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avromatic
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Generate Ruby models from Avro schemas
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# Avromatic [][circleci] [](https://badge.fury.io/rb/avromatic) [circleci]: https://circleci.com/gh/salsify/avromatic `Avromatic` generates Ruby models from [Avro](http://avro.apache.org/) schemas and provides utilities to encode and decode them. **This README reflects Avromatic 2.0. Please see the [1-0-stable](https://github.com/salsify/avromatic/blob/1-0-stable/README.md) branch for Avromatic 1.0.** ## Installation Add this line to your application's Gemfile: ```ruby gem 'avromatic' ``` And then execute: $ bundle Or install it yourself as: $ gem install avromatic See the [Logical Types](#logical-types) section below for details on using Avromatic with unreleased Avro features. ## Usage ### Configuration `Avromatic` supports the following configuration: #### Model Generation * **schema_store**: A schema store is required to load Avro schemas from the filesystem. It should be an object that responds to `find(name, namespace = nil)` and returns an `Avro::Schema` object. An `AvroTurf::SchemaStore` can be used. The `schema_store` is unnecessary if models are generated directly from `Avro::Schema` objects. See [Models](#models). * **nested_models**: An optional [ModelRegistry](https://github.com/salsify/avromatic/blob/master/lib/avromatic/model_registry.rb) that is used to store, by full schema name, the generated models that are embedded within top-level models. By default a new `Avromatic::ModelRegistry` is created. * **eager_load_models**: An optional array of models, or strings with class names for models, that are added to `nested_models` at the end of `Avromatic.configure` and during code reloading in Rails applications. This option is useful for defining models that will be extended when the load order is important. * **allow_unknown_attributes**: Optionally allow model constructors to silently ignore unknown attributes. Defaults to `false`. WARNING: Setting this to `true` will result in incorrect union member coercions if an earlier union member is satisfied by a subset of the latter union member's attributes. #### Custom Types See the section below on configuring [Custom Types](#custom-type-configuration). #### Using a Schema Registry/Messaging API The configuration options below are required when using a schema registry (see [Confluent Schema Registry](http://docs.confluent.io/2.0.1/schema-registry/docs/intro.html)) and the [Messaging API](#messaging-api). * **schema_registry**: An `AvroSchemaRegistry::Client` or `AvroTurf::ConfluentSchemaRegistry` object used to store Avro schemas so that they can be referenced by id. Either `schema_registry` or `registry_url` must be configured. If using `build_schema_registry!`, only `registry_url` is required. See example below. * **registry_url**: URL for the schema registry. This must be configured when using `build_schema_registry!`. The `build_schema_registry!` method may be used to create a caching schema registry client instance based on other configuration values. * **use_schema_fingerprint_lookup**: Avromatic supports a Schema Registry [extension](https://github.com/salsify/avro-schema-registry#extensions) that provides an endpoint to lookup existing schema ids by fingerprint. A successful response from this GET request can be cached indefinitely. The use of this additional endpoint can be disabled by setting this option to `false` and this is recommended if using a Schema Registry that does not support the endpoint. * **messaging**: An `AvroTurf::Messaging` object to be shared by all generated models The `build_messaging!` method may be used to create a `Avromatic::Messaging` instance based on the other configuration values. * **logger**: The logger to use for the schema registry client. Example using a schema registry: ```ruby Avromatic.configure do |config| config.schema_store = AvroTurf::SchemaStore.new(path: 'avro/schemas') config.registry_url = Rails.configuration.x.avro_schema_registry_url config.build_messaging! end ``` NOTE: `build_messaging!` ultimately calls `build_schema_registry!` so you don't have to call both. #### Decoding * **use_custom_datum_reader**: `Avromatic` includes a modified subclass of `Avro::IO::DatumReader`. This subclass returns additional information about the index of union members when decoding Avro messages. This information is used to optimize model creation when decoding. By default this information is included in the hash returned by the `DatumReader` but can be omitted by setting this option to `false`. #### Encoding * **use_custom_datum_writer**: `Avromatic` includes a modified subclass of `Avro::IO::DatumWriter`. This subclass supports caching avro encodings for immutable models and uses additional information about the index of union members to optimize the encoding of Avro messages. By default this information is included in the hash passed to the encoder but can be omitted by setting this option to `false`. ### Models Models are defined based on an Avro schema for a record. The Avro schema can be specified by name and loaded using the schema store: ```ruby class MyModel include Avromatic::Model.build(schema_name: :my_model) end # Construct instances by passing in a hash of attributes instance = MyModel.new(id: 123, name: 'Tesla Model 3', enabled: true) # Access attribute values with readers instance.name # => "Tesla Model 3" # Models are immutable by default instance.name = 'Tesla Model X' # => NoMethodError (private method `name=' called for #<MyModel:0x00007ff711e64e60>) # Booleans can also be accessed by '?' readers that coerce nil to false instance.enabled? # => true # Models implement ===, eql? and hash instance == MyModel.new(id: 123, name: 'Tesla Model 3', enabled: true) # => true instance.eql?