Event Sourcing: Eric Evans Interviews Greg Young

I follow Eric Evans (@ericevans0) and Greg Young (@gregyoung) on twitter.  I really respect their opinion and knowledge.

Eric tweeted about an interview he did with Greg over 5 years ago about Event Sourcing.  It’s great to hear Greg describe how Event Sourcing fits with DDD concepts.  This is video is one of the first Event Sourcing + DDD talks that I’m aware of.   I highly recommend checking  it out.

http://www.infoq.com/interviews/Architecture-Eric-Evans-Interviews-Greg-Young

If you are new to Event Sourcing, here is a snippet from Martin Fowler article.

Capture all changes to an application state as a sequence of events.

We can query an application’s state to find out the current state of the world, and this answers many questions. However there are times when we don’t just want to see where we are, we also want to know how we got there.

Event Sourcing ensures that all changes to application state are stored as a sequence of events. Not just can we query these events, we can also use the event log to reconstruct past states, and as a foundation to automatically adjust the state to cope with retroactive changes.

Domain Driven Design Quickly

I just realized that it has been almost 10 years since Eric Evans wrote Domain-Driven Design: Tackling Complexity in the Heart of Software.  Although it’s not new, it is still gaining awareness  and is still very relevant.  I’ve mentioned DDD in many of my posts so I figured I needed to provide some resources.

If you are new to Domain Driven Design and want a summary instead of jumping into Eric Evans book, then Domain Driven Design Quickly is for you.  Produced by InfoQ.com, this cliff notes book provide introductions to all the fundamentals of DDD.

DDD Quickly can be downloaded free from InfoQ.

Also, while browsing InfoQ, I found an older video of Eric Evans discussing what he has learned since he wrote the book.  It’s a tad old, but still provides great insight.

CQRS: Read Model

Keep it simple.  There isn’t much of a need to go out of your way adding extra layers of abstraction.  Any data access framework or ORM you might be using will already be creating a good enough level of abstraction.  With linq (IQueryable) being implemented in NHibernate, Entity Framework, or Linq to SQL, ORM du jour, you already have your a level of abstraction to your data source.

Need to query your read model from the client? Single page application maybe?  Ok… you need one more layer.  OData to the rescue.  WCF Data Services provides an incredibly easy implementation with Entity Framework.  ASP.NET Web API does have an OData implementation but has limitations.

Keep it simple:

  • Data Source
  • Data Access Layer (ORM)
  • OData Layer – only if needed if client is making the query
  • Client