0081. Event sourcing and CQRS are two extensions

0081. Event sourcing and CQRS are two extensions

Status

Accepted. Row: DOM-24. No row is open.

Date

2026-10-11

Deciders

operator (Saulo Vallory), 2026-10-11.

Context

Event sourcing keeps events as the source of truth and rebuilds state by folding them; CQRS splits writes from reads and keeps read models. The study (D14) asked whether either belongs in core. Core holds only what Mesh cannot run without (ADR-0001).

Decision

Event sourcing and CQRS are two extensions, @meshfw/event-sourcing and @meshfw/cqrs (DOM-24). Core provides domain events and the outbox (ADR-0079); the extensions build on them through declared points (ADR-0020).

Options considered

Both in core; both as extensions (chosen); event sourcing as an extension and CQRS as a pattern only; neither.

Trade-off analysis

Most applications need neither. As extensions they cost nothing to those who do not use them. The price is that the extensions must rely on seams core gives them, so core’s event design in M6 has to leave room.

Consequences

• The apply word: the study’s event-sourcing sketch folds events into state with apply, while ADR-0079 uses apply for a handler that updates a read-model row. The extension decides its own words; core’s apply is the read-model one. This is not yet confirmed and stays an open question.
• Not built. No row dates them. RDM-01 applies: not before the port unless the port needs them, and as many planned constructs as possible before 1.0.

Action items

• After M6: design both extensions against the outbox and the seams.