AIP Protocol Buffers & API Design Skill

This skill summarizes Google API Improvement Proposals (AIPs) for designing and working with protocol buffers, RPCs, services, messages, fields, and naming for APIs that follow resource-oriented design.

Scope: General AIP guidance from the google.aip.dev codebase (AIPs in approved state). Domain-specific AIPs (Cloud, Firebase, Auth, etc.) may add further rules.

Bundled references

references/full-example.md: Read when implementing a new service or checking request/response shape for standard methods.
references/naming-and-methods.md: Read when checking naming conventions or RPC/method patterns for standard methods.

1. Protocol Buffers (Protos)

1.1 Package and file structure

Package: Each API must live in a single package that ends in a major version (e.g. google.library.v1). Directory layout must match the package (e.g. google/library/v1).
Syntax: Use proto3 only.
File names: Use snake_case. Avoid language keywords (e.g. not import.proto). Must not use the version as the filename (e.g. no v1.proto).
File layout order: Syntax → package → imports (alphabetical) → file options → services (standard methods before custom, grouped by resource) → resource messages (parent before child) → request/response messages (matching RPC order, request before response) → other messages → top-level enums.

1.2 API-specific vs common protos

API-specific protos belong in the versioned package (e.g. google.library.v1). Do not create shared "API-specific common" packages across versions; duplicate the proto per version if needed.
Cross-API references: Use resource names (strings), not the resource message type from another API.
Common components: APIs may import:
- google.api.* (not subpackages)
- google.longrunning.Operation
- google.protobuf.* (e.g. Timestamp, Duration, Struct, FieldMask, Empty)
- google.rpc.*
- google.type.* (e.g. Date, Money, LatLng, PostalAddress, TimeOfDay)
- google.iam.v1.* where relevant
Organization-specific common packages must end with .type (e.g. google.geo.type), be published in the googleapis repo, and follow AIP-213; do not put generic types there.

2. Services (interfaces)

Interface name: Use an intuitive noun in PascalCase (e.g. Library, Calendar, BlobStore). Avoid names that clash with common language/runtime concepts (e.g. File). If needed, add a suffix like Api or Service to disambiguate.
Methods: Prefer standard methods (Get, List, Create, Update, Delete) over custom methods. Group RPCs by resource; list standard methods before custom ones.

3. Resources and resource names

3.1 Resource-oriented design

Model the API as a resource hierarchy: resources (nouns) and collections; use a small set of standard methods plus custom methods when necessary.
Each resource must support Get. All non-singleton resources must support List.
Resource schema (message shape) for a given resource must be the same across all methods that take or return that resource.
Relationships must form a directed acyclic graph; each resource has at most one canonical parent. Use parent for collection scope and optional filter for other associations (AIP-124, AIP-160).

3.2 Resource names

Format: Path-like, without leading slash: publishers/123/books/les-miserables. Use / only as segment separator; no / inside a segment.
Segments: Alternate collection identifiers (plural, camelCase, e.g. publishers, books) and resource ID segments. Collection IDs must be unique within a single resource name (no people/xyz/people/abc).
Characters: Prefer DNS-safe (RFC 1123); avoid uppercase in IDs; avoid URL-encoding; if Unicode is needed, use NFC (AIP-210).
User-specified IDs: Document format; prefer lowercase letters, numbers, hyphen; first character letter, last letter or number; max 63 chars (RFC 1034 style). Duplicate name → ALREADY_EXISTS (or PERMISSION_DENIED if user cannot see the duplicate).
Full resource name (cross-API): schemeless URI with service name + relative name, e.g. //library.googleapis.com/publishers/123/books/les-miserables.

3.3 Resource types and annotations

Resource type: {ServiceName}/{Type} (e.g. library.googleapis.com/Book). Type must match the message name, singular, PascalCase, alphanumeric.
Annotation: Use google.api.resource with type, pattern, singular, plural. Pattern variables: snake_case, no _id suffix, unique within pattern. Pattern variables match singular resource type (e.g. {topic} for Topic).
Nested collections: Child collection segment may drop redundant prefix (e.g. users/{user}/events/{event} instead of userEvents in path); message/type name stays e.g. UserEvent; singular/plural not shortened.

