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Douglas ReithOSDC 2015 Hobart

CQRS / DDD

About Me

I use random presentation transitions

< 4 years old = I lived with penguins

About Me

Tech guy at Engagement Innovation

Glowfeed.com

What's This About?

Software development and...

A way of thinking

Basic Truths - Language

We're wired for language

We play 'language games'

Basic Truths – Software Development

Change is constant

Especially at the start!

'Agile' Methodology

Codebase will GROW

Basic Truths – Software Development

BIG BALL OF MUD

What To Do?

It's not a new problem!

GNU/Linux: lots of binaries and authors

What do they do?

Interfaces/contracts

Isolated behaviour

High internal cohesion, low external coupling

Building an application...

it's still hard: time and 'resources'

HELP! - CQRS / DDD

Domain Driven Design (DDD)

'Domain-driven design (DDD) is an approach to developing software for complex needs by deeply connecting the implementation to an evolving model of the core business concepts.'

http://dddcommunity.org/learning-ddd/what_is_ddd/

'Domain-driven design (DDD) is an approach to developing software for complex needs by deeply connecting the implementation to an evolving model of the core business concepts.'

Domain Driven Design (DDD)

Ubiquitous language – agreed terms that are well-meaning and consistent within a bounded

context

Develop domain models with a reference to the bounded context

Domain Driven Design (DDD)

Domain models contain:

Value Objects (VOs)

Entities

Aggregate Roots (ARs)

- that emit Events

Command and Query Responsibility Segregation (CQRS)

Split your system -

Writing via Commands

Reading via Queries

CQRS / DDD – The Messaging Marriage

Commands distributed via a Bus

Domain Events distributed via a Bus

Commands and Domain Events are Messages

The Example

How does all this help?

The Example

Hi Doug, we need to build a cinema booking application. Our film-goers can book a seat through our application and before the film starts we'll send them an SMS or app notification telling them to get to their seats. Let's send them a notification if they change their password too, that'd be nice :)

The Example

package cinema­app; 

class User {

     * register(email, password);

     * authenticate(email, password);

     * updatePassword(password);

     * Notification[] notifications;

     * addNotification(notification);

     * Booking[] bookings;

     * addBooking(booking);

}

More requirements please!

Hi Doug, we want the SMS to go if they haven't logged into the app recently.

Oh and our customers sometimes make group bookings, can you just add that please?

The Example

package cinema­app; 

class User {

     * register(string email, string password);

     * authenticate(string email, string password);

     * Notification[] notifications;

     * addNotification(Notification notification);

     * Booking[] bookings;

     * addNotification(Notification notification);

}

Hmmmm... add it h

ere?

Break It Down

Registration/Authentication

Notifications

Bookings

User

User

User

Now I am repeating myself

package users; 

class User {

     * register(email, password);

     * authenticate(email, password);

     * updatePassword(password);

}

package bookings;

class User {

     * Group group;

     * Booking[] bookings;

     * addBooking(booking);

}

package notifications; 

class User {

     * Notification[] notifications;

     * addNotification(notification);

}

Packages, packages, packages

'Package' can mean:● Business Domain● PHP Bundle/Library (definitely namespaced)● Bower update● Separate persistence store● Separate source control repository?● Indepedent – you can run unit tests without any other

packages

But, but, but

But how do we send the notification when they update their password?

How do we know when to send them a notification based on a booking?

Messages, buses and handlers

package users; 

class User {

     * updatePassword(password) {

          raiseEvent(

              new PasswordUpdated(userId)

          )

       }

}

package bookings;

class User {

     * addBooking(booking) {

          raiseEvent(

              new BookingCreated(userId)

          )

       }

}

package notifications; 

class PasswordChangeListener {

     * handle(PasswordUpdated event);

}

The framework bus will find the right handler/listeners for you.

Language!

package users; 

class AppUser {

     * register(email, password);

     * authenticate(email, password);

     * updatePassword(password);

}

package bookings;

class FilmGoer {

     * Group group;

     * Booking[] bookings;

     * addBooking(booking);

}

package notifications; 

class Recipient {

     * Notification[] notifications;

     * addNotification(notification);

}

Consequences:● pushing language

up

Downsides

● Write more code● Lot's of packages/bundles/repositories/databases● Context switching● Developer onboarding – new names for things

(AR's, events etc)● Few reference sites?

Benefits

● 'Phil, write unit tests for cinema-app' vs 'write them for notifications'

● Dependencies are very clear● Straight forward progressions to microservices when you

want to● 'Deletability'● 'Swapability' – not the right language or persistence store

– don't worry!

Resources

PHP frameworks:● https://github.com/PhpFriendsOfDdd/state-of-the-union

● https://github.com/szjani/predaddy

A talk I gave previously including a full request/response journey:http://www.slideshare.net/DouglasReith/php-melb-cqrsdddpredaddy

Thanks

Organisers, Sponsors, and you

János Szurovecz, Greg Young, Eric Evans, Vaughn

Vernon

Extras?

The Journey