Architecting a web application in a functional style

Chris Birchall

Agenda

• Hand waving
• Write some code
• Summarise

http://lambdale.org/

@lambd_ale

1st September 2018

Functional style?

Obstacles to writing in functional style

• Disconnect between theory and practice
• Decision paralysis
• Nagging doubts

Disconnect between theory and practice

Disconnect between theory and practice

theory

practice

Kan

extensions

corepresentable functors

Monads

Yoneda lemma

Type classes

Kleisli

StateT

"build a working, well-tested, maintainable piece of software to deliver business value"

Decision paralysis

Somebody told me to use free monads,

but somebody else told they were passé

Should this thing be a class or an object?

Is it ok to use traits?

I don't even know what my package structure

should look like

Nagging doubt #1

Am I doing it right?

Nagging doubt #2

Is FP actually better than the alternatives?

Benefits of FP

• Local reasoning
• Abstraction
• Composition

Nagging doubt #3

Don't FP and OOP boil down to the same thing anyway?

Yes!

But one or the other might still be

a better tool for the job

Worked example

Photo storage service

Requirements

User can upload a photo and get back a unique ID

User can retrieve a photo by ID

Detailed requirements: upload photo

• Check that uploaded data is a valid photo
  • If validation fails, log it and return a 400 error

• Store to a durable filestore service
  • If this fails, log a warning and return a 500 error
• Also store to a remote cache
  • If this fails, just log a warning
• Send a "photo uploaded" Kafka event
• Return a 201 response containing the photo ID

Detailed requirements: retrieve photo

• First lookup the file in the cache
  • If this fails, log a warning and treat as cache miss
• On cache hit, return a 200 response
• On cache miss:
  • Retrieve file from filestore
    • If file exists:
      • Write it to the cache
        • If this fails, just log a warning
      • Return a 200 response
    • Else, return a 404 response

Non-functional requirements

• Request ID should be extracted from request header (or generated if not present)
  • Should be propagated to all backend requests
  • Should be included in all logs
• We expect a lot of concurrent requests, so all I/O must be asynchronous

Domain-driven design

Domain

Translation

External world

Functional design

Pure

Impure

External world

Functional design

Abstract

Concrete

External world

"DDD-inspired" package structure

api

application

domain

infrastructure

HTTP controllers

Concrete effects, interpreters

Domain models and algebras

HTTP clients, config loading, ...

Functional domain modelling

Data     Behaviour = Domain

ADTs     Algebras   = Domain

+

+

Errors

• Expected errors

  • Part of the domain
  • Model with Either
• Unexpected errors
  • Network errors, cosmic rays, ...
  • Handle with Try, IO, Task, ...
• In general you need to handle both
  • EitherT can be handy

Enough talk!

Let's write some code

Summary

✓ Local reasoning

✓ Abstraction

✓ Composition