Jens Rantil's blog

Posted Sön 19 Maj 2013

CQRS - Time to Rewind

For the last year I've been thinking a lot about CQRS [1], Event Sourcing and distributed architecture using ZeroMQ. I first stumbled across the concept of CQRS through the mailing list for a nerd meetup in Helsingborg, Sweden. This led me to a lot of CQRS reading and videos, and with that the Event Sourcing data model.

To try out CQRS and Event Sourcing in practise, I started writing an open source event store about a year ago. I named it Rewind. This also opened up for me to try some concepts that I had never tried before [2].

[2]See below.

CQRS and Event Sourcing

But first, let's Rewind ;). Rewind's README does a fairly good job at an elevator speech for CQRS:

Have you ever been nervous of all those DBMSs schema changes when you are deploying your applications? They are gonna take too long, or break backward compatibility? Have you ever thought "Crap, I wish I had stored that information since earlier"? Have you ever felt your writing patterns and your reading patterns differ a lot, making things harder to scale? Issues like these can be solved using CQRS and event sourcing.

CQRS (Command-Query Response Segregation) is an architectural pattern that aims to solve these issues by splitting up your architectural system into two parts:

  • A write side that takes care of validating input and optimizes for fast writes. The write side takes commands and outputs corresponding events if the command validates correctly.
  • A read side that listens to incoming events from the write side. The read side is optimized for fast reads and incrementally build up state that can be queried fast.

While not required, it is common to use messaging between the write and read sides. This means that the system will be in an inconsistent state from time to time. This is usually not an issue and came be overcome in various ways.

A couple of additional things to note about CQRS:

  • As I see it, it is the architectural equivalent of the CQS design pattern.
  • It is a pattern that decouples systems very well. This can have huge implications when it comes to testability.
  • It's also worth noting that there are some similarities between Domain Driven Design (DDD) and CQRS. Most vocabularies in DDD are used in CQRS; ubiquous language, aggregates, aggregrate root, value objects, bounded contexts etc.

The README then states about Event Sourcing:

A common pattern used together with CQRS is event sourcing. The concept can be summarized as using state changes as primary persistence, instead of the final state. The state changes are called events and they are generated by the write side and delivered to the read side.

The events are persisted in an event store that sits inbetween the read and write side of things. It takes care of three things:

  • persisting all events to disk.
  • being a hub/broker replicating all events from the write to the read side of things.
  • allowind fast querying of events so that different parts of the system can be synced back on track and new components can be brought back in play.

Rewind

Question was, how would a CQRS/event sourced system behave in production? Could it scale out? Could writes be partitioned? What about fault tolerance? I was tired of heavy database schema changes, I wanted a nouvaeu way of testing and question some of the common practices.

Enter Rewind; Rewind was my pet project for a major part of 2012. It was a Python implementation of an event store that supported multiple backends. It really gave me an opportunity to try everything I wanted.

The development of Rewind halted at the beginning of 2013. From thereon, I instead used the lessons I had learnt and rewrote Rewind's functionality in Go. The project was named GoRewind.

Within the next couple of blog posts I plan to write about what I've learnt from these two projects; design decisions, testability, ZeroMQ, developing in Go among other things.

Category: misc
Tags: cqrs distributed-architecture
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