Event Driven Systems
EdPug - July 2018
$(whoami)
Scott Pringle
Lead Software Developer & Technical Team Lead
@ People's Postcode Lottery
Former DDDEdinburgh Organiser
why?
What I'll cover
- What Is An Event?
- Advantages & Disadvantages of events
- Thinking In Events
- Event Driven Architectures
- Event Sourcing
- A Practical Example
What Is An Event?
An event is an action recognized by software, that may be handled by the software
https://en.wikipedia.org/wiki/Event_(computing)#Description
An Event is the encapsulation of a recorded action within an application
Me, probably plagiarising someone...
Event structure
<?php
namespace Acme\Demo\Event;
use Acme\Demo\Entity\User;
use Acme\Demo\Entity\Transaction;
class UserRefundRequestedEvent
{
/**
* @var User
*/
private $user;
/**
* @var Transaction
*/
private $transaction;
public function __construct(User $user, Transaction $transaction)
{
$this->user = $user;
$this->transaction = $transaction;
}
public function getUser(): User
{
return $this->user;
}
public function getTransaction(): Transaction
{
return $this->transaction;
}
}{
"timestamp": "2018-07-18T19:00:00Z",
"payload": {
"user": {
"forename": "Scott",
"surname": "Pringle",
"id": 1
},
"transaction": {
"timestamp": "2018-06-01 11:55:00",
"totalCost": "1099",
"items": [],
"shipping": "99"
}
},
"type": "UserRefundRequested",
"version": 1,
"identifier": "7683ba44-bd0a-49ac-82b7-ce41b38d4a2a"
}Cloud Events
http://cloudevents.io
{
"eventTime": "2018-07-18T19:00:00Z",
"data": {
"user": {
"forename": "Scott",
"surname": "Pringle",
"id": 1
},
"transaction": {
"timestamp": "2018-06-01 11:55:00",
"totalCost": "1099",
"items": [],
"shipping": "99"
}
},
"eventType": "UserRefundRequested",
"eventTypeVersion": 1,
"cloudEventsVersion": "0.1",
"source": "ProducerUri/Component",
"eventID": "7683ba44-bd0a-49ac-82b7-ce41b38d4a2a",
"schemaURL": "OptionalUrlForData", # Soap did something good...
"contentType": "application/json",
"extensions": {
"key1": "value1",
}
}Advantages of using events
Decoupling
Additional flexibility
Scalability
Disadvantages of Event Driven Systems
Additional "Plumbing"
no centralised rules for system behaviour
Complexity
Thinking In Events
As Developers we...
- Think in Commands
- Want to know the result of a request
thinking in events is hard!
Mindset change from "will" to "has"
Think In Reactions, Not Actions
Treat events As first class citizens
MAP TO THINGS THAT REALLY HAPPEN IN YOUR ORGANISATION
Domain Event
Command
Actor
Aggregate
Event Storming
Event Driven Architecture
4 Principal Layers
- Event Generator
- Event Channel
- Event Processing
- Downstream Activity
Event Generator
- Senses A Change
- Represents The Change As An Event
Event Channel
- Middleware from Event Generator
to Event Processor - TCP/IP connection / Input file
- Stores events into queues for Processors
Event Processing
- Where the magic happens
- 0 or more reactions to an event
- Not necessarily under your control
Downstream Activity
Consequence of Event Processing, e.g.
- Order confirmed emails being sent
- Stock count being updated
Events & Messaging
Event handling becomes asynchronous
Events are meant to be immutable
Tell other parts of your application to do something
other applications may react
Event Sourcing
Events used to Build state
State becomes
re-playable
Create alternative state with new "faked" events
Complete application history
You don't think of an audit log until you're audited
Practical example Of using an event
REFUNDING A PAYMENT
User Requests Refund
$$$
Side effects
- Review required
- Approver sent an email
Email sent to reviewer
refund approved
refund sent
Recap
Events allow for a decoupled platform
thinking in events exposes side effects
using events opens many architectural doors
events don't solve all problems...just most
Questions?
Thank you
Twitter: @Luciam91
Event Driven Systems - EdPug
By Scott Pringle
