Microservices
Event Sourcing and CQRS
Chandan Jhunjhunwal
Tech Architect @
https://github.com/indyarocks
@ChandanJ
What is Event-Store?
- It is a very old concept. Many domains in real world works like that. For example: Ledger, Banking, Databases
- Only Write and Read. No Update/Delete like CRUD.
- Choreography vs Orchestration
Why Event-Store?
- We can go to any state of an object/aggregate for any moment in time by replaying events.
- No coupling between current state in the domain and in storage.
- Read side optimised for fast queries and write side highly optimised for writes .
- Fool proof Audit Log.
- No impedance mismatch between object oriented and relational world like RDBMS.
- Easier to undo things.
When to Event-Source?
- Accountability/debugability is critical
- you're building rich object-oriented domain models (e.g. DDD)
- you have a lot of behaviour that isn't CRUD
- you need version control/undo for data (e. G. Wikis, Google Docs)
- your business derives value or competitive advantage from event data
- your domain is inherently event driven (e. G. basketball game tracking)
- you're building a scaleable system based on the CQRS pattern
What is CQRS?
Every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a question should not change the answer.
Bertrand Meyer
- CQRS stands for Command and Query Responsibility Segregation
- CQRS is architectural style.
IMAGE FROM MARTIN FOWLER BLOG
Standard Application DB Design
IMAGE FROM MARTIN FOWLER BLOG
CQRS
Event-Sourcing and CQRS
You can use CQRS without Event Sourcing, but with Event Sourcing you must use CQRS.
Greg Young
How?
Technology Set
- Event Store
- Open Source database for Event Sourcing (geteventstore.com ) - rails_event_store gem
Domain Events
IMAGE FROM MARTIN FOWLER BLOG
IMAGE FROM MARTIN FOWLER BLOG
Normal Flow
IMAGE FROM MARTIN FOWLER BLOG
With Event-Sourcing
IMAGE FROM MARTIN FOWLER BLOG
Data with Event-Sourcing
IMAGE FROM MARTIN FOWLER BLOG
Domain Event Design
- Greg Young's Talk: https://goo.gl/WCvPAf
- Open-source, functional database: https://geteventstore.com/
- Rails Event Store: http://goo.gl/uwyu1C
- Rails Event Store Gem: https://github.com/arkency/rails_event_store
- Martin Fowler's Blog:
- CQRS (http://martinfowler.com/bliki/CQRS.html)
- Event Sourcing (http://www.martinfowler.com/eaaDev/EventSourcing.html)
- Domain Events (http://www.martinfowler.com/eaaDev/DomainEvent.html)
REFERENCES
THANK YOU!
Event-Sourcing and CQRS Introduction
By Chandan Jhunjhunwal
Event-Sourcing and CQRS Introduction
A detailed comparison between Monolith architecture and Microservices. The factors for choosing one architecture over other. The key concepts in Microservice architecture.
