Editing conflict

Use this when the client loaded a document, someone else may have changed it, and a save must not overwrite that change.

Rule	A rename is rejected if the document changed after the editor loaded it
Mechanism	An explicit scope built from the revision the client sent, returned with EventsWithConcurrencyScopes
Alternatives	A read model parameter when the conflict only matters within one request
Not	concurrency: true alone: it reads the tail after Handle(), so it cannot carry the revision the user saw

using Cratis.Arc.Commands.ModelBound;
using Cratis.Chronicle.Events;
using Cratis.Chronicle.EventSequences;
using Cratis.Chronicle.EventSequences.Concurrency;

public record DocumentId(string Value) : EventSourceId(Value);

[EventType]
public record DocumentTitleChanged(string Title);

[Command]
public record RenameDocument(DocumentId Document, string Title, ulong ExpectedRevision)
{
    public EventsWithConcurrencyScopes Handle() => new(
        [new(Document, new DocumentTitleChanged(Title))],
        [new(Document, new ConcurrencyScope(
            new EventSequenceNumber(ExpectedRevision),
            EventSourceId: Document,
            EventTypes: [typeof(DocumentTitleChanged).GetEventType()]))]);
}

guard document: types=[DocumentTitleChanged], after <ExpectedRevision>

The scope fails the append if any DocumentTitleChanged for this document has a sequence number above ExpectedRevision. How the client learns the revision is part of your read model and query.

Under read protection

If the command also reads a model of the same document, the scope must cover what the read watches and must not have been captured after it. Otherwise the command is refused with ExplicitScopeDoesNotCoverReads. The scope above covers a read of DocumentTitleChanged events. A read model that folds more types than the scope names needs a wider scope. ARCCHR0036 reports it at build time.

Related

• Concurrency
• Chronicle: explicit scopes