API style guidelines

Generally follow guidance at https://cloud.google.com/apis/design/, in particular for proto3 as described at:

A key aspect of our API style is maintaining stability by following the API versioning guidelines. All developers must familiarize themselves with these guidelines, any PR which makes breaking changes to the API will not be merged.

In addition, the following conventions should be followed:

Every proto directory should have a README.md describing its content. See for example envoy.service.
The data plane APIs are primarily intended for machine generation and consumption. It is expected that the management server is responsible for mapping higher level configuration concepts to concrete API concepts. Similarly, static configuration fragments may be generated by tools and UIs, etc. The APIs and tools used to generate xDS configuration are beyond the scope of the definitions in this repository.
Use wrapped scalar types where there is a real need for the field to have a default value that does not match the proto3 defaults (0/false/""). This should not be done for fields where the proto3 defaults make sense. All things being equal, pick appropriate logic, e.g. enable vs. disable for a bool field, such that the proto3 defaults work, but only where this doesn't result in API gymnastics.
Use a [#not-implemented-hide:] protodoc annotation in comments for fields that lack Envoy implementation. These indicate that the entity is not implemented in Envoy and the entity should be hidden from the Envoy documentation.
Always use plural field names for repeated fields, such as filters.
Due to the fact that we consider JSON/YAML to be first class inputs, we cannot easily change a a singular field to a repeated field (both due to JSON/YAML array structural differences as well as singular vs. plural field naming). If there is a reasonable expectation that a field may need to be repeated in the future, but we don't need it to be repeated right away, consider making it repeated now but using constraints to enforce a maximum repeated size of 1. E.g.:
```
repeated OutputSink sinks = 1 [(validate.rules).repeated = {min_items: 1, max_items: 1}];
```
Always use upper camel case names for message types and enum types without embedded acronyms, such as HttpRequest.

Prefer oneof selections to boolean overloads of fields, for example, prefer:

oneof path_specifier {
  string simple_path = 1;
  string regex_path = 2;
}

string path = 1;
bool path_is_regex = 2;

This is more efficient, extendable and self-describing.

The API includes two types for representing percents. Percent is effectively a double value in the range 0.0-100.0. FractionalPercent is an integral fraction that can be used to create a truncated percentage also in the range 0.0-100.0. In high performance paths, FractionalPercent is preferred as randomness calculations can be performed using integral modulo and comparison operations only without any floating point conversions. Typically, most users do not need infinite precision in these paths.
For enum types, if one of the enum values is used for most cases, make it the first enum value with 0 numeric value. Otherwise, define the first enum value like TYPE_NAME_UNSPECIFIED = 0, and treat it as an error. This design pattern forces developers to explicitly choose the correct enum value for their use case, and avoid misunderstanding of the default behavior.
Proto fields should be sorted logically, not by field number.

Package organization

API definitions are layered hierarchically in packages from top-to-bottom as following:

envoy.extensions contains all definitions for the extensions, the package should match the structure of the source directory.
envoy.service contains gRPC definitions of supporting services and top-level messages for the services. e.g. envoy.service.route.v3 contains RDS, envoy.service.listener.v3 contains LDS.
envoy.config contains other definitions for service configuration, bootstrap and some legacy core types.
envoy.data contains data format declaration for data types that Envoy produces.
envoy.type contains common protobuf types such as percent, range and matchers.

Extensions should use the regular hierarchy. For example, configuration for network filters belongs in a package under envoy.extensions.filter.network.

Adding an extension configuration to the API

Extensions must currently be added as v3 APIs following the package organization above. To add an extension config to the API, the steps below should be followed:

If this is still WiP and subject to breaking changes, use vNalpha instead of vN in steps below. Refer to the Cache filter config as an example of v3alpha, and the Buffer filter config as an example of v3.

Place the v3 extension configuration .proto in api/envoy/extensions, e.g. api/envoy/extensions/filter/http/foobar/v3/foobar.proto together with an initial BUILD file:

load("@envoy_api//bazel:api_build_system.bzl", "api_proto_package")

licenses(["notice"])  # Apache 2

api_proto_package(
    deps = ["@com_github_cncf_udpa//udpa/annotations:pkg"],
)

Add to the v3 extension config proto import "udpa/annotations/migrate.proto"; and import "udpa/annotations/status.proto";
If this is still WiP and subject to breaking changes, set option (udpa.annotations.file_status).work_in_progress = true;.
Add to the v3 extension config proto a file level option (udpa.annotations.file_status).package_version_status = ACTIVE;. This is required to automatically include the config proto in api/versioning/BUILD.
Add a reference to the v3 extension config in (1) in api/versioning/BUILD under active_protos.
Run ./tools/proto_format/proto_format.sh fix. This should regenerate the BUILD file, reformat foobar.proto as needed and also generate the v4alpha extension config (if needed), together with shadow API protos.
git add api/ generated_api_shadow/ to add any new files to your Git index.

