Lint Rules and Categories

Lint rules and categories have been simplified between v1beta1 and v1. The old v1beta1 documentation is included here for posterity, but you are strongly encouraged to migrate to v1. See the migration guide to get started. You can find the v1 lint rules here.

buf provides a carefully curated set of lint rules designed to provide consistency and maintainability across a Protobuf schema of any size and any purpose, but without being so opinionated as to restrict organizations from making the design decisions they need to make for their individual APIs.

buf lint applies individual lint rules across your Protobuf schema, reporting any violations as errors. All lint rules have an ID, and belong to one or more categories. On this page, we'll discuss the available categories, and the individual rules within each category.

Although categories are not required to be in tree form, they can be represented as such. Note this is just a human representation and is not actual configuration.

• DEFAULT
  • STYLE_DEFAULT
  • BASIC
    • STYLE_BASIC
    • MINIMAL
      • FILE_LAYOUT
      • PACKAGE_AFFINITY
      • SENSIBLE
• COMMENTS
• UNARY_RPC
• OTHER

Style guide

Our style guide provides a concise document that effectively includes all rules in the DEFAULT category, as well as additional recommendations that are not enforced. We provide this for ease of consumption across your various teams, while linking back to this document for rationale for individual rules.

Categories

Buf provides three "main top-level" categories of increasing strictness:

• MINIMAL
• BASIC
• DEFAULT

These provide the majority of lint rules you may want to apply.

Additionally, Buf provides "extra top-level" categories, currently:

• COMMENTS
• UNARY_RPC
• OTHER

These enforce additional constraints that users may want to apply to their Protobuf schema.

We will add a STRICT lint category in the near future. All user-requested rules will go in a special category OTHER.

MINIMAL

The MINIMAL category represents what we consider to be fundamental rules for modern Protobuf development, regardless of style. We find these rules so important that if it were up to us (which it is not), and protoc could make breaking changes (which it can't, and shouldn't), these would be required for protoc to produce valid output.

Not applying these rules can lead to a myriad of bad situations across the variety of available Protobuf plugins, especially plugins not built into protoc itself. There is no downside to applying these rules. If you can't tell, we highly recommend abiding by the MINIMAL group for your development sanity.

The MINIMAL category includes three "sub-categories".

FILE_LAYOUT

The FILE_LAYOUT category includes three rules:

• DIRECTORY_SAME_PACKAGE checks that all files in a given directory are in the same package.
• PACKAGE_SAME_DIRECTORY checks that all files with a given package are in the same directory.
• PACKAGE_DIRECTORY_MATCH checks that all files with are in a directory that matches their package name.

In short, this verifies that all files that declare a given package foo.bar.baz.v1 are in the directory foo/bar/baz/v1 relative to root, and that only one such directory exists. For example, assuming we have a single root, proto:

.
└── proto
    └── foo
        └── bar
            ├── bat
            │   └── v1
            │       └── bat.proto // package foo.bar.bat.v1
            └── baz
                └── v1
                    ├── baz.proto         // package foo.bar.baz.v1
                    └── baz_service.proto // package foo.bar.baz.v1

protoc doesn't enforce file structure in any way, however you're like to have a rough time with many Protobuf plugins across various languages if you do not do this.

This also has the effect of allowing imports to self-document their package, for example you know that the import foo/bar/bat/v1/bat.proto has types in the package foo.bar.bat.v1.

There is no downside to maintaining this structure, and in fact many languages explicitly or effectively enforce such a file structure anyways (for example, Golang and Java).

PACKAGE_AFFINITY

Buf does not lint file option values, but it is important to make sure that certain file option values are consistent across all files in a given Protobuf package if you do use them.

The PACKAGE_AFFINITY category includes these rules:

• PACKAGE_SAME_CSHARP_NAMESPACE checks that all files with a given package have the same value for the csharp_namespace option.
• PACKAGE_SAME_GO_PACKAGE checks that all files with a given package have the same value for the go_package option.
• PACKAGE_SAME_JAVA_MULTIPLE_FILES checks that all files with a given package have the same value for the java_multiple_files option.
• PACKAGE_SAME_JAVA_PACKAGE checks that all files with a given package have the same value for the java_package option.
• PACKAGE_SAME_PHP_NAMESPACE checks that all files with a given package have the same value for the php_namespace option.
• PACKAGE_SAME_RUBY_PACKAGE checks that all files with a given package have the same value for the ruby_package option.
• PACKAGE_SAME_SWIFT_PREFIX checks that all files with a given package have the same value for the swift_prefix option.

Each of these rules also verifies that if a given option is used in one file in a given package, it is used in every file.

