the-frey~/blog

Very functional python and AWS Lambda

01 October 2018

So in the last few posts on Python and AWS Lambda, I’ve been talking through the evolution of a Python serverless project.

Coming from a Clojure background, I wish that I had some of the syntactic sugar of that language, namely ->, ->> and comp, at a minimum.

Enter the python functools library, and an easy snippet to generate a working comp function.

This has two interesting side effects - first, that we can now chain together any number of arbitrary operations so long as:

  1. The inputs are the same shape (a list)
  2. The items in the list are the same shape (a JSON schema)
  3. We rescue from errors and then handle at the end of operations on the list

What’s interesting about this pattern is it means that you can think about each Lambda as a set of transformations and then set of Lambdas as conceptually similar to a comp - after all, they’re basically operating over the same shape of data the entire time.

We have a structure a little like this, FWIW:

{
  "payload": {
    "field1": 1,
    "field2": "foo"
    "optional_properties": {}
  },
  "errors": {}
}

A great side effect of this is that you also get errors persisting between Lambdas. In order to maximise throughput we tend to DLQ errors when they occur, and then monitor those metrics, but it’s possible something would error and then be valid enough to cross a system (Lambda) boundary successfully. It’s admittedly an edge case, but nice to know we’re covered.

My colleague cooked this up as our utility fn, based on the docs:

import functools

def compose(*functions):
    def compose2(f, g):
        return lambda x: f(g(x))
    return functools.reduce(compose2, functions, lambda x: x)

So then, let’s say we want to transform our payload above - we want to flatten out the optional_properties into the top level, so we have a series of operations:

We of course would like to accomplish this without breaking the contract of input shape, which should be a list of what we’ll call ‘records’, after the AWS convention for which key they’re under in the JSON payload.

To do this, we will use a partial function to apply our configuration to an insert function, but leave an arity of 1 that takes only a list of records.

Hopefully that makes sense.

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