Decode: layering, boundaries, and naming

Three of Rai’s reviewers pushed the same underlying idea from different angles: logic belongs to the layer that owns it, and decisions get made at the boundary. This lesson decodes all three, because they’re one principle.

The comment (Nisha Nambissan, PR #491, on a frontend retry):

“the pattern of retrying on the browser side looks like an antipattern. This should not have been a frontend concern in the first place. We have surfaced it backwards and made it a client concern, when it should have stayed in the backend and been addressed there.”

And (Nick Ferrell, PR #355, on a nullable date threaded downstream):

“Why allow null at all? Doesn’t it just use the current date anywhere? I would think about making this explicit upstream rather than buried downstream.”

And (Nick, same PR, on the query store):

“nit: the querystore should NOT be making this business decision.”

What they meant. A resilience concern (retrying a flaky dependency) is owned by the backend, which makes the call; pushing it to the browser inverts the layering and, worse, stacks client retries onto a cold-start timeout. A business default (what “no date” means) is owned by the boundary, the endpoint; defaulting it with ?? today deep in the query store buries a product decision in the persistence layer, where it’s invisible and untestable. Both are the same rule: push each decision to the layer that owns it, and make it explicit at the edge.

// What Nick flagged: a nullable date threaded down, defaulted deep inside.
public Task<FlightsView> Execute(DateOnly? day) { var d = day ?? Today; /* ... */ }

// The fix: the endpoint resolves the default; Execute takes a non-null DateOnly.
public Task<FlightsView> Execute(DateOnly day, CancellationToken ct) { /* ... */ }
// endpoint: useCase.Execute(day ?? DateOnly.FromDateTime(clock.UtcNow), ct)

And the boundary-validation flavour of the same idea, from Nick on PR #397:

“Data should be cleaned/valid before being sent/used in the domain.”

Rai’s fix: “the query store drops (and logs) any row without a unique_flight_key, so the aggregate receives a guaranteed-present, valid id.” Clean at the boundary so the domain never re-checks; its invariants are guaranteed by the fact that nothing invalid can reach it.