming in is a stream of characters (referred to as standard input or STDIN), and that all they can return is also a stream of characters (standard output or STDOUT). This is one of the reasons why we chose to use JSON as the standard format for sending and receiving data to/from functions. JSON is a widely used format, particularly for transferring data over the web. If you are not familiar with JSON, we do provide some explanation/examples in later sections, but you can familiarise yourself with online resources like this brief description.
Our templates combined with OpenFaaS abstract away most of this, and you can assume that (depending on the template language) the function will:
  • receive a request object / dictionary / list containing parameters
  • need to return something which can be serialised as JSON
The next section will walk you through examples of how to write and package a function in either R or Python.