Ressourcen entwerfen

Getting the verbs right (last lesson) is half of REST; the other half is modeling the resources those verbs act on. Good resource design makes an API feel obvious - a developer guesses the URL correctly on the first try. Bad design leaks your database schema, buries actions in query strings, and forces consumers to memorize idiosyncrasies. The craft is choosing the right nouns, shaping representations that hide your internals, and following the boring conventions for collections.

Resources are nouns; hierarchy shows relationships

A resource is a thing the API exposes - an account, a transaction, an invoice. URLs name these nouns, and nesting expresses containment:

/accounts                     collection of accounts
/accounts/42                  a single account
/accounts/42/transactions     the transactions belonging to account 42 (a sub-collection)
/accounts/42/transactions/9   one specific transaction of that account

Conventions that keep this predictable:

• Plural nouns for collections (/accounts, not /account) - consistency beats grammar debates.
• Nest to show relationship, but not too deep. /accounts/42/transactions is clear; nesting four levels (/banks/1/branches/2/accounts/42/transactions/9) is painful - once a resource has its own stable id, prefer /transactions/9 over deep nesting.
• No verbs (last lesson) - the action is the HTTP method.

The awkward case: actions that aren't CRUD

Not everything is create/read/update/delete. "Freeze this account," "transfer money," "retry this job" are operations, and forcing them into pure CRUD is unnatural. Two accepted approaches:

• Model the operation as a resource. A transfer is a thing: POST /transfers with a body describing it, returning a Transfer you can later GET. This is often the cleanest - the operation becomes a first-class, inspectable resource.
• A controller sub-resource for state transitions: POST /accounts/42/freeze. Pragmatic and readable for simple state changes; use it sparingly and consistently.

Prefer modeling operations as resources when they have their own data or lifecycle; the sub-resource verb is fine for simple, stateless transitions.

Collection conventions: paging, filtering, sorting

Any collection large enough to matter must never return everything at once (recall the batch/streaming lessons - unbounded results kill servers and clients). Use consistent query parameters:

GET /transactions?accountId=42&status=SETTLED     ← filtering
GET /transactions?sort=-createdAt                 ← sorting (- for descending)
GET /transactions?page=2&size=50                  ← pagination (or cursor-based)

• Filtering narrows a collection via query params - these are not new resources, just views of one.
• Sorting via a sort param; a leading - for descending is a common convention.
• Pagination is mandatory for large collections. Offset pagination (page/size) is simple but drifts if data changes between pages; cursor pagination (an opaque ?after=<cursor>) is stable and scales better for large or live datasets. Return metadata (total count or a next-page link) so clients can navigate.

Representations should not leak your database

The JSON you return is a representation you design deliberately - it is not your JPA entity serialized:

// ❌ leaking the entity: exposes internal column names, an internal FK, a nullable audit field
{ "acct_id": 42, "bal": 100.00, "usr_fk": 7, "internal_risk_flag": null, "db_version": 3 }

// ✅ a designed DTO: consumer-friendly names, only what clients should see, computed fields
{ "id": 42, "balance": "100.00", "currency": "USD", "status": "ACTIVE",
  "ownerId": 7, "createdAt": "2026-07-22T10:00:00Z" }

Serializing entities directly couples your public contract to your schema (rename a column, break every client), exposes fields consumers shouldn't see, and leaks internal concepts. Map to a purpose-built response DTO at the boundary - the same "map at the edge" discipline from the hexagonal module, now protecting your API contract.