For example, if we have file foo_one.proto:

syntax = "proto3";

package foo.v1;

option go_package = "foov1";
option java_multiple_files = true;
option java_package = "com.foo.v1";

Another file foo_two.proto with package foo.v1 must have these three options set to the same value, and the other options unset:

syntax = "proto3";

package foo.v1;

option go_package = "foov1";
option java_multiple_files = true;
option java_package = "com.foo.v1";

SENSIBLE

The SENSIBLE category outlaws certain Protobuf features that you should never use in modern Protobuf development. It includes these rules:

• ENUM_NO_ALLOW_ALIAS checks that enums do not have the allow_alias option set.
• FIELD_NO_DESCRIPTOR checks that field names are not name capitalization of "descriptor" with any number of prefix or suffix underscores.
• IMPORT_NO_PUBLIC checks that imports are not public.
• IMPORT_NO_WEAK checks that imports are not weak.
• PACKAGE_DEFINED checks that all files with have a package defined.

ENUM_NO_ALLOW_ALIAS

This rule outlaws aliased enums like this:

enum Foo {
  option allow_alias = true;
  FOO_UNSPECIFIED = 0;
  FOO_ONE = 1;
  FOO_TWO = 1; // no!
}

The allow_alias option allows multiple enum values to have the same number. This can lead to issues when working with the JSON representation of Protobuf, a first-class citizen of proto3. If you get a serialized Protobuf value over the wire in binary format, it is unknown what specific value in the enum it applies to, and JSON usually serialized enum values by name. While in practice, if you declare an alias, you expect names to be interchangeable, this can lead to hard-to-track bugs.

Instead of having an alias, we recommend deprecating your current enum, and making a new one with the enum value name you want. Or just stick with the current name for your enum value.

FIELD_NO_DESCRIPTOR

This rules outlaws field names being any capitalization of "descriptor", with any number of prefix or suffix underscores. For example:

// ALL FIELDS ARE INVALID
message Foo {
  string descriptor = 1;
  string Descriptor = 2;
  string descRiptor = 3;
  string _descriptor = 4;
  string __descriptor = 5;
  string descriptor_ = 6;
  string descriptor__ = 7;
  string __descriptor__ = 8;
}

This prevents a long-standing issue with Protobuf where certain languages generate an accessor named descriptor that conflicts with generated code for this field name. There is actually an option no_standard_descriptor_accessor on MessageOptions that allows mitigation of this issue for fields that are named descriptor. Following the documentation there, developers should avoid naming fields "descriptor". This actually happens more often than you may think.

IMPORT_NO_PUBLIC, IMPORT_NO_WEAK

These rules outlaw declaring imports as public or weak. If you didn't know this was possible, forget what you just learned in this sentence, and regardless do not use these.

PACKAGE_DEFINED

This rule requires all Protobuf files to specify a package. It is possible to have a Protobuf file that does not declare a package. If you did not know this was possible, forget what you just learned, and regardless do not do this.

BASIC

The BASIC category includes everything from the MINIMAL category, as well as the STYLE_BASIC category. That is, this configuration...

version: v1beta1
lint:
  use:
    - BASIC

...is equivalent to:

version: v1beta1
lint:
  use:
    - MINIMAL
    - STYLE_BASIC

STYLE_BASIC

The STYLE_BASIC category includes basic style checks that are widely accepted as standard Protobuf style. These checks should generally be applied for all Protobuf schemas.

These checks represent the "old" Google Style Guide that has been around for years, before elements from the Uber Style Guide were merged in during the spring of 2019.

The STYLE_BASIC category includes these rules:

• ENUM_PASCAL_CASE checks that enums are PascalCase.
• ENUM_VALUE_UPPER_SNAKE_CASE checks that enum values are UPPER_SNAKE_CASE.
• FIELD_LOWER_SNAKE_CASE checks that field names are lower_snake_case.
• MESSAGE_PASCAL_CASE checks that messages are PascalCase.
• ONEOF_LOWER_SNAKE_CASE checks that oneof names are lower_snake_case.
• PACKAGE_LOWER_SNAKE_CASE checks that packages are lower_snake.case.
• RPC_PASCAL_CASE checks that RPCs are PascalCase.
• SERVICE_PASCAL_CASE checks that services are PascalCase.

DEFAULT

The DEFAULT category includes everything from the BASIC category, as well as the STYLE_DEFAULT category. That is, this configuration...

version: v1beta1
lint:
  use:
    - DEFAULT

...is equivalent to:

version: v1beta1
lint:
  use:
    - BASIC
    - STYLE_DEFAULT

Consistent with its name, DEFAULT is also the default set of lint rules used by Buf if no configuration is present, and represents the our baseline enforced recommendations for modern Protobuf development without being overly burdensome.