3.4 Fields for resource names

On the resource message: First field should be string name with resource name; annotate with (google.api.field_behavior) = IDENTIFIER and google.api.resource.
In request messages (acting on one resource): Use string name for the resource; annotate with REQUIRED and google.api.resource_reference (type or child_type). Comment should document the pattern (e.g. Format: publishers/{publisher}/books/{book}).
In request messages (collection scope): Use string parent for the collection's parent resource; same annotations and pattern comment.
Referencing another resource: Prefer string with resource name; field name ≈ referenced type in snake_case (e.g. shelf); use google.api.resource_reference. Do not embed the other resource's message type (except internal/revisions cases in AIP-162). Use _name suffix only if needed to avoid ambiguity (e.g. crypto_key_name).
Reserved: Use name only for resource name; use parent only for parent collection. For other concepts use a different name or adjective (e.g. display_name).

4. RPCs (methods)

4.1 HTTP and transcoding

Every RPC must have an HTTP mapping via google.api.http (except bi-directional streaming, which should have a non-streaming alternative if possible).
Verbs: Use get, post, patch, or delete as prescribed; should not use put or custom.
URI: Use {field=path/*} for variables; use * for one segment (no /), ** only if needed for path segments. For Update, use nested field in path (e.g. {book.name=...}).
Body: No body for GET/DELETE. For Create/Update, body must point to the resource field; must not be nested, a URI param, or repeated. Prefer not using json_name except for compatibility.
Method signatures: Use google.api.method_signature as specified per method type below.

4.2 Standard methods

See references/naming-and-methods.md for the standard-methods table.

Get: Request has name (required, resource reference). URI single variable for resource; method_signature = "name". Response is the resource (fully populated unless partial response per AIP-157).
List: Request has parent (required when not top-level), page_size (int32), page_token (string); optional filter, order_by. Response has repeated resource field + next_page_token; optional total_size. Pagination is mandatory from the start (AIP-158). method_signature = "parent" (or "" for top-level).
Create: Request has parent, {resource}_id, and resource field (body). Resource name ignored on input. method_signature = "parent,book,book_id" (or without _id if optional). Management plane must allow user-specified ID; data plane should.
Update: Request has resource field (with name) + google.protobuf.FieldMask update_mask. URI uses {resource.name=...}. Support partial update (PATCH). Omitted mask = all populated fields. Support * for full replacement with documented caveats. State fields must not be writable in Update (use custom methods). Optional allow_missing for upsert when using client-assigned names.
Delete: Request has name. Optional force for cascading delete; optional etag for conditional delete; optional allow_missing for no-op when missing. Child resources present without force → FAILED_PRECONDITION.

4.3 Custom methods

Use only when standard methods do not fit; prefer standard methods.
Name: Verb + noun, UpperCamelCase (e.g. ArchiveBook). Must not include prepositions or "Async"; may use LongRunning suffix. Should not reuse standard verbs (Get, List, Create, Update, Delete).
HTTP: GET for read-only; POST for side effects. URI must include :customVerb in camelCase (e.g. :archive).
Body: Prefer body: "*".
Request/response: {RpcName}Request and {RpcName}Response (or return the resource when operating on one resource).
Resource-based: Single resource → name in path. Collection-based → parent in path + literal collection segment. Stateless → scope field (e.g. project) in path; verb after : (e.g. :translateText).

4.4 Long-running operations (LRO)

For operations that take significant time, return google.longrunning.Operation with google.longrunning.operation_info (response_type, metadata_type). Implement the standard Operations service; do not define a custom LRO interface.

5. Messages

Names: Short, PascalCase; no prepositions (e.g. "With", "For"). Omit redundant adjectives if there is no contrasting type.
Request/response: Request = RPC name + Request; response = resource for Get/Create/Update (and some custom), or ListXResponse for List, or XResponse for custom. Response usually holds the full resource unless partial response (AIP-157).
Conflict: A message should not have a field with the same name as the message (after case normalization).

6. Fields

6.1 Field names (AIP-140)

Case: lower_snake_case in proto. No leading/trailing/adjacent underscores; no word starting with a number.
Language: Correct American English; same concept = same name across APIs; avoid overloaded or overly generic names (e.g. "instance", "info", "service" only with clear context).
Repeated: Use plural (books); non-repeated use singular (book). Resource names in field names use singular.
Prepositions: Avoid ("for", "with", "at", "by"); e.g. error_reason not reason_for_error. "Per" is allowed in units (AIP-141).
Abbreviations: Use common ones: config, id, info, spec, stats; for units use common abbreviations (e.g. _km, _px).
Adjective + noun: Adjective before noun: collected_items not items_collected.
Verbs: Field names must not be verbs or intent/action; use nouns (current or desired value).
Booleans: Omit is_ prefix: disabled not is_disabled (exception: reserved words, e.g. is_new).
URIs: Prefer uri; if only URLs, use url. Optional prefix (e.g. image_url).
Display: Human-readable name → display_name (no uniqueness). Formal/official name → title (no uniqueness).
Reserved words: Avoid names that conflict with common language keywords.

