Introduction to Event Sourcing
Event Sourcing is a design pattern that captures the history of an application’s state as a sequence of events. Instead of storing just the current state, you store every state change as an immutable event. This approach provides a robust mechanism for auditing, debugging, and even recovering from errors. In this article, we’ll dive into implementing Event Sourcing in Go, with practical examples and step-by-step instructions.
Why Event Sourcing?
Before we dive into the implementation, let’s quickly discuss why Event Sourcing is valuable:
- Audit Trail: Every change to the system is recorded, providing a complete history.
- Debugging: Easier to debug issues by replaying the sequence of events.
- Recovery: System state can be rebuilt from the event history.
- Scalability: Decouples the write model from the read model, allowing for better scalability.
Key Components of Event Sourcing
1. Events
Events are the core of Event Sourcing. They represent state changes and are stored in an event store.
type Event struct {
EventType string
Data interface{}
}
2. Aggregates
Aggregates are the entities that hold the state and the history of events. They are responsible for applying events to their state.
type Account struct {
ID string
Balance int
Events []Event
}
func (a *Account) Deposit(amount int) {
a.Balance += amount
event := Event{EventType: "Deposit", Data: amount}
a.Events = append(a.Events, event)
}
func (a *Account) Withdraw(amount int) {
if amount <= a.Balance {
a.Balance -= amount
event := Event{EventType: "Withdraw", Data: amount}
a.Events = append(a.Events, event)
} else {
fmt.Println("Insufficient funds")
}
}
func (a *Account) Rebuild(events []Event) {
for _, evt := range events {
switch evt.EventType {
case "Deposit":
a.Balance += evt.Data.(int)
case "Withdraw":
a.Balance -= evt.Data.(int)
}
}
}
3. Event Store
The event store is responsible for saving and retrieving events. It can be implemented using various storage systems like SQL, BoltDB, or even an in-memory store.
type EventStore interface {
Save(events []Event) error
Get(id string, aggregateType string, afterVersion int) ([]Event, error)
}
Implementing Event Sourcing in Go
Step 1: Define Your Events and Aggregates
First, define your events and aggregates. For example, let’s consider a simple banking system where we have Deposit and Withdraw events.
type DepositEvent struct {
Amount int
}
type WithdrawEvent struct {
Amount int
}
type Account struct {
ID string
Balance int
Events []Event
}
func (a *Account) ApplyEvent(event Event) {
switch event.EventType {
case "Deposit":
deposit := event.Data.(DepositEvent)
a.Balance += deposit.Amount
case "Withdraw":
withdraw := event.Data.(WithdrawEvent)
if withdraw.Amount <= a.Balance {
a.Balance -= withdraw.Amount
} else {
fmt.Println("Insufficient funds")
}
}
}
Step 2: Implement the Event Store
Next, implement the event store. Here’s a simple example using an in-memory store.
type InMemoryEventStore struct {
events map[string][]Event
}
func NewInMemoryEventStore() *InMemoryEventStore {
return &InMemoryEventStore{events: make(map[string][]Event)}
}
func (es *InMemoryEventStore) Save(events []Event) error {
for _, event := range events {
es.events[event.ID] = append(es.events[event.ID], event)
}
return nil
}
func (es *InMemoryEventStore) Get(id string, aggregateType string, afterVersion int) ([]Event, error) {
events, ok := es.events[id]
if !ok {
return nil, errors.New("aggregate not found")
}
return events, nil
}
Step 3: Rebuild the Aggregate State
To rebuild the aggregate state, you need to replay the events from the event store.
func RebuildAggregate(es EventStore, id string) (*Account, error) {
events, err := es.Get(id, "Account", 0)
if err != nil {
return nil, err
}
account := &Account{ID: id}
for _, event := range events {
account.ApplyEvent(event)
}
return account, nil
}
Example Workflow
Here’s a sequence diagram illustrating the workflow of depositing money into an account using Event Sourcing:
Testing and Acceptance Criteria
Testing is crucial in Event Sourcing. You should test both the command side (how events are generated and applied) and the query side (how the state is rebuilt and queried).
Acceptance Tests
Acceptance tests ensure that the system behaves as expected from a user’s perspective. Here’s an example of how you might write acceptance tests:
func TestDeposit(t *testing.T) {
es := NewInMemoryEventStore()
account := &Account{ID: "123"}
account.Deposit(100)
es.Save(account.Events)
rebuiltAccount, err := RebuildAggregate(es, "123")
if err != nil {
t.Fatal(err)
}
if rebuiltAccount.Balance != 100 {
t.Errorf("Expected balance 100, got %d", rebuiltAccount.Balance)
}
}
Conclusion
Event Sourcing is a powerful pattern that offers many benefits, including a complete audit trail, easier debugging, and better scalability. By following the steps outlined in this article, you can implement Event Sourcing in your Go applications effectively. Remember, the key is to store every state change as an immutable event and to rebuild the state by replaying these events.
Final Thoughts
Event Sourcing might seem complex at first, but once you grasp the concept, it’s like having a superpower in your development toolkit. It’s not just about storing data; it’s about telling the story of how your application’s state evolved over time. So, the next time you’re designing a system, consider Event Sourcing—it might just be the missing piece to your architectural puzzle. Happy coding