Exercises in Programming Style–Hollywood

NOTE : read the rest of the series, or check out the source code.

If you enjoy read­ing these exer­cises then please buy Crista’s book to sup­port her work.

exercises-prog-styles-cover

Fol­low­ing on from the last post, we will look at the Hollywood style today.

 

Style 14 – Hollywood

Constraints

  • Larger problem is decomposed into entities using some form of abstraction
  • The entities are never called on directly for actions
  • The entities provide interfaces for other entities to be able to register callbacks
  • At certain points of the computation, the entities call on the other entities that have registered for callbacks

 

I’m not overly fond of Crista’s Python interpretation of this style, so I decided to do something slightly different using F#’s built-in support for Observables. You could also use Rx via FSharp.Control.Reactive, though it seemed to be overkill for this particular problem.

Sticking with the same entities Crista defined in her example:

  • DataStorage
  • StopWordsFilter
  • WordFrequencyCounter

in our F# version, each will subscribe to an upstream IObservable, does whatever it needs to do and provide the output also as an IObservable for downstream entities.

Style14_designv2

And upstream of all the entities is an attempt to run a term frequency analysis against a data file and a corresponding stop words file:

Style14_01

Our version of DataStorage would depend on an IObservable<RunArgs>. It’ll in turn expose an IObservable<RunArgs * string[]> as member so downstream entities can subscribe to and be notified when DataStorage is able to load the words from the specified data file.

Style14_02

Next, we’ll implement a StopWordsFilter type that will:

  1. subscribe to an IObservable<RunArgs * string[]>;
  2. on a new value, load the stop words and use them to filter the words from the data file;
  3. make the filtered words available to downstream entities via an IObservable<string[]>

Style14_03

Finally we have the WordFrequencyCounter, which takes an IObservable<string[]> and print the top 25 most frequent words:

Style14_04

To string everything together, we’ll create an instance of IObservable<RunArgs> via an F# Event (Event.Publish gives us an instance of IEvent<‘T> which inherits from IObservable<T>).

This IOservable will act as the upstream to DataStorage and to kick things off we just have to trigger a new event with an instance of RunArgs:

Style14_05

 

You can find the source code for this exer­cise here.

Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter


Hi, I’m Yan. I’m an AWS Serverless Hero and the author of Production-Ready Serverless.

I specialise in rapidly transitioning teams to serverless and building production-ready services on AWS.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.


Check out my new course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. Including basic concepts, HTTP and event triggers, activities, callbacks, nested workflows, design patterns and best practices.

Get Your Copy