API annotations

A number of annotations are used in the Envoy APIs to provide additional API metadata. We describe these annotations below by category.

Field level

[deprecated = true] to denote fields that are deprecated in a major version. These fields are slated for removal at the next major cycle and follow the breaking change policy.
[envoy.annotations.disallowed_by_default = true] to denote fields that have been disallowed by default as per the breaking change policy.
[(udpa.annotations.field_migrate).rename = "<new field name>"] to denote that the field will be renamed to a given name in the next API major version.
[(udpa.annotations.field_migrate).oneof_promotion = "<oneof name>"] to denote that the field will be promoted to a given oneof in the next API major version.
[(udpa.annotations.sensitive) = true] to denote sensitive fields that should be redacted in output such as logging or configuration dumps.
PGV annotations to denote field value constraints.

Enum value level

[(udpa.annotations.enum_value_migrate).rename = "new enum value name"] to denote that the enum value will be renamed to a given name in the next API major version.

Message level

option (udpa.annotations.versioning).previous_message_type = "<message type name>"; to denote the previous type name for an upgraded message. You should never have to write these manually, they are generated by protoxform.

Service level

option (envoy.annotations.resource).type = "<resource type name>"; to denote the resource type for an xDS service definition.

File level

option (udpa.annotations.file_migrate).move_to_package = "<package name>"; to denote that in the next major version of the API, the file will be moved to the given package. This is consumed by protoxform.
option (udpa.annotations.file_status).work_in_progress = true; to denote a file that is still work-in-progress and subject to breaking changes.

Principles

The following principles should be adhered to when extending or modifying the xDS APIs:

The xDS APIs have a logical distinction between transport and data model:
- The xDS transport protocol describes the network transport on which xDS configuration resources are delivered to clients. A versioned gRPC streaming protocol with support for ACK/NACK is provided by xDS; this is known as the xDS transport protocol (xDS-TP). xDS configuration resources can also be delivered on other transports, e.g. HTTP or filesystem, with some limitations (e.g. no version feedback).
- The xDS data model describes the xDS configuration resources themselves, e.g. listeners, route configurations, clusters, endpoints, secrets.
The xDS APIs are directionally client and server neutral. While many aspects of the APIs reflect the history of their origin as Envoy's control plane APIs, API decisions going forward should reflect the principle of client neutrality.
The xDS APIs are expressed canonically as Proto3. Both JSON and YAML are also supported formats, with the standard JSON-proto3 conversion used during client configuration ingestion.
xDS APIs are eventual consistency first. For example, if RDS references a cluster that has not yet been supplied by CDS, it should be silently ignored and traffic not forwarded until the CDS update occurs. Stronger consistency guarantees are possible if the management server is able to sequence the xDS APIs carefully (for example by using the ADS API below). By following the [CDS, EDS, LDS, RDS] sequence for all pertinent resources, it will be possible to avoid traffic outages during configuration update.
The API is primarily intended for machine generation and consumption. It is expected that the management server is responsible for mapping higher level configuration concepts to API responses. Similarly, static configuration fragments may be generated by templating tools, etc. With that consideration, we also aim to have API artifacts readable by humans for debugging and understanding applied configuration. This implies that APIs do not have to have ergonomics as the main driver, but should still be reasonable to read by humans. The APIs and tools used to generate xDS configuration are beyond the scope of the definitions in this repository.
All supported transports (xDS-TP, HTTP, filesystem) support basic singleton xDS subscription services CDS/EDS/LDS/RDS/SDS. Advanced APIs such as HDS, ADS and EDS multi-dimensional LB are xDS-TP only. This avoids having to map complicated bidirectional stream semantics onto REST, etc..
Versioning follows the scheme described here. A key principle that we target is that API consumers should not be exposed to breaking changes where there is no substantial gain in functionality, performance, security or implementation simplification. We will tolerate technical debt in the API itself, e.g. in the form of vestigial deprecated fields or reduced ergnomics (such as not using oneof when we would prefer to), in order to meet this principle.
Namespaces for extensions, metadata, etc. use a reverse DNS naming scheme, e.g. com.google.widget, com.lyft.widget. Client built-ins may be prefixed with client name, e.g. envoy.foo, grpc.bar.

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