STRANGLER APPLICATION PATTERNS AND ANTIPATTERNS

Matteo Vaccari @xpmatteo

Get a whiteboard, put on the colored stickies, and try not to be a pain in the arse.

#agile #inanutshell

A company and its software

Legacy code == code that is difficult to change

The core domain of a profitable company is always complex

Refactor or rewrite?

What to do with legacy code?

Why do we fear the big rewrite?

http://www.bonkersworld.net/building-software/

Things going against the big rewrite

• Same people and same methods: they will get the same results

• The old software contains knowledge and details that we’re not aware of

• While we are busy rewriting the old features, competitors are busy implementing new ones

• Why is there always time to do things over, but there is never time to do things right?

A tale of two companies• There once were two competing companies, with

equal market share

• One company rewrote their product…

• … and many customers migrated to the other!

• The company who kept the old codebase now has twice the market share

A digression on software design

A non-obvious fact

This is more complex…

…than these

Doing it wrong…

What are the factors that make the split work well?

Simplicity is achieved when…

• Each box is understandable in isolation

• Each box does only one thing

• Each box represents a single concept

• Each box is cohesive

How to connect A & B

Simplicity is achieved when…

• Things are loosely coupled

• Or, rather…

• … they are coupled appropriately

Design is fractal!

And, similarly…

Or worse!

How to connect services?

Domain Events to the rescue!

A Domain Event

How to connect services?REST apis

RSS-like event queue

Messaging service

So how do we deal with legacy code?

Patterns for incremental improvement

1. Pay down your technical debt

2. Bubble context

3. Strangler application

Strangler application

Database

nuovo

App Nuova

Database

legacy

App Legacy

Batch Sync

Utenti

ApachePerl filter

Utenti migrati Utenti non migrati

Router

Old and new apps

Update mechanism

A Strangler application is composed of:

• A router

• The old application

• The new application(s)

• An update mechanism

Another example

Strangler app antipatterns

The router is inside the legacy app

The new app is aware of the old one

For best results, the new app and the old

should not be aware of each other!

• A router

• The old application

• The new application(s)

• An update mechanism

Conclusions?

twitter.com/xpmatteo thoughtworks.com

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