12 Things You Should Know About Event Sourcing

Are you aware that storing and updating current state means loosing important data?

Event sourcing is a way to solve this problem. It is the technique of storing state transitions rather than updating the current state itself.

Event sourcing has some more benefits:

  • Complete audit-proof log for free
  • Complete history of every state change ever
  • No more mapping objects to tables
  • Distribution support
  • CQRS (Command Query Responsibility Segregation) support
  • Natural fit for domain-driven design and functional programming
  • Be prepared for unanticipated use cases in the future (for free)

State transitions are an important part of our problem space and should be modelled within our domain — Greg Young

When I first encountered the concept of event sourcing and CQRS and looked at some sample applications, I had the impression that it must be extremely difficult to implement. But later I found out that event sourcing is easier than I first thought, especially when it is expressed with functional programming.

Here are 12 things about event sourcing that should help you to get started today.

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Purity in an impure language with the free monad – by example of a Tic-Tac-Toe backend with CQRS and event sourcing

This post is part of the F# Advent Calendar in English 2016. Please also checkout the other posts or the F# Advent Calendar 2016 eBook.

Pure code intermingled with impure code.

This is not a very good separation of concerns and has many other disadvantages.

Here is an example of how many programs look like:


In Haskell e.g. this would not be possible. But how should we deal with this in an impure programming language that does not enforce side effects to be made explicit, like F# e.g.?

There are a few approaches that will be presented in this post, one of which is the free monad pattern.

We will also examine a proof of concept implementation of a Tic-Tac-Toe backend following the command query responsibility segregation pattern (CQRS) together with event sourcing (ES).

See how you can implement a program in F# that is entirely pure and free from any effects and side effects!
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