# Avromatic

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`Avromatic` generates Ruby models from [Avro](http://avro.apache.org/) schemas
and provides utilities to encode and decode them.

## Installation

Add this line to your application's Gemfile:

```ruby
gem 'avromatic'
```

And then execute:

    $ bundle

Or install it yourself as:

    $ gem install avromatic

## 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).
  
#### 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 `AvroTurf::SchemaRegistry` object used to store Avro schemas 
  so that they can be referenced by id. Either `schema_registry` or 
  `registry_url` must be configured.
* **registry_url**: URL for the schema registry. Either `schema_registry` or 
  `registry_url` must be configured.
* **messaging**: An `AvroTurf::Messaging` object to be shared by all generated models.
  The `build_messaging!` method may be used to create a `Messaging` instance based
  on the other configuration values.
* **logger**: The logger to use for the schema registry client.
* [Custom Types](#custom-types)

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
```

### 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
```

Or an `Avro::Schema` object can be specified directly:

```ruby
class MyModel
  include Avromatic::Model.build(schema: schema_object)
end
```

Models are generated as [Virtus](https://github.com/solnic/virtus) value
objects. `Virtus` attributes are added for each field in the Avro schema
including any default values defined in the schema. `ActiveModel` validations
are used to define validations on certain types of fields ([see below](#validations)).

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.

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)
```

#### Custom Types

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)
```

#### 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

The following validations are supported:

- The size of the value for a fixed type field.
- The value for an enum type field is in the declared set of values.
- Presence of a value for required fields.

### Unsupported/Future

The following types/features are not supported for generated models:

- Generic union fields: The special case of an optional field, the union of `:null` and
  another type, is supported.
- Reused models for nested records: Currently an anonymous model class is
  generated for each subrecord.

## 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).