Protocol Buffers Architecture

Language-agnostic schema standards. Proto files target proto3 + buf toolchain. Validation uses protovalidate (CEL). Code generation is per-language and out of scope here — see the language architect skill for the generated-code idioms (e.g. go-architect when generating for Go). File-layout, validation, codegen, and breaking-check snippets in RECIPES.md; pinned tool versions in STACK.md.

1. File layout & package naming

Buf-style hierarchical packages — <org>.<product>.<resource>.<version>. Mirrors the on-disk path. One service or coherent message group per file. Tree + package declaration in RECIPES.md.

• Package = directory path. acme/shop/users/v1/ → package acme.shop.users.v1;. Enforced by buf lint.
• *Per-language `option _package`** declarations live in every file. Set them up-front; don't refactor later (it's a breaking change).
• One `service` definition per file; supporting messages can live alongside or in sibling files.

2. Message design

• All fields are `optional` semantically in proto3. Use the explicit optional keyword (proto3.15+) when "field absent" must be distinguishable from "field present with zero value" — common for PATCH-style partial updates.
• Field names: `snake_case`. Enforced by buf lint. The language plugin converts to the target idiom (Go CamelCase, Python snake_case, etc.).
• Message names: `PascalCase` singular. User, not Users (the collection is repeated User).
• Enum names: `PascalCase`. Enum value names: UPPER_SNAKE_CASE prefixed with the enum name to keep them unambiguous across imports. Example in RECIPES.md.
• *`_UNSPECIFIED = 0` is mandatory** — proto3 default is 0; making the zero value an explicit "unspecified" sentinel prevents accidental defaults from being mistaken for real states.
• No primitive wrappers (`google.protobuf.StringValue` etc.) at API boundaries. Use optional instead — it's the modern equivalent and doesn't require importing wrappers.proto.
• Avoid `map<K, V>` when ordering or evolution matters. repeated KeyValue with explicit key/value fields gives you ordering, validation, and the ability to add metadata per entry later.

3. Field numbering & reservation discipline

• Field numbers 1–15 use 1 byte on the wire; 16–2047 use 2 bytes. Reserve 1–15 for fields read on every request (IDs, status, frequently-accessed metadata).
• Never reuse a field number. When deleting a field, reserved it forever (example in RECIPES.md).
• Never change a field's type. Changing int32 age = 5; to string age = 5; is a breaking change at the wire level. Delete + reserve + introduce a new field with a new number instead.
• `buf breaking` enforces both rules in CI (see §8).

4. Versioning

• Version is part of the package path (acme.shop.users.v1), never appended to message names (UserV1 is wrong).
• A new major version = a new `vN` package + a new directory. v1/ and v2/ coexist; clients migrate at their pace.
• Additive changes stay in the same version. Adding a new optional field, a new enum value, or a new service method does not break clients — no version bump needed.
• Breaking changes always go to a new major version. Field removal, type change, semantic change, required-field tightening.
• Deprecate before remove. Add [deprecated = true] on the field; communicate via Deprecation headers when wrapped in REST; remove only in the next major version after migration is complete.

5. Validation — protovalidate (CEL)

Buf's protovalidate is the modern, declarative replacement for the older protoc-gen-validate (PGV). Constraints are CEL expressions attached as field options; validation runs at runtime via per-language protovalidate libraries. Field-level and cross-field examples in RECIPES.md.

• Validate at the boundary — server-side, in the gRPC interceptor (see grpc-architect §4). Don't rely on clients to validate.
• CEL allows cross-field rules via (buf.validate.message).cel.
• Skip the old `protoc-gen-validate` (PGV). It's deprecated; protovalidate's CEL is more expressive and the runtime is maintained.

6. Code generation — buf generate + buf.gen.yaml

One config file at the repo root drives all generators. Versions of the plugins are pinned in buf.gen.yaml itself; no protoc invocation by hand. Full config in RECIPES.md.

• Generated code is committed in monorepos and in any repo whose consumers don't run buf themselves. Commit it under gen/<lang>/... clearly separated from hand-written source.
• Generated code is never edited by hand. If you need to add helpers, write them in sibling hand-written files.
• Generator plugin versions are pinned in buf.gen.yaml — never :latest.

7. Linting — buf lint

buf lint runs the STANDARD rule set out of the box (proto3 conventions, naming, file structure). Add to CI. buf.yaml config in RECIPES.md.

Common lint catches:

• File package doesn't match directory path
• Field name isn't snake_case
• Enum value not prefixed with enum name
• Missing *_UNSPECIFIED = 0
• Missing per-language option *_package

8. Breaking-change detection — buf breaking in CI

buf breaking --against '.git#branch=main,subdir=proto' blocks merges that introduce breaking changes without an intentional new vN package. FILE-level rules (the default) treat the file as the unit of compatibility — appropriate when generated code is consumed file-by-file. Stricter PACKAGE and WIRE modes exist; pick FILE unless you have a reason to differ.

• Run on every PR.
• The check compares against the main branch state. Workflows that want to compare against a tagged release can pass --against '.git#tag=v1.5.0,subdir=proto'.

9. Well-known types

Use the Google-provided well-known types instead of inventing equivalents.

• Always import via the canonical path. Don't copy these into your own packages.
• `Timestamp` over `int64 unix_ts` for any human-relevant time. Wire size is similar; human readability wins.
• `Struct` is a hint of bad design. Schemaless data in a typed schema is a smell — model it relationally if you can. Same opinion as sql-architect §10 on JSONB.

10. Common types / shared messages

When two services need the same domain concept (Money, Address, CustomerId), put it in a shared types/v1/ sub-package — example layout in RECIPES.md.

• `types/v1/` is its own versioned package. It evolves on the same major-version contract as anything else.
• Avoid premature sharing. Two services with similar-looking Address messages might genuinely have different semantics; force the sharing only when the model is provably the same.
• Never put services in the shared package. Only messages.