STYLE_DEFAULT

The STYLE_DEFAULT category includes everything in STYLE_BASIC, as well as style checks that we recommend for consistent, maintainable Protobuf schemas. We recommend applying all of these checks to any schema you develop.

The STYLE_DEFAULT category includes these rules on top of STYLE_BASIC:

• ENUM_VALUE_PREFIX checks that enum values are prefixed with ENUM_NAME_UPPER_SNAKE_CASE.
• ENUM_ZERO_VALUE_SUFFIX checks that enum zero values are suffixed with _UNSPECIFIED (suffix is configurable).
• FILE_LOWER_SNAKE_CASE checks that filenames are lower_snake_case.
• RPC_REQUEST_RESPONSE_UNIQUE checks that RPCs request and response types are only used in one RPC (configurable).
• RPC_REQUEST_STANDARD_NAME checks that RPC request type names are RPCNameRequest or ServiceNameRPCNameRequest (configurable).
• RPC_RESPONSE_STANDARD_NAME checks that RPC response type names are RPCNameResponse or ServiceNameRPCNameResponse (configurable).
• PACKAGE_VERSION_SUFFIX checks that the last component of all packages is a version of the form v\d+, v\d+test.*, v\d+(alpha|beta)\d+, or v\d+p\d+(alpha|beta)\d+, where numbers are >=1.
• SERVICE_SUFFIX checks that services are suffixed with Service (suffix is configurable).

ENUM_VALUE_PREFIX

This rule requires that all enum value names are prefixed with the enum name. For example:

enum Foo {
  FOO_UNSPECIFIED = 0;
  FOO_ONE = 1;
}

message Bar {
  enum Baz {
    BAZ_UNSPECIFIED = 0;
    BAZ_ONE = 1;
  }
}

Protobuf enums use C++ scoping rules, which makes it not possible to have two enums in the same package with the same enum value name (an exception is when enums are nested, in which case this rule applies within the given message). While you may expect a given enum value to always be unique across a package, APIs can develop over years, and there are countless examples of developers having to compromise on their enum names due to backwards compatibility issues. For example, you might have this enum:

enum Scheme {
  // Right off the bat, you can't use "UNSPECIFIED" in multiple enums
  // in the same package, so you always would have to prefix this anyways.
  SCHEME_UNSPECIFIED = 0;
  HTTP = 1;
  HTTPS = 2;
  ...
}

Two years later, you have an enum in the same package you want to add, but can't:

// This is a made up example, bear with us.
enum SecureProtocol {
  SECURE_PROTOCOL_UNSPECIFIED = 0;
  // If this enum is in the same package as Scheme, this produces
  // a protoc compile-time error!
  HTTPS = 1;
  ...
}

ENUM_ZERO_VALUE_SUFFIX

This rule requires that all enum values have a zero value of ENUM_NAME_UNSPECIFIED. For example:

enum Foo {
  FOO_UNSPECIFIED = 0;
}

The suffix is configurable.

All enums should have a zero value. Proto3 does not differentiate between set and unset fields, so if an enum field is not explicitly set, it defaults to the zero value. If an explicit zero value is not part of the enum definition, this defaults to the actual zero value of the enum. For example, if you had:

enum Scheme {
  // *** DO NOT DO THIS ***
  SCHEME_FTP = 0
}

message Uri {
  Scheme scheme = 1;
}

Any Uri with scheme not explicitly set defaults to SCHEME_FTP.

FILE_LOWER_SNAKE_CASE

This rule says that all .proto files must be named in lower_snake_case.proto. This is the widely accepted standard.

RPC_REQUEST_STANDARD_NAME, RPC_RESPONSE_STANDARD_NAME, RPC_REQUEST_RESPONSE_UNIQUE

These rules enforce the message name of RPC request/responses, and that all request/responses are unique.

One of the single most important rules to enforce in modern Protobuf development is to have a unique request and response message for every RPC. Separate RPCs should not have their request and response parameters controlled by the same Protobuf message, and if you share a Protobuf message between multiple RPCs, this results in multiple RPCs being affected when fields on this Protobuf message change. Even in straightforward cases, best practice is to always have a wrapper message for your RPC request and response types. Buf enforces this with these three rules by verifying that:

• All request and response messages are unique across your Protobuf schema.
• All request and response messages are named after the RPC, either by naming them MethodNameRequest, MethodNameResponse or ServiceNameMethodNameRequest, ServiceNameMethodNameResponse.

