CHR0026: [Key] or [Subject] on an EventSourceId<T> is redundant

Rule Description

A property or record parameter whose type derives from EventSourceId<T> already is both the key (stream identity) and the compliance subject. [Key] and [Subject] exist for non-EventSourceId<T> values; on an EventSourceId<T> they are at best redundant and at worst mask an ambiguous multi-identity design that should be split into distinct types rather than annotated around.

Remove the attribute.

Severity

Warning

Example

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Keys;

public record Chr0026AccountId(Guid Value) : EventSourceId<Guid>(Value);

public record Chr0026Account(
    // Warning CHR0026: Chr0026AccountId derives from EventSourceId<T>, so it already is the
    // key/stream identity and compliance subject — the [Key] attribute is redundant.
    [Key] Chr0026AccountId Id,
    string Name);

Why This Rule Exists

EventSourceId<T> is the strongly-typed event-source identity. Being that identity, it is automatically the key Chronicle resolves the stream by, and it is automatically the compliance subject a value’s PII is encrypted under. [Key] and [Subject] exist precisely to designate those roles on values that would not otherwise carry them — a plain Guid, a string, another concept. On an EventSourceId<T> there is nothing to designate; the type already says it.

At best the attribute is noise. At worst it is a signal that the design is trying to carry more than one identity on a single type — for example annotating a second EventSourceId<T>-derived property as the [Key] to disambiguate. That ambiguity should be resolved by splitting the identities into distinct types, not papered over with an attribute. Removing the attribute keeps the identity’s meaning where it belongs: in the type.

Related Rules

• Event source — how EventSourceId<T> identifies a stream.
• Subject — how the compliance subject is resolved.