event-sourcing-workshop

A workshop for hands-on introduction to Event Sourcing.

What is Event Sourcing?

The fundamental idea of Event Sourcing is that changes to state (events) are persisted in sequence. As opposed to storing the current state. A common use case for Event Sourcing is to persist events from an aggregate.

Component Types

Input/Output	Events	Commands	External
Events	Translator (ACL)	Process Manager	Projector (Projection)
Commands	Aggregator (Aggregate)	Controller (Translator/ACL)	Actuator
External	Sensor	Actor	System

Prerequisites

• .NET
  • Visual Studio Code (or anything that can be used for dotnet development)
  • .NET Core SDK 3.1
  • C# for Visual Studio Code version 1.17.1 or later (If you are using VSCode and prefer C#)
  • F#: Ionide-fsharp (If you are using VSCode and prefer F#)
• Node.js
  • Visual Studio Code (or anything that can be used for Node development)
  • Node.js with npm

Concepts

In this workshop we will be working with the inventory item example. Assume this example is for a store and a user can be create, stock, sell, activate, and deactivate inventory items.

State

InventoryItem is a container that holds state.

inventoryItem:
    id: e85c1ec3-f077-4625-adf2-5a593a293988
    name: "Item 1"
    count: 212
    active: true

Events

InventoryEvent defines the events that can happen to an InventoryItem state.

- Created:
    InventoryId: e85c1ec3-f077-4625-adf2-5a593a293988
    Name: "Item 1"

- Stocked:
    Count: 123

- Sold:
    Count: 43

- Activated

- Deactivated

Commands

InventoryCommand defines the commands that a user can ask to execute on an InventoryItem state.

- Create:
    InventoryId: e85c1ec3-f077-4625-adf2-5a593a293988
    Name: "Item 1"

- Stock:
    Count: 123

- Sell:
    Count: 43

- Activate

- Deactivate

Event Sourcing Functions

The basic function to describe event sourcing is: (state, event) -> state'
This function is described as: given some state, when an event is applied, then a new state is given.
This function is often referred to as the Apply function.

When utilizing commands the execute function is: (state, command) -> event
This function is describe as: given some state, when a command is executed, then an event is given.
This function is where your business logic belongs and is often referred to as the Execute function.

Another function that is useful for event sourcing is an initialization function. () -> state
This function allows for a known initialization state before applying events.

Workshop

This workshop is implemented as using Quality Assurance and test writing to verify an Event Sourcing implementation.

Steps

• Clone this repo and navigate to the directory of your chosen framework/language.
• Build if using dotnet dotnet build; Install packages if using node npm i
• Run the tests dotnet test or npm test
  • A number of the tests will fail.
• Open the Domain.EventTests.fs, EventTests.cs, or event.test.js file under the tests directory.
• Fix the broken event tests by following the Given,When,Then pattern of the test names. You will need to use the Apply function.
• Open the Domain.CommandTests.fs, CommandTests.cs or command.test.js file under the tests directory.
• Fix the broken command tests by following the Given,When,Then pattern of the test names. You will need to use the Execute function.

Bonus Concepts

• Compensating Events
• Error Events