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 in v2 as following:

envoy.service contains gRPC definitions of supporting services;
envoy.config contains definitions for service configuration, filter configuration, and bootstrap;
envoy.api.v2 contains definitions for EDS, CDS, RDS, LDS, and top-level resources such as Cluster;
envoy.api.v2.endpoint, envoy.api.v2.cluster, envoy.api.v2.route, envoy.api.v2.listener, envoy.api.v2.ratelimit define sub-messages of the top-level resources;
envoy.api.v2.core and envoy.api.v2.auth hold core definitions consumed throughout the API.

In Envoy API v3, 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.

Dependencies are enforced from top-to-bottom using visibility constraints in the build system to prevent circular dependency formation. Package group //envoy/api/v2:friends selects consumers of the core API package (services and configs) and is the default visibility for the core API packages. The default visibility for services and configs should be //docs (proto documentation tool).

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

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API style guidelines

Package organization

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