Appending events

When you start working with KurrentDB, your application streams are empty. The first meaningful operation is to add one or more events to the database using this API.

Tips

Check the Getting Started guide to learn how to configure and use the client SDK.

Append your first event

The simplest way to append an event to KurrentDB is to create an EventData object and call appendToStream method.

class OrderPlaced {
    private String orderId;
    private String customerId;
    private double totalAmount;
    private String status;

    public OrderPlaced(String orderId, String customerId, double totalAmount, String status) {
        this.orderId = orderId;
        this.customerId = customerId;
        this.totalAmount = totalAmount;
        this.status = status;
    }

    public String getOrderId() {
        return orderId;
    }

    public String getCustomerId() {
        return customerId;
    }

    public double getTotalAmount() {
        return totalAmount;
    }

    public String getStatus() {
        return status;
    }
}


EventData eventData = EventData
        .builderAsJson(
                UUID.randomUUID(),
                "OrderPlaced",
                new OrderPlaced("order-456", "customer-789", 249.99, "confirmed"))
        .build();

AppendToStreamOptions options = AppendToStreamOptions.get()
        .streamState(StreamState.noStream());

client.appendToStream("orders", options, eventData)
        .get();

appendToStream takes a collection or a single object that can be serialized in JSON or binary format, which allows you to save more than one event in a single batch.

Outside the example above, other options exist for dealing with different scenarios.

Tips

If you are new to Event Sourcing, please study the Handling concurrency section below.

Events appended to KurrentDB must be wrapped in an EventData object. This allows you to specify the event's content, the type of event, and whether it's in JSON format. In its simplest form, you need three arguments: eventId, eventType, and eventData.

eventId

This takes the format of a UUID and is used to uniquely identify the event you are trying to append. If two events with the same UUID are appended to the same stream in quick succession, KurrentDB will only append one of the events to the stream.

For example, the following code will only append a single event:

EventData eventData = EventData
        .builderAsJson(
                UUID.randomUUID(),
                "OrderPlaced",
                new OrderPlaced("order-456", "customer-789", 249.99, "confirmed"))
        .build();

AppendToStreamOptions options = AppendToStreamOptions.get()
        .streamState(StreamState.any());

client.appendToStream("orders", options, eventData)
        .get();

// attempt to append the same event again
client.appendToStream("orders", options, eventData)
        .get();

eventType

Each event should be supplied with an event type. This unique string is used to identify the type of event you are saving.

It is common to see the explicit event code type name used as the type as it makes serialising and de-serialising of the event easy. However, we recommend against this as it couples the storage to the type and will make it more difficult if you need to version the event at a later date.

eventData

Representation of your event data. It is recommended that you store your events as JSON objects. This allows you to take advantage of all of KurrentDB's functionality, such as projections. That said, you can save events using whatever format suits your workflow. Eventually, the data will be stored as encoded bytes.

userMetadata

Storing additional information alongside your event that is part of the event itself is standard practice. This can be correlation IDs, timestamps, access information, etc. KurrentDB allows you to store a separate byte array containing this information to keep it separate.

contentType

The content type indicates whether the event is stored as JSON or binary format. This is automatically set when using the builder methods like builderAsJson() or builderAsBinary().

Handling concurrency

When appending events to a stream, you can supply a stream state. Your client uses this to inform KurrentDB of the state or version you expect the stream to be in when appending an event. If the stream isn't in that state, an exception will be thrown.

For example, if you try to append the same record twice, expecting both times that the stream doesn't exist, you will get an exception on the second:

EventData eventDataOne = EventData
        .builderAsJson(
                UUID.randomUUID(),
                "OrderPlaced",
                new OrderPlaced("order-456", "customer-789", 249.99, "confirmed"))
        .build();

EventData eventDataTwo = EventData
        .builderAsJson(
                UUID.randomUUID(),
                "OrderPlaced",
                new OrderPlaced("order-457", "customer-789", 249.99, "confirmed"))
        .build();

AppendToStreamOptions options = AppendToStreamOptions.get()
        .streamState(StreamState.noStream());

client.appendToStream("no-stream-stream", options, eventDataOne)
        .get();

// attempt to append the same event again
client.appendToStream("no-stream-stream", options, eventDataTwo)
        .get();

There are several available expected revision options:

StreamState.any() - No concurrency check
StreamState.noStream() - Stream should not exist
StreamState.streamExists() - Stream should exist
StreamState.streamRevision(long revision) - Stream should be at specific revision

This check can be used to implement optimistic concurrency. When retrieving a stream from KurrentDB, note the current version number. When you save it back, you can determine if somebody else has modified the record in the meantime.

First, let's define the event classes for our ecommerce example:

public class PaymentProcessed {
    private String orderId;
    private String paymentId;
    private double amount;
    private String paymentMethod;

    public PaymentProcessed(String orderId, String paymentId, double amount, String paymentMethod) {
        this.orderId = orderId;
        this.paymentId = paymentId;
        this.amount = amount;
        this.paymentMethod = paymentMethod;
    }
    // getters omitted for brevity
}

public class OrderCancelled {
    private String orderId;
    private String reason;
    private String comment;

    public OrderCancelled(String orderId, String reason, String comment) {
        this.orderId = orderId;
        this.reason = reason;
        this.comment = comment;
    }
    // getters omitted for brevity
}

Now, here's how to implement optimistic concurrency control:

ReadStreamOptions readOptions = ReadStreamOptions.get()
        .forwards()
        .fromStart();

ReadResult result = client.readStream("order-12345", readOptions)
        .get();

// Get the current revision to use for optimistic concurrency
long currentRevision = result.getLastStreamPosition();

// Two concurrent operations trying to update the same order
EventData paymentProcessedEvent = EventData
        .builderAsJson(
                UUID.randomUUID(),
                "PaymentProcessed",
                new PaymentProcessed("order-12345", "payment-789", 149.99, "VISA"))
        .build();

EventData orderCancelledEvent = EventData
        .builderAsJson(
                UUID.randomUUID(),
                "OrderCancelled",
                new OrderCancelled("order-12345", "customer-request", "Customer changed mind"))
        .build();

// Process payment (succeeds)
AppendToStreamOptions appendOptions = AppendToStreamOptions.get()
        .streamState(currentRevision);

WriteResult paymentResult = client.appendToStream("order-12345", appendOptions, paymentProcessedEvent)
        .get();

// Cancel order (fails due to concurrency conflict)
AppendToStreamOptions cancelOptions = AppendToStreamOptions.get()
        .streamState(currentRevision);

client.appendToStream("order-12345", cancelOptions, orderCancelledEvent)
        .get();

User credentials

You can provide user credentials to append the data as follows. This will override the default credentials set on the connection.

UserCredentials credentials = new UserCredentials("admin", "changeit");

AppendToStreamOptions options = AppendToStreamOptions.get()
        .authenticated(credentials);

client.appendToStream("some-stream", options, eventData)
        .get();