Event Sourcing

Raza Z Sayed

Lead Developer @ Idyllic Software

https://github.com/razasayed

https://twitter.com/razasayed

Pune Microservices Meetup

Problem 1: CRUD does not work always...

Source : http://slides.com/stefankutko/nodejs-microservices-event-sourcing-cqrs#/24

Solution

Dont save the current state
Save the "events" that lead to the current state

Problem 2: Monitoring Changes in a real-time app

In a music application for example the push notification service needs to know when an album has received a comment to notify the artist
Involves traditional approaches like 'polling'.

Solution: Event Sourcing

Dont store current state !
Store every data transition as an 'event' that occured separately. (solves the CRUD problem by being able to replay the events to determine how current state was reached)
Notify interested parties of events by publish-subscribe mechanism using for example a message-bus. (solves the problem with monitoring for changes in a real-time application)

Event Sourcing is 'not' a new idea !

Examples of event sourcing in everyday life...

Bank Account Statement
Patient Medical Records
etc..

How to 'efficiently' query the events ?

CQRS (Command and Query Responsibility Segregation). This makes event sourcing practical !
'Projections' is a technique used to implement CQRS and it works by converting raw json serialized event data into a form suitable for querying by storing it in an 'appropriate form'.
Projections provide the beneficial side effect of 'Polyglot Data' !

Simple Example of Event Sourcing Architecture

CreateAlbum (Command)

CreateComment (Command)

Event Store e.g. EventStore

CreatedAlbum Event

{"name": "The Beatles"}

CreatedComment Event

{"text": "Its amazing !"}

Message Bus e.g. RabbitMQ

Albums microservice

Comments microservice

Store event and publish to a message bus using AMQP for example

Subscribe to Album events

Subscribe to Comment events

Albums Database

Comments Database

Using Projection

Using Projection

Notify Artist

Query

Query

Event Stores/Event Sourcing Libraries

For Ruby On Rails developers (Theres a gem for that !):

rails_event_store (Ruby port of Greg Youngs EventStore)
event_sourced_record (Event Sourcing library)
rails-disco (Event Sourcing library)
etc ! (http://fhwang.net/g/Event-Sourcing-Libraries-in-Ruby-A-Guide)

For .Net, Javascript developers

EventStore (https://geteventstore.com /)

For Java developers

EventStore.JVM (https://github.com/EventStore/EventStore.JVM)

Questions

Thank You !

We are hiring awesome Ruby, Node, iOS and Android developers !

Get in touch at : commons@idyllic-software.com