Protobuf to JSON-Schema compiler

This takes protobuf definitions and converts them into JSONSchemas, which can be used to dynamically validate JSON messages.

Useful for people who define their data using ProtoBuf, but use JSON for the "wire" format.

"Heavily influenced" by Google's protobuf-to-BigQuery-schema compiler.

Generated Schemas

• One JSONSchema file is generated for each root-level proto message and ENUM. These are intended to be stand alone self-contained schemas which can be used to validate a payload derived from their source proto message
• Nested message schemas become referenced "definitions". This means that you know the name of the proto message they came from, and their schema is not duplicated (within the context of one JSONSchema file at least)

Logic

• For each proto file provided
  • Generates schema for each ENUM
    • JSONSchema filename deried from ENUM name
  • Generates schema for each Message
    • Builds a list of every nested message and converts them to JSONSchema
    • Recursively converts attributes and nested messages within the root message
      • Optionally makes all fields required
      • Optionally allows NULL values
      • Optionally allows additional properties
      • Optionally marks all fields required
      • Specially marked fields are labelled required (options.proto)
      • Specially marked fields are omitted (options.proto)
      • Special handling for "OneOf"
      • Special handling for arrays
      • Special handling for maps
    • Injects references to nested messages
    • JSONSchema filename derived from Message name
  • Bundles these into a protoc generator response

Installation

Note: This tool requires Go 1.11+ to be installed.

Install this plugin using Go:

go install github.com/chrusty/protoc-gen-jsonschema/cmd/protoc-gen-jsonschema@latest

Usage

Note: This plugin requires the protoc CLI to be installed.

protoc-gen-jsonschema is designed to run like any other proto generator. The following examples show how to use options flags to enable different generator behaviours (more examples in the Makefile too).

protoc \ # The protobuf compiler
--jsonschema_out=. \ # jsonschema out directory
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto # proto input directories and folders

Configuration Parameters

The following configuration parameters are supported. They should be added to the protoc command and can be combined as a comma-delimited string. Some examples are included in the following Examples section.

Options can also be provided in this format (which is easier on the eye):

protoc \
  --plugin=${HOME}/go/bin/protoc-gen-jsonschema \
  --jsonschema_opt=enforce_oneof
  --jsonschema_opt=file_extension=schema.json \
  --jsonschema_opt=disallow_additional_properties \
  --jsonschema_out=schemas \
  --proto_path=proto

Custom Proto Options

If you don't want to use the configuration parameters (admittedly quite a nasty cli syntax) then some of the generator behaviour can be controlled using custom proto options. These are defined in options.proto, and your protoc command will need to include this file. See the sample protos and generator commands in the Makefile.

Enum Options

These apply to specifically marked enums, giving you more finely-grained control than with the CLI flags.

• enums_as_constants: Encode ENUMs (and their annotations) as CONST
• enums_as_strings_only: ENUM values are only strings (not the numeric counterparts)
• enums_trim_prefix: ENUM values have enum name prefix removed

Field Options

These apply to specifically marked fields, giving you more finely-grained control than with the CLI flags.

• ignore: Ignore (omit) a specific field
• required: Mark a specific field as being REQUIRED

File Options

These options apply to an entire proto file.

• ignore: Ignore (skip) a specific file
• extension: Specify a custom file-extension for the generated schema for this file

Message Options

These options apply to a specific proto message.

• ignore: Ignore (skip) a specific message
• all_fields_required: Mark all fields in a specific message as "required"
• allow_null_values: Additionally allow null values for all fields in a message
• disallow_additional_properties: Only accept the specific properties, no extras
• enums_as_constants: Encode ENUMs (and their annotations) as CONST

Validation Options

We are also beginning to support validation options from protoc-gen-validate.

At the moment the following are supported (but export more in the future):

• Arrays
  • MaxItems
  • MinItems
• Strings
  • MaxLength
  • MinLength
  • Pattern

Examples

Require all fields

Because proto3 doesn't accommodate this.

protoc \
--jsonschema_out=all_fields_required:. \
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto

Allow NULL values

By default, JSONSchemas will reject NULL values unless we explicitly allow them

protoc \
--jsonschema_out=allow_null_values:. \
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto

Enable debug logging

protoc \
--jsonschema_out=debug:. \
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto

Disallow additional properties

JSONSchemas won't validate JSON containing extra parameters

protoc \
--jsonschema_out=disallow_additional_properties:. \
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto

Disallow permissive validation of big-integers as strings

(eg scientific notation)

protoc \
--jsonschema_out=disallow_bigints_as_strings:. \
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto

Prefix generated schema files with their package name (as a directory)

protoc \
--jsonschema_out=prefix_schema_files_with_package:. \
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto

Target specific messages within a proto file

# Generates MessageKind10.jsonschema and MessageKind11.jsonschema
# Use this to generate json schema from proto files with multiple messages
# Separate schema names with '+'
protoc \
--jsonschema_out=messages=[MessageKind10+MessageKind11]:. \
--proto_path=testdata/proto testdata/proto/TwelveMessages.proto

Generate fields with JSON names

protoc \
--jsonschema_out=json_fieldnames:. \
--proto_path=testdata/proto testdata/proto/ArrayOfPrimitives.proto

Custom schema file extension

The default file extension is json. You can override that with the "file_extension" parameter.

protoc \
--jsonschema_out=file_extension=jsonschema:. \
--proto_path=internal/converter/testdata/proto internal/converter/testdata/proto/ArrayOfPrimitives.proto

Generate type names without fully qualified package

By default, referenced type names will be generated using the fully qualified package and type name. e.g packageName.TypeName. Setting this option will generate type names and their references only as TypeName

protoc \
--jsonschema_out=type_names_with_no_package:. \
--proto_path=internal/converter/testdata/proto internal/converter/testdata/proto/ArrayOfPrimitives.proto

Sample protos (for testing)

• Proto with a simple (flat) structure: samples.PayloadMessage
• Proto containing a nested object (defined internally): samples.NestedObject
• Proto containing a nested message (defined in a different proto file): samples.NestedMessage
• Proto containing an array of a primitive types (string, int): samples.ArrayOfPrimitives
• Proto containing an array of objects (internally defined): samples.ArrayOfObjects
• Proto containing an array of messages (defined in a different proto file): samples.ArrayOfMessage
• Proto containing multi-level enums (flat and nested and arrays): samples.Enumception
• Proto containing a stand-alone enum: samples.ImportedEnum
• Proto containing 2 stand-alone enums: samples.FirstEnum, samples.SecondEnum
• Proto containing 2 messages: samples.FirstMessage, samples.SecondMessage
• Proto containing 12 messages: samples.MessageKind1 - samples.MessageKind12

Links

• About JSON Schema
• Popular GoLang JSON-Schema validation library
• Another GoLang JSON-Schema validation library