# Edition Zero: Converged Semantics **Authors:** [@perezd](https://github.com/perezd), [@haberman](https://github.com/haberman) **Approved:** 2021-10-07 ## Background The Protobuf Team has been exploring potential facilities for introducing breaking API and semantic changes. This document is an attempt to make use of these facilities to unify proto semantics from this point forward, while giving customers the ability to more-granularly manage their project's specific needs. ## Objective We want to reduce complications of API semantics that are coarsely managed through the syntax keyword, and instead default to converged proto2/proto3 when opting into editions. Where needed, customers will be able to opt out of specific semantics that are incompatible with their existing usages, at a fine-grained level, using the new capabilities provided by editions + features. ### Why Now? As we introduce new facilities for managing breaking changes, we have an additional opportunity to cutover and rectify a long-standing vision of converging proto 2/3 semantics as a natural extension of this. Doing this in lockstep with the introduction of editions provides the protobuf team with a few valuable outcomes: * Editions provides us with more granular specification of intent than the existing coarse knob of "proto2" or "proto3." By opting into our first edition, customers are upgrading to what we've referred to in the past as "converged semantics," and if needed can reversibly downgrade back to proto2 or proto3 semantics respectively by opt-ing out of the specific features that are incompatible with their existing needs. * The protobuf team can avoid the n^2 complexity of considering how an edition/feature will interplay with an explicit syntax designation of "proto2" vs "proto3" for all impacted runtimes. This allows us to transition our thinking/support model to be explicitly feature-centric. * The introduction of editions will almost certainly cause a major version bump and gives us ample justification to make breaking changes as we transition to this granular specification. ## Introduction of the `edition` keyword to proto IDL The `edition` keyword is used to define which semantic version a particular file and all of its contents will adhere to as a baseline. Whenever a proto file declares an `edition` keyword, it automatically defaults to converged proto2/3 semantics. An edition's value is represented as a string, encoded by convention as a year. ## Introduction of `features` option to `descriptor.proto` This option will be uniformly defined as a repeated set of strings which can be used to encode the ability to opt-out of a specific feature (eg: `"-string_view"`), or to potentially opt-in to a future/experimental feature (eg: `"string_view"`). The `features` option will be added to `descriptor.proto` for the following descriptor options: * File * Message * Field * Enum * Enum Value * Oneof * Service * Method * Stream (internal repositories only) Features are only respected when used in conjunction with the `edition` keyword. They are not validated for correctness to ensure they are forward/backward compatible with releases. Features may be declared at any descriptor level, however, a feature definition may influence descendant types at the discretion of the protobuf team. (e.g., a file-level feature opt-out could impact all fields within the file, if it was desired). ## A taxonomy of features Features can be broken down into two main categories: language-specific and semantic. ### Language-specific features Language-specific features pertain to the generated API for a given language. Referring to the protobuf breaking changes backlog we can see some examples: * (C++) Changing string fields to return `string_view`. * (Java) Removing the confusing `Enum#valueOf(int)` API. * (Java) Rename oneof enums to do appropriate camel casing. Language-specific features have no meaning for any other language: they can be ignored entirely. They are, in essence, a private (tunneled) interface between protobuf IDL and the respective code generator. Each language's code generator can independently decide what the "base" set of features is for any given edition. Each language defines the migration path between editions independently. ### Semantic Features Semantic features define behavior changes that apply to the protobuf data model, independent of language. These can also have API implications, but their meaning goes deeper than just a surface-level API. Some examples of semantic features: * Open enums (enums are placed directly into the field instead of the `UnknownFieldSet`). * Packed (whether repeated fields are packed on the wire) Semantic features have significantly broader scope, since they must be respected across languages, and each language must implement the semantic correctly. This also implies that every language must either (1) know the canonical set of "base features" for each edition, or (2) that the set of "default" features for the edition must be resolved in protoc itself and propagated explicitly into the descriptor. ## Rev'ing the protobuf IDL vs. descriptor.proto Changing `descriptor.proto` to reflect editions is a much more intrusive change than changing just the protobuf IDL. The protobuf IDL is parsed and resolved in protoc, and we have only a single implementation of that parser. Any change that can be resolved in the parser alone is relatively unintrusive (though there are build horizon issues since GCL parses protos in prod). Rev'ing `descriptor.proto` is a far more intrusive change that affects many downstream systems. Many systems access descriptors through either a descriptor API (for example, `proto2::Descriptor` in C++) or by directly accessing a proto from `descriptor.proto` (eg. `google.protobuf.DescriptorProto`). Any changes here need to be managed much more delicately. ## Deprecation of the `syntax` keyword from proto IDL The `syntax` keyword shall no longer be required/observed when an `edition` keyword is present, as it is now considered redundant. If `edition` and `syntax` are both present, `edition` takes precedence and `syntax` is ignored. ## Migrating from `proto2` and `proto3` to Editions + Features Today's usage of syntax opaquely bundles a collection of implied feature flags that are set based on the presence of `proto2` or `proto3`. This is often a source of confusion for customers (eg: what am I gaining by moving to proto3? What am I losing?). By deciding that editions/features exist in a state of proto2/3 convergence, this enables customers to decide for themselves what features are important to their usage of protos. Migrating existing users of proto2 and proto3 to editions w/converged semantics would mean we'd need to execute a large-scale change to make their implicit/implied behavior explicit. Here are examples of implied behavior. today:
Feature proto2 implied behavior proto3 implied behavior
packed_repeated_primitives 🚫 ✅
extensions ✅ 🚫
required ✅ 🚫
groups ✅ 🚫
cpp_string_view 🚫 🚫
java_enum_no_value_of 🚫 🚫
open_enums 🚫 ✅
MORE STUFF ...
### Managing Complexity of `features` for Large Deployments A separate concept has been established to help mitigate the complexity of editions and progressive feature rollouts and synchronizations for larger proto projects. This facility could be used to migrate existing usages of the `syntax` keyword to use Editions + Features across google3, for example. ## Prior Work * proto{2,3} Convergence Vision (not available externally) * Epochs for descriptor.proto (not available externally) * Rust editions: https://doc.rust-lang.org/edition-guide/editions/index.html