(MyModel.new(id: 123, name: 'Tesla Model 3', enabled: true)) # => true instance.hash # => -1279155042741869898 # Retrieve a hash of the model's attributes via to_h, to_hash or attributes instance.to_h # => {:id=>123, :name=>"Tesla Model 3", :enabled=>true} ``` Or an `Avro::Schema` object can be specified directly: ```ruby class MyModel include Avromatic::Model.build(schema: schema_object) end ``` A specific subject name can be associated with the schema: ```ruby class MyModel include Avromatic::Model.build(schema_name: 'my_model', schema_subject: 'my_model-value') end ``` Models are generated as immutable value objects by default, but can optionally be defined as mutable: ```ruby class MyModel include Avromatic::Model.build(schema_name: :my_model, mutable: true) end ``` Generated models include attributes for each field in the Avro schema including any default values defined in the schema. A model may be defined with both a key and a value schema: ```ruby class MyTopic include Avromatic::Model.build(value_schema_name: :topic_value, key_schema_name: :topic_key) end ``` When key and value schemas are both specified, attributes are added to the model for the union of the fields in the two schemas. By default, optional fields are not allowed in key schemas since their values may be accidentally omitted leading to problems if data is partitioned based on the key values. This behavior can be overridden by specifying the `:allow_optional_key_fields` option for the model: ```ruby class MyTopic include Avromatic::Model.build(value_schema_name: :topic_value, key_schema_name: :topic_key, allow_optional_key_fields: true) end ``` A specific subject name can be associated with both the value and key schemas: ```ruby class MyTopic include Avromatic::Model.build(value_schema_name: :topic_value, value_schema_subject: 'topic_value-value', key_schema_name: :topic_key, key_schema_subject: 'topic_key-value') end ``` A model can also be generated as an anonymous class that can be assigned to a constant: ```ruby MyModel = Avromatic::Model.model(schema_name :my_model) ``` #### Experimental: Union Support Avromatic contains experimental support for unions containing more than one non-null member type. This feature is experimental because Avromatic may attempt to coerce between types too aggressively. For now, if a union contains [nested models](#nested-models) then it is recommended that you assign model instances. Some combination of the ordering of member types in the union and relying on model validation may be required so that the correct member is selected, especially when deserializing from Avro. In the future, the type coercion used in the gem will be enhanced to better support the union use case. #### Nested Models Nested models are models that are embedded within top-level models generated using Avromatic. Normally these nested models are automatically generated. By default, nested models are stored in `Avromatic.nested_models`. This is an `Avromatic::ModelRegistry` instance that provides access to previously generated nested models by the full name of their Avro schema. ```ruby Avromatic.nested_models['com.my_company.test.example'] #=> <model class> ``` The `ModelRegistry` can be customized to remove a namespace prefix: ```ruby Avromatic.nested_models = Avromatic::ModelRegistry.new(remove_namespace_prefix: 'com.my_company') ``` The `:remove_namespace_prefix` value can be a string or a regexp. By default, top-level generated models reuse `Avromatic.nested_models`. This allows nested models to be shared across different generated models. A `:nested_models` option can be specified when generating a model. This allows the reuse of nested models to be scoped: ```ruby Avromatic::Model.model(schema_name, :my_model nested_models: ModelRegistry.new) ``` Only models without a key schema can be used as nested models. When a model is generated with just a value schema then it is automatically registered so that it can be used as a nested model. To extend a model that will be used as a nested model, you must ensure that it is defined, which will register it, prior it being referenced by another model. Using the `Avromatic.eager_load_models` option allows models that are extended and will be used as nested models to be defined at the end of the `.configure` block. In Rails applications, these models are also re-registered after `nested_models` is cleared when code reloads to ensure that classes load in the correct order: ```ruby Avromatic.configure do |config| config.eager_load_models = [ # reference any extended models that should be defined first 'MyNestedModel' ] end ``` #### Custom Type Configuration Custom types can be configured for fields of named types (record, enum, fixed). These customizations are registered on the `Avromatic` module. Once a custom type is registered, it is used for all models with a schema that references that type. It is recommended to register types within a block passed to `Avromatic.configure`: ```ruby Avromatic.configure do |config| config.register_type('com.example.my_string', MyString) end ``` The full name of the type and an optional class may be specified. When a class is provided then values for attributes of that type are defined using the specified class. If the provided class responds to the class methods `from_avro` and `to_avro` then those methods are used to convert values when assigning to the model and before encoding using Avro respectively. `from_avro` and `to_avro` methods may be also be specified as Procs when registering the type: ```ruby Avromatic.configure do |config| config.register_type('com.example.updown_string') do |type| type.from_avro = ->(value) { value.upcase } type.to_avro = ->(value) { value.downcase } end end ``` Nil handling is not required as the conversion methods are not be called if the inbound or outbound value is nil. If a custom type is registered for a record-type field, then any `to_avro` method/Proc should return a Hash with string keys for encoding using Avro. ### Encoding and Decoding `Avromatic` provides two different interfaces for encoding the key (optional) and value associated with a model. #### Manually Managed Schemas The attributes for the value schema used to define a model can be encoded using: ```ruby encoded_value = model.avro_raw_value ``` In order to decode this data, a copy of the value schema is required. If a model also has an Avro schema for a key, then the key attributes can be encoded using: ```ruby encoded_key = model.avro_raw_key ``` If attributes were encoded using the same schema(s) used to define a model, then the data can be decoded to create a new model instance: ```ruby MyModel.avro_raw_decode(key: encoded_key, value: encoded_value) ``` If the attributes where encoded using a different version of the model's schemas, then a new model instance can be created by also providing the schemas used to encode the data: ```ruby MyModel.avro_raw_decode(key: encoded_key, key_schema: writers_key_schema, value: encoded_value, value_schema: writers_value_schema) ``` #### Messaging API The other interface for encoding and decoding attributes uses the `AvroTurf::Messaging` API. This interface leverages a schema registry and prefixes the encoded data with an id to identify the schema. In this approach, a schema registry is used to ensure that the correct schemas are available during decoding. The attributes for the value schema can be encoded with a schema id prefix using: ```ruby message_value = model.avro_message_value ``` If a model has an Avro schema for a key, then those attributes can also be encoded prefixed with a schema id: ```ruby message_key = model.avro_message_key ``` A model instance can be created from a key and value encoded in this manner: ```ruby MyTopic.avro_message_decode(message_key, message_value) ``` Or just a value if only one schema is used: ```ruby MyValue.avro_message_decode(message_value) ``` The schemas associated with a model can also be added to a schema registry without encoding a message: ```ruby MyTopic.register_schemas! ``` #### Avromatic::Model::MessageDecoder A stream of messages encoded from various models using the messaging approach can be decoded using `Avromatic::Model::MessageDecoder`. The decoder must be initialized with the list of models to decode: ```ruby decoder = Avromatic::Model::MessageDecoder.new(MyModel1, MyModel2) decoder.decode(model1_messge_key, model1_message_value) # => instance of MyModel1 decoder.decode(model2_message_value) # => instance of MyModel2 ``` ### Validations and Coercions An exception will be thrown if an attribute value cannot be coerced to the corresponding Avro schema field's type. The following coercions are supported: | Ruby Type | Avro Type | | --------- | --------- | | String, Symbol | string | | Array | array | | Hash | map | | Integer | int | | Integer | long | | Float, Integer | float | | Float, Integer | double | | String | bytes | | Date, Time, DateTime | date | | Time, DateTime | timestamp-millis | | Time, DateTime | timestamp-micros | | Float, Integer, BigDecimal | decimal | | TrueClass, FalseClass | boolean | | NilClass | null | | Hash | record | Validation of required fields is done automatically when serializing a model to Avro. It can also be done explicitly by calling the `valid?` or `invalid?` methods from the [ActiveModel::Validations](https://edgeapi.rubyonrails.org/classes/ActiveModel/Validations.html) interface. ### RSpec Support This gem also includes an `"avromatic/rspec"` file that can be required to support using Avromatic with a fake schema registry during tests. Requiring this file configures a RSpec before hook that directs any schema registry requests to a fake, in-memory schema registry (instead of port 21001) and rebuilds the `Avromatic::Messaging` object for each example. Note: Use of `avromatic/rspec` requires installing the `sinatra` gem for the in-memory schema registry to work properly. ## Development After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake spec` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment. To install this gem onto your local machine, run `bundle exec rake install`. To release a new version, update the version number in `version.rb`, and then run `bundle exec rake release`, which will create a git tag for the version, push git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org). ## Contributing Bug reports and pull requests are welcome on GitHub at https://github.com/salsify/avromatic. ## License The gem is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT).
