Oneof, 맵 및 잘 알려진 유형

Real schemas need more than strings and ints: mutually exclusive choices, key/value collections, and standard date/time types. Protobuf provides oneof, maps, and well-known types.

A oneof groups fields where at most one can be set at a time. Setting one clears the others, and only the active field is sent on the wire.

Generated code gives a way to check which case is set, usually a case enum or accessor, so you branch on the active field.

A map stores key/value pairs. Keys must be scalar (string/int); values can be any type except another map.

Maps are sugar over a repeated message of key/value entries. Order is not guaranteed, and duplicate keys keep the last value — important when evolving schemas.

Protobuf ships standard types in google/protobuf/:

• Timestamp — a point in time
• Duration — a length of time
• Any — a packed arbitrary message
• Struct — dynamic JSON-like data

Prefer these over raw int seconds for clarity and tooling support. They map to native time types in most languages.

Any wraps any message plus a type URL, letting you carry heterogeneous payloads. The rich error model and reflection both use it.

FieldMask lists which fields a request touches, enabling partial updates (PATCH-style) without ambiguity over unset vs default values.

Proto3 scalars cannot distinguish unset from zero. Wrapper types like Int32Value and StringValue add explicit nullability when you need it.

When evolving these constructs:

• Add new fields to a oneof safely; do not reuse tag numbers
• Maps: avoid changing value type
• Well-known types are stable; prefer them over hand-rolled equivalents

You learned advanced protobuf constructs:

• oneof models mutually exclusive fields
• map stores key/value pairs (sugar over repeated entries)
• Well-known types: Timestamp, Duration, Any, Struct
• FieldMask enables partial updates; wrappers add nullability
• Evolve carefully without reusing tag numbers