For example, this service definition abides by these rules:

// request/response message definitions omitted for brevity

service FooService {
  rpc Bar(BarRequest) returns (BarResponse) {}
  rpc Baz(FooServiceBazRequest) returns (FooServiceBazResponse) {}
}

There are configuration options associated with these three rules.

PACKAGE_VERSION_SUFFIX

This rule enforces that the last component of a package must be a version of the form v\d+, v\d+test.*, v\d+(alpha|beta)\d*, or v\d+p\d+(alpha|beta)\d*, where numbers are >=1.

Examples (all taken directly from buf testing):

foo.v1
foo.v2
foo.bar.v1
foo.bar.v1alpha
foo.bar.v1alpha1
foo.bar.v1alpha2
foo.bar.v1beta
foo.bar.v1beta1
foo.bar.v1beta2
foo.bar.v1p1alpha
foo.bar.v1p1alpha1
foo.bar.v1p1alpha2
foo.bar.v1p1beta
foo.bar.v1p1beta1
foo.bar.v1p1beta2
foo.bar.v1test
foo.bar.v1testfoo

One of the core promises of Protobuf API development is to never have breaking changes in your APIs, and Buf helps enforce this through the breaking change detector. There are scenarios, however, where you do want to properly version your API. Instead of making changes, the proper method to do so is to make a completely new Protobuf package that is a copy of your existing Protobuf package, serve both packages server-side, and manually migrate your callers. This rule enforces that all packages have a version attached so that it is clear when a package represents a new version.

A common idiom is to use alpha and beta packages for packages that are still in development and can have breaking changes. You can configure the breaking change detector to ignore breaking changes in files for these packages with the ignore_unstable_packages option:

version: v1beta1
breaking:
  ignore_unstable_packages: true

SERVICE_SUFFIX

This rule enforces that all services are suffixed with Service. For example:

service FooService {}
service BarService {}
service BazService {}

Service names inherently end up having a lot of overlap with package names, and service naming often ends up inconsistent as a result across a larger Protobuf schema. Enforcing a consistent suffix takes away some of this inconsistency.

The suffix is configurable via the lint.service_suffix option. For example, if you have this configuration in your buf.yaml...

version: v1beta1
lint:
  service_suffix: Endpoint

...the SERVICE_SUFFIX rule enforces this naming instead:

service FooEndpoint {}
service BarEndpoint {}
service BazEndpoint {}

COMMENTS

This is an "extra top-level" category that enforces that comments are present on various parts of your Protobuf schema.

The COMMENTS category includes these rules:

• COMMENT_ENUM checks that enums have non-empty comments.
• COMMENT_ENUM_VALUE checks that enum values have non-empty comments.
• COMMENT_FIELD checks that fields have non-empty comments.
• COMMENT_MESSAGE checks that messages have non-empty comments.
• COMMENT_ONEOF checks that oneof have non-empty comments.
• COMMENT_RPC checks that RPCs have non-empty comments.
• COMMENT_SERVICE checks that services have non-empty comments.

Note that only leading comments are considered - trailing comments do not count towards passing these rules.

Buf intends to host a documentation service (both public and on-prem) in the future, which may include a structured documentation scheme, however for now you may want to at least enforce that certain parts of your schema contain comments. This group allows such enforcement. Of note is that general usage may be not to use all rules in this category, instead selecting the rules you specifically want. For example:

version: v1beta1
lint:
  use:
    - DEFAULT
    - COMMENT_ENUM
    - COMMENT_MESSAGE
    - COMMENT_RPC
    - COMMENT_SERVICE

UNARY_RPC

This is an "extra top-level" category that outlaws streaming RPCs.

This UNARY_RPC category includes these rules:

• RPC_NO_CLIENT_STREAMING checks that RPCs are not client streaming.
• RPC_NO_SERVER_STREAMING checks that RPCs are not server streaming.

Some RPC protocols do not allow streaming RPCs, for example Twirp. This extra category enforces that no developer accidentally adds a streaming RPC if your setup does not support them. Additionally, streaming RPCs have a number of issues in general usage. See this discussion for more details.

OTHER

This is an "extra top-level" category that includes lint rules not in a main collection.

This category can be modified between collection versions.

ENUM_FIRST_VALUE_ZERO

This rule enforces that the first enum value is the zero value.

This is a proto3 requirement on build, but is not required in proto2 on build. This rule enforces that this is also followed in proto2.

As an example:

syntax = "proto2";

enum Scheme {
  // *** DO NOT DO THIS ***
  SCHEME_FTP = 1;
  SCHEME_UNSPECIFIED = 0;
}

The above results in generated code in certain languages defaulting to SCHEME_FTP instead of SCHEME_UNSPECIFIED.