Introduction to Event Sourcing
Event Sourcing is a design pattern that has been gaining traction in the software development world, and for good reason. It allows you to capture changes to an application’s state as a sequence of events, rather than just storing the current state. This approach provides a complete audit trail of all changes, making it easier to debug, audit, and even revert to previous states if needed.
What is EventStoreDB?
EventStoreDB is an event-native database specifically designed to store, process, and deliver application state changes, known as events. It offers features such as an append-only event log, streams to organize and speed up event retrieval, subscriptions and connectors to deliver events to external systems, and projections to transform and filter events[3].
Setting Up EventStoreDB
Before diving into the Go implementation, let’s quickly set up EventStoreDB. You can run EventStoreDB locally or use a managed service like Event Store Cloud.
Running EventStoreDB Locally
To run EventStoreDB locally, you can use Docker:
docker run -d --name eventstoredb -p 2113:2113 -p 1113:1113 eventstore/eventstore:latest
This command starts an EventStoreDB instance on your local machine.
Connecting to EventStoreDB from Go
To connect to EventStoreDB from a Go application, you need to use the EventStoreDB client SDK. Here’s how you can create a client and connect to the database:
package main
import (
"context"
"fmt"
"log"
"github.com/eventstore/eventstore-client-go/v3/esdb"
)
func main() {
ctx := context.Background()
client, err := esdb.NewClient(esdb.ConnectionSettings{
Addresses: []string{"esdb://localhost:2113?tls=false"},
})
if err != nil {
log.Fatal(err)
}
defer client.Close()
fmt.Println("Connected to EventStoreDB")
}
Implementing Event Sourcing in Go
For a more robust implementation of Event Sourcing in Go, you can use a library like the Go Event Sourcing Library, which leverages Go’s generics for type-safe and reusable components.
Using the Go Event Sourcing Library
Here’s an example of how you might use this library to manage aggregates and apply events:
Aggregate Management
package main
import (
"context"
"fmt"
"log"
"github.com/thefabric-io/eventsourcing"
)
type ToDoItem struct {
ID string
Tasks []string
Completed bool
}
type ToDoItemCreated struct {
ID string
}
type TaskAdded struct {
Task string
}
type ToDoItemCompleted struct{}
func (t *ToDoItem) Apply(event eventsourcing.Event) error {
switch e := event.(type) {
case ToDoItemCreated:
t.ID = e.ID
case TaskAdded:
t.Tasks = append(t.Tasks, e.Task)
case ToDoItemCompleted:
t.Completed = true
default:
return fmt.Errorf("unknown event type: %T", event)
}
return nil
}
func main() {
ctx := context.Background()
// Initialize the event store
eventStore, err := eventsourcing.NewPostgreSQLEventStore(ctx, "your_db_connection_string", "your_schema_name")
if err != nil {
log.Fatal(err)
}
// Create a new to-do item
toDoItem := &ToDoItem{}
events := []eventsourcing.Event{
ToDoItemCreated{ID: "123"},
TaskAdded{Task: "Buy milk"},
TaskAdded{Task: "Walk the dog"},
ToDoItemCompleted{},
}
// Apply the events to the aggregate
for _, event := range events {
err := toDoItem.Apply(event)
if err != nil {
log.Fatal(err)
}
}
// Save the events to the event store
err = eventStore.Save(ctx, toDoItem.ID, events)
if err != nil {
log.Fatal(err)
}
fmt.Println("To-Do Item:", toDoItem)
}
Sequence Diagram for Event Sourcing
Here is a sequence diagram illustrating the process of creating and applying events to an aggregate using the Go Event Sourcing Library:
Using EventStoreDB with the Go Event Sourcing Library
To integrate EventStoreDB with the Go Event Sourcing Library, you need to implement the event store interface provided by the library using EventStoreDB.
Example Implementation
Here’s an example of how you might implement the event store interface using EventStoreDB:
package main
import (
"context"
"fmt"
"log"
"github.com/eventstore/eventstore-client-go/v3/esdb"
"github.com/thefabric-io/eventsourcing"
)
type eventStoreDB struct {
client *esdb.Client
}
func (e *eventStoreDB) Save(ctx context.Context, aggregateID string, events []eventsourcing.Event) error {
streamName := fmt.Sprintf("aggregate-%s", aggregateID)
var eventDatas []esdb.EventData
for _, event := range events {
eventData, err := eventsourcing.ToEventData(event)
if err != nil {
return err
}
eventDatas = append(eventDatas, eventData)
}
_, err := e.client.AppendToStream(ctx, streamName, esdb.AppendToStreamOptions{}, eventDatas...)
return err
}
func main() {
ctx := context.Background()
client, err := esdb.NewClient(esdb.ConnectionSettings{
Addresses: []string{"esdb://localhost:2113?tls=false"},
})
if err != nil {
log.Fatal(err)
}
defer client.Close()
eventStore := &eventStoreDB{client: client}
// Use the event store as shown in the previous example
}
Conclusion
Implementing the Event Sourcing pattern in a Go application with EventStoreDB provides a robust and scalable way to manage application state. By leveraging the Go Event Sourcing Library and integrating it with EventStoreDB, you can ensure that your application’s state changes are captured and stored efficiently.
Remember, Event Sourcing is not just about storing events; it’s about creating a system that is transparent, auditable, and highly maintainable. With the right tools and a bit of creativity, you can build systems that are not only functional but also delightful to work with.
Final Thoughts
As you embark on this journey of implementing Event Sourcing, keep in mind that it’s a pattern that requires careful planning and execution. But the rewards are well worth the effort. So, go ahead, dive into the world of Event Sourcing, and watch your application’s state unfold like a beautifully written story.
And as always, happy coding