Returning EventSourceId from a command

Return EventSourceId from Handle() when the command decides which event source to append to at runtime. This is the pattern to use when the command does not already carry the final identity as part of its input.

Return EventSourceId together with the event

When Handle() returns a tuple that contains both an event and an EventSourceId, Chronicle uses the returned id for the automatic append.

using Cratis.Arc.Commands;
using Cratis.Chronicle.Events;

[Command]
public record RegisterCustomer(string Email, string DisplayName)
{
    public (EventSourceId, CustomerRegistered) Handle()
    {
        var customerId = EventSourceId.New();
        return (customerId, new CustomerRegistered(Email, DisplayName));
    }
}

[EventType]
public record CustomerRegistered(string Email, string DisplayName);

The tuple order does not matter. Chronicle looks for the EventSourceId value anywhere in the tuple.

using Cratis.Arc.Commands;
using Cratis.Chronicle.Events;

[Command]
public record RegisterCustomer(string Email, string DisplayName)
{
    public (CustomerRegistered, EventSourceId) Handle()
    {
        var customerId = EventSourceId.New();
        return (new CustomerRegistered(Email, DisplayName), customerId);
    }
}

[EventType]
public record CustomerRegistered(string Email, string DisplayName);

Use typed ids

If your solution uses a type that derives from EventSourceId, you can return that type in the tuple as well.

using Cratis.Arc.Commands;
using Cratis.Chronicle.Events;

public record CustomerId(Guid Value) : EventSourceId<Guid>(Value);

[Command]
public record RegisterCustomer(string Email)
{
    public (CustomerRegistered, CustomerId) Handle()
    {
        var customerId = new CustomerId(Guid.NewGuid());
        return (new CustomerRegistered(Email), customerId);
    }
}

[EventType]
public record CustomerRegistered(string Email);

Important behavior

• Chronicle treats the returned EventSourceId as append metadata and as the command response value.
• If the command record already had an event source id, the returned tuple value wins.
• If you need to target several different event sources from one command, use Events and return EventForEventSourceId values instead.