6.2 Field behavior (AIP-203)

Must apply google.api.field_behavior on every field of messages used in requests. Use at least one of: REQUIRED, OPTIONAL, OUTPUT_ONLY. Never use FIELD_BEHAVIOR_UNSPECIFIED.
IDENTIFIER: Only on the resource's name field (identifies resource; not input on Create; immutable on Update).
REQUIRED / OPTIONAL: For input fields; required = must be present and non-empty where applicable.
OUTPUT_ONLY: Response-only; server clears on input; ignore in update_mask.
INPUT_ONLY: Request-only; not in response (rare; e.g. some TTL patterns).
IMMUTABLE: Cannot be changed after creation; ignore if unchanged on update, error if change requested.
UNORDERED_LIST: Repeated field order not guaranteed.

6.3 Standard and special fields

Timestamps: Use google.protobuf.Timestamp; names end in _time (e.g. create_time, update_time). Use imperative form: publish_time not published_time (AIP-142).
Durations: Use google.protobuf.Duration; e.g. flight_duration (AIP-142).
State: Use enum State (or XxxState) nested in the resource; values like ACTIVE, SUCCEEDED, FAILED, CREATING, DELETING. Field should be output-only; state changes via custom methods, not Update (AIP-216).
Enums: Values in UPPER_SNAKE_CASE; first value {Enum}_UNSPECIFIED (or UNKNOWN if zero). Nested in message when used only there (AIP-126).
Quantities: Include unit suffix (e.g. distance_km, node_count); use _count for counts, not num_ (AIP-141). No unsigned integers.
Etag: Optional string etag for optimistic concurrency; mismatch on update/delete → ABORTED.
Format / FieldInfo: Use google.api.field_info format (e.g. UUID4, IPv4, IPv6) only when specified by an AIP; document and compare per spec (AIP-202).

6.4 Pagination (List)

Request: int32 page_size (optional; document default and max; coerce over max; error on negative), string page_token.
Response: repeated items + string next_page_token (set only when there is a next page).

7. Naming summary

See references/naming-and-methods.md for the naming summary table.

8. Errors and documentation

Errors: Return google.rpc.Status with google.rpc.Code. Include ErrorInfo in details with machine-readable reason (UPPER_SNAKE_CASE, ≤63 chars). Message: developer-facing, English, brief and actionable; dynamic parts in details.
Comments: Every message, RPC, and field should have a clear comment (valid American English). Document resource name patterns (Format: ...) and constraints (e.g. page_size default/max).
Precedent violations: If deviating from AIP guidance, document with comment prefixed aip.dev/not-precedent and justify per AIP-200.

9. Best practices (concise)

Run the API Linter and fix reported issues (or document why a rule is disabled).
Keep APIs self-contained: versioned package, resource names for cross-API refs, shared types only from approved common packages.
Use standard methods and standard request/response shapes; add custom methods only when necessary.
Annotate all request fields with field_behavior; annotate resource and reference fields with google.api.resource / resource_reference.
Use consistent vocabulary: same concept → same name; avoid overloaded terms; prefer standard fields (name, parent, create_time, update_time, display_name, etag, state).
Design for many clients: CLIs, SDKs, declarative/IaC, UIs; avoid special-casing that hurts one client for marginal gain.
Preserve backwards compatibility: no breaking changes to field behavior, required fields, or resource names within a major version; pagination and LRO from day one where applicable.

10. Example (minimal CRUD resource)

For a full minimal CRUD proto example (service, Book resource, and all request/response messages), see references/full-example.md.

11. References

AIP index: google.aip.dev
API Linter: github.com/googleapis/api-linter
Key AIPs: 1 (purpose), 8 (style), 121 (resource design), 122 (resource names), 123 (resource types), 127 (HTTP transcoding), 131–136 (standard + custom methods), 140 (field names), 158 (pagination), 190 (naming), 203 (field behavior), 213/215 (common/API-specific protos)

When in doubt, open the specific AIP on google.aip.dev for full guidance and rationale.