Domain Events in Distributed Architecture

Michał Łukaszewski

/m_lukaszewski

How to pretend that our application is reliable and fast and still get paid

Basics

Reasons

Methods

Events

Dark places

Rules

Basics

Performance Budget0.1 sec – human response to changes

2 sec – impatience

3 sec – 30% leaves a page

10 sec - users forget what they want to do (a short-term memory feature)

GUI

Action

Domain A Domain B

Responder

Renderer

Reasons

Performance issues

Performance issues

Performance issues

Application integration

Premature optimization

Overengineering

https://twitter.com/ziobrando/status/741186560373186560

Methods

http://www.pc-freak.net/blog/vertical-horizontal-server-services-scaling-vertical-horizontal-hardware-scaling/

Bigger is better?

Optimization

3 rules of optimization

Extract bottleneck to a service

Don’t

Don’t yet

Measure before do anything

3 rules of optimization

voilà

Events

Calling remote services

Calling remote services

You know nothing about state of remote service

but you want to use it aspart of real-time processing

Async everything

Event is a fact from the past

Event cannot be ordered

UserCommandRegisteredAddBlogPost

BlogPostAddedSendNotification

Domain events

Domain event is something that happened that

domain experts care about.Wise anonymous

Domain events

And now what?

Confirmation of action has been sent

Dark places

What the heck can I answer to a user?

Receiving messages in a right order

Sending messages in a right order

Sending messages in a right order

Concurrency conflicts

https://speakerdeck.com/ulff/event-sourcing-in-php

Concurrency conflicts

• You have no idea where conflict will rise• But you have to handle it

Events sequence

When, how and how many times we should inform a user

about progress?

UserCommandRegisteredAddBlogPost

BlogPostAddedSendNotification

Complex event trigger Distributed Dependency Hell

Complex event trigger Distributed Dependency Hell

Who the heck sent this s..t?!

Who the heck sent this s..t?!

• How to confirm–A sender–If message was sent–Is data are valid

QoS

FIFO

QoS

• How to–Mark an event as important

*by type*–Mark an event as important

*because source*–Mark some events as important

*because business logic*

There is no „the right way”

Know your enemy.

Rules

#1

I don’t know• how many of you are listening• in what order you will finish your

job• what you will return (and when)

#2

Event is a fact, not a wish.

#3

Event (data) have to be small and serializable.

#4

Each event has its own source.

#5

Don't model an event for a specific handler

Thanks!

?

Credits• https://speakerdeck.com/mat3u/domain-events-czyli-jak-radzic-sobie-z-rzeczywistoscia• https://speakerdeck.com/ulff/event-sourcing-in-php• http://blog.best.net.pl/16-key-points-wydajnosc-w-budowaniu-aplikacji-internetowych/• https://www.linkedin.com/profile/portfolio?id=31205064&authType=name&authToken=YnLc&med

iaID=51178139&trk=landing_share• http://danielwhittaker.me/2014/09/29/handling-concurrency-issues-cqrs-event-sourced-system/