fn-serverless/docs/function-format.md

Open Function Format

This document will describe the details of how a function works, inputs/outputs, etc.

Input

STDIN and Environment Variables

While wanting to keep things simple, flexible and expandable, we decided to go back to the basics, using Unix input and output. Standard in is easy to use in any language and doesn't require anything extra. It also allows streaming input so we can do things like keeping a container running some time and stream requests into the container.

Configuration values, environment information and other things will be passed in through environment variables.

Input Formats

The goals of the input format are the following:

• Very easy to use and parse
• Streamable for increasing performance (more than one call per container execution)
• Ability to build higher level abstractions on top (ie: Lambda syntax compatible)

The format is still up for discussion and in order to move forward and remain flexible, it's likely we will just allow different input formats and the function creator can decide what they want, on a per function basis. Default being the simplest format to use.

Default Input Format

The default input format is simply the request body itself plus some environment variables. For instance, if someone were to post a JSON body, the unmodified body would be sent in via STDIN.

Pros:

• Very simple to use

Cons:

• Not streamable

HTTP/1 Input Format (Not implemented)

--input-format http

HTTP format could be a good option as it is in very common use obviously, most languages have some semi-easy way to parse it, and it's streamable. The basic format is:

REQUEST LINE
HEADER
BLANK LINE
BODY

The header keys and values would be populated with information about the function call such as the request URL and query parameters.

Body length is determined by the Content-Length header, which is mandatory.

Pros:

• Streamable
• Common format

Cons:

• Requires a parsing library or fair amount of code to parse headers properly
• Double parsing - headers + body (if body is to be parsed, such as json)

JSON/HTTP Input Format (Not implemented)

--input-format json-http

The idea here is to keep the HTTP base structure, but make it a bit easier to parse by making the request line and headers a JSON struct. Eg:

{
  "request_url":"http://....",
  "params": {
    "blog_name": "yeezy"
  }
}
BLANK LINE
BODY

Pros:

• Streamable
• Easy to parse headers

Cons:

• New, unknown format

Output

STDOUT

For synchronous: True to form, whatever is written to standard out is returned as the response. If you want to return some JSON output, just write it directly to STDOUT.

TODO: How to change response headers? Perhaps a similar style as input? Headers, then body. Default headers can be defined on the route and overridden on output.

For asynchronous: STDOUT will be written to /dev/null until further notice. We do not want to write this to the logs now, then change it later, otherwise people will start to depend on it.

STDERR

Standard error is reserved for logging, like it was meant to be. Anything you output to STDERR will show up in the logs. And if you use a log collector like logspout, you can collect those logs in a central location. See logging.