Field behavior documentation
When defining fields in protocol buffers, it is customary to explain to users certain aspects of the field's behavior (such as whether it is required or optional). Additionally, it can be useful for other tools to understand this behavior (for example, to optimize client library signatures).
APIs use the
google.api.field_behavior annotation to describe well-understood
field behavior, such as a field being required or immutable.
// The audio data to be recognized. RecognitionAudio audio = 2 [(google.api.field_behavior) = REQUIRED];
- APIs must apply the
google.api.field_behaviorannotation on every field on a message or sub-message used in a request.
- The annotation must include any google.api.FieldBehavior values that
accurately describe the behavior of the field.
FIELD_BEHAVIOR_UNSPECIFIEDmust not be used.
- APIs must at minimum use one of
field_behavior does not impact proto-level behavior,
many clients (e.g. CLIs and SDKs) rely on them to generate code. Thoroughly
review and consider which values are relevant when adding a new field.
Fields with no annotation are interpreted as
backwards-compatility. Nontheless, this annotation must not be omitted.
Note: The vocabulary given in this document is for descriptive purposes only, and does not itself add any validation. The purpose is to consistently document this behavior for clients.
The use of
REQUIRED indicates that the field must be present (and set to
a non-empty value) on the request or resource.
A field should only be described as required if either:
- It is a field on a resource that a user provides somewhere as input. In this case, the resource is only valid if a "truthy" value is stored.
- It is a field on a request message (a message that is an argument to an RPC,
with a name usually ending in
Request). In this case, a value must be provided as part of the request, and failure to do so must cause an error (usually
We define the term "truthy" above as follows:
- For primitives, values other than
0.0, empty string/bytes, and
- For repeated fields maps, values with at least one entry
- For messages, any message with at least one "truthy" field.
Fields should not be described as required in order to signify:
- A field which will always be present in a response.
- A field which is conditionally required in some situations.
- A field on any message (including messages that are resources) which is never used as user input.
Note: In most cases, empty values (such as
false for booleans,
integers, or the unspecified value for enums) are indistinguishable from unset
values, and therefore setting a required field to a falsy value yields an
error. A corollary to this is that a required boolean must be set to
The use of
OPTIONAL indicates that a field is not required.
A field may be described as optional if it is a field on a request message
(a message that is an argument to an RPC, usually ending in
Request), or a
field on a submessage.
The use of
OUTPUT_ONLY indicates that the field is provided in responses, but
that including the field in a message in a request does nothing (the server
must clear out any value in this field and must not throw an error as a
result of the presence of a value in this field on input). Similarly, services
must ignore the presence of output only fields in update field masks.
Additionally, a field should only be described as output only if it is a
field in a resource message, or a field of a message farther down the tree.
Notably, fields in response messages (a message which only ever acts as a
return value to an RPC, usually ending in
Response) should not be
described as output only because this is already implied.
Output only fields may be set to empty values if appropriate to the API.
Potential use cases for output only fields (this is not an exhaustive list) are:
- Create or update timestamps.
- Derived or structured information based on original user input.
- Properties of a resource assigned by the service which can not be altered.
The use of
INPUT_ONLY indicates that the field is provided in requests and
that the corresponding field will not be included in output.
Additionally, a field should only be described as input only if it is a
field in a resource message or a field of a message included within a resource
message. Notably, fields in request messages (a message which only ever acts as
an argument to an RPC, with a name usually ending in
Request) should not
be described as input only because this is already implied.
Potential use cases for input only fields (this is not an exhaustive list) are:
ttlfield as described in AIP-214.
Warning: Input only fields are rare and should be considered carefully before use.
The use of
IMMUTABLE indicates that a field on a resource cannot be changed
after it's creation. This can apply to either fields that are input or outputs,
required or optional.
When a service receives an immutable field in an update request (or similar),
even if included in the update mask, the service should ignore the field if
the value matches, but should error with
INVALID_ARGUMENT if a change is
Potential use cases for immutable fields (this is not an exhaustive list) are:
- Names or IDs which are set on creation and then used as a primary key.
Note: Fields which are "conditionally immutable" must not be given the immutable annotation.
The use of
UNORDERED_LIST on a repeated field of a resource indicates that
the service does not guarantee the order of the items in the list.
A field should be described as an unordered list if the service does not guarantee that the order of the elements in the list will match the order that the user sent, including a situation where the service will sort the list on the user's behalf.
A resource with an unordered list may return the list in a stable order, or may return the list in a randomized, unstable order.
Required set of annotations
A field used in a request message must be either an input or an output.
In the case of an output, the
OUTPUT_ONLY annotation is sufficient.
In the case of an input, a field is either required or optional, and therefore
should have at least the
OPTIONAL annotation, respectively.
Requiring field behavior
By including the field behavior annotation for each field, the overall behavior that the resource exhibits is more clearly defined. Clearly defined field behavior improves programmatic clients and user understanding.
Requiring the annotation also forces the API author to explicitly consider the behavior when initially authoring of the API.
Modifying field behavior after initial authoring results in backwards-incompatible changes in clients. For example, making an optional field required, results in backwards-incompatible changes in the method signature of an RPC or a resource in an IaC client.
In 2023-05 field_behavior was made mandatory. Prior to this change, the annotation was often omitted. Its values, e.g. REQUIRED, OUTPUT_ONLY, and IMMUTABLE, are relied upon to produce high quality clients. Further, when the value is added after the fact or changes, within a major version, it is backwards-incompatible, which is likely to break clients.
The benefits of requiring field_behavior, at the time that the API is authored, surpass the costs to clients and API users of not doing so.
- 2023-05-24: Clarify that
IMMUTABLEdoes not imply input nor required.
- 2023-05-10: Added guidance to require the annotation.
- 2020-12-15: Added guidance for
- 2020-05-27: Clarify behavior when receiving an immutable field in an update.
- 2019-12-05: Added guidance on output only fields in field masks.
- 2019-06-18: Use the machine-readable annotation, not comments.