Domain Discovery and Integration on Brian Faust

An Open Host Service Is Not a Published Language

Fri, 01 Jul 2022 00:00:00 +0000

Inventory exposes one endpoint to Storefront, Fulfilment, and Replenishment:

GET /availability/{sku}?location={location}

The route, authentication, media type, and JSON shape remain unchanged. One release quietly redefines available_quantity. Yesterday it meant physical stock at the location. Today it means stock which may still be promised after reservations and holds.

Every request still returns 200. Schema validation still sees an integer. Storefront, Fulfilment, and Replenishment now act on a different fact.

An API Cannot Tell You Who Has Influence

Wed, 01 Jun 2022 00:00:00 +0000

Two Order Management features call the same Payments endpoint. They use the same schema, service credential, and provider-managed release process.

The first feature needs a reversal_reason. Orders raises the request, Payments accepts it into quarterly planning, and a Payments owner commits to a compatibility window.

The second feature asks for another Orders-specific field. Payments declines it as outside the product roadmap. Orders decides that the generic refund status is adequate for its support view and accepts the cost of adapting when the provider contract changes.

An Anti-Corruption Layer Must Own the Translation

Sun, 01 May 2022 00:00:00 +0000

Payments adds a refund state named reversed. Order Management already has a default branch for provider failures, so the new event keeps moving:

anything except completed ──▶ provider-refund-failed

Nothing crashes. That is the problem.

The provider has reported a fact, but Orders has invented its consequence. A reversed refund might reopen the order, create a debt, require manual recovery, or leave the order cancelled while Finance resolves the money. The provider enum cannot choose among those policies.

A Context Map Must Say What the Arrow Means

Fri, 01 Apr 2022 00:00:00 +0000

The cancellation workshop ends with three boxes: Orders, Payments, and Fulfilment. Someone connects them in event order. The diagram looks plausible.

It is also impossible to read.

Does the arrow mean an HTTP request, a domain event, a dependency, ownership, or influence over another model? Does it describe the system that exists or the one the team hopes to build? Who translates “refund declined” into an order state? The line answers none of these questions.

Shared Kernel or Published Language: Choose Where Coordination Lives

Tue, 01 Mar 2022 00:00:00 +0000

Billing and fulfilment both use the word “order.” Sharing an Order class looks like the obvious way to keep them consistent.

Then billing adds credit notes, tax evidence, and settlement state. Fulfilment adds picking waves, split shipments, and packing exceptions. The shared class either accumulates both models or becomes a negotiation every time one team changes it.

Integration always creates coordination. A Shared Kernel and a Published Language place that coordination in different locations. The first shares a small piece of model and requires teams to change it together. The second shares a versioned language at the boundary and lets each side translate it into its own model.

EventStorming Finds Disagreement Before It Finds Services

Tue, 01 Feb 2022 00:00:00 +0000

Ask sales, support, finance, and engineering when an order becomes cancelled and the answers may sound compatible until they share a wall.

Sales says cancellation happens when the customer asks. Support says it happens after a case is approved. Finance says a paid order remains active until the refund clears. Engineering points to a cancelled_at column written before the payment provider responds.

That disagreement is not workshop noise. It is the model.