# Racecar

Introducing Racecar, your friendly and easy-to-approach Kafka consumer framework!

Using [ruby-kafka](https://github.com/zendesk/ruby-kafka) directly can be a challenge: it's a flexible library with lots of knobs and options. Most users don't need that level of flexibility, though. Racecar provides a simple and intuitive way to build and configure Kafka consumers that optionally integrates seemlessly with Rails.

## Installation

Add this line to your application's Gemfile:

```ruby
gem 'racecar'
```

And then execute:

    $ bundle

Or install it yourself as:

    $ gem install racecar

Then execute (if you're in a Rails application):

    $ bundle exec rails generate racecar:install

This will add a config file in `config/racecar.yml`.

## Usage

Racecar is built for simplicity of development and operation. If you need more flexibility, it's quite straightforward to build your own Kafka consumer executables using [ruby-kafka](https://github.com/zendesk/ruby-kafka#consuming-messages-from-kafka) directly.

First, a short introduction to the Kafka consumer concept as well as some basic background on Kafka.

Kafka stores messages in so-called _partitions_ which are grouped into _topics_. Within a partition, each message gets a unique offset.

In Kafka, _consumer groups_ are sets of processes that collaboratively process messages from one or more Kafka topics; they divide up the topic partitions amongst themselves and make sure to reassign the partitions held by any member of the group that happens to crash or otherwise becomes unavailable, thus minimizing the risk of disruption. A consumer in a group is responsible for keeping track of which messages in a partition it has processed – since messages are processed in-order within a single partition, this means keeping track of the _offset_ into the partition that has been processed. Consumers periodically _commit_ these offsets to the Kafka brokers, making sure that another consumer can resume from those positions if there is a crash.

### Creating consumers

A Racecar consumer is a simple Rails class that inherits from `Racecar::Consumer`:

```ruby
class UserBanConsumer < Racecar::Consumer
  subscribes_to "user_banned"

  def process(message)
    data = JSON.parse(message.value)
    user = User.find(data["user_id"])
    user.banned = true
    user.save!
  end
end
```

In order to create your own consumer, run the Rails generator `racecar:consumer`:

    $ bundle exec rails generate racecar:consumer TapDance

This will create a file at `app/consumers/tap_dance_consumer.rb` which you can modify to your liking. Add one or more calls to  `subscribes_to` in order to have the consumer subscribe to Kafka topics.

Now run your consumer with `bundle exec racecar TapDanceConsumer`.

Note: if you're not using Rails, you'll have to add the file yourself. No-one will judge you for copy-pasting it.

#### Initializing consumers

You can optionally add an `initialize` method if you need to do any set-up work before processing messages, e.g.

```ruby
class PushNotificationConsumer < Racecar::Consumer
  subscribes_to "notifications"
  
  def initialize
    @push_service = PushService.new # pretend this exists.
  end
  
  def process(message)
    data = JSON.parse(message.value)
    
    @push_service.notify!(
      recipient: data.fetch("recipient"),
      notification: data.fetch("notification"),
    )
  end
end
```

This is useful to do any one-off work that you wouldn't want to do for each and every message.

#### Setting the starting position

When a consumer is started for the first time, it needs to decide where in each partition to start. By default, it will start at the _beginning_, meaning that all past messages will be processed. If you want to instead start at the _end_ of each partition, change your `subscribes_to` like this:

```ruby
subscribes_to "some-topic", start_from_beginning: false
```

Note that once the consumer has started, it will commit the offsets it has processed until and in the future will resume from those.

### Running consumers

Racecar is first and foremost an executable _consumer runner_. The `racecar` executable takes as argument the name of the consumer class that should be run. Racecar automatically loads your Rails application before starting, and you can load any other library you need by passing the `--require` flag, e.g.

    $ bundle exec racecar --require dance_moves TapDanceConsumer

### Configuration

Racecar provides a flexible way to configure your consumer in a way that feels at home in a Rails application. If you haven't already, run `bundle exec rails generate racecar:install` in order to generate a config file. You'll get a separate section for each Rails environment, with the common configuration values in a shared `common` section.

The possible configuration keys are:

* `brokers` (_optional_) – A list of Kafka brokers in the cluster that you're consuming from. Defaults to `localhost` on port 9092, the default Kafka port.
* `client_id` (_optional_) – A string used to identify the client in logs and metrics.
* `group_id_prefix` (_optional_) – A prefix used when generating consumer group names. For instance, if you set the prefix to be `kevin.` and your consumer class is named `BaconConsumer`, the resulting consumer group will be named `kevin.bacon_consumer`.
* `offset_commit_interval` (_optional_) – How often to save the consumer's position in Kafka.
* `heartbeat_interval` (_optional_) – How often to send a heartbeat message to Kafka.
* `pause_timeout` (_optional_) – How long to pause a partition for if the consumer raises an exception while processing a message.
* `connect_timeout` (_optional_) – How long to wait when trying to connect to a Kafka broker. Default is 10 seconds.
* `socket_timeout` (_optional_) – How long to wait when trying to communicate with a Kafka broker. Default is 30 seconds.
* `max_wait_time` (_optional_) – How long to allow the Kafka brokers to wait before returning messages. A higher number means larger batches, at the cost of higher latency. Default is 5 seconds.

Note that many of these configuration keys correspond directly with similarly named concepts in [ruby-kafka](https://github.com/zendesk/ruby-kafka) for more details on low-level operations, read that project's documentation.

It's also possible to configure Racecar using environment variables. For any given configuration key, there should be a corresponding environment variable with the prefix `RACECAR_`, in upper case. For instance, in order to configure the client id, set `RACECAR_CLIENT_ID=some-id` in the process in which the Racecar consumer is launched. You can set `brokers` by passing a comma-separated list, e.g. `RACECAR_BROKERS=kafka1:9092,kafka2:9092,kafka3:9092`.

### Testing consumers

Since consumers are merely classes that implement a simple interface, they're dead simple to test.

Here's an example of testing a consumer class using [RSpec](http://rspec.info/) and Rails:

```ruby
# app/consumers/create_contacts_consumer.rb
#
# Creates a Contact whenever an email address is written to the
# `email-addresses` topic.
class CreateContactsConsumer < Racecar::Consumer
  subscribes_to "email-addresses"
  
  def process(message)
    email = message.value
    
    Contact.create!(email: email)
  end
end

# spec/consumers/create_contacts_consumer_spec.rb
describe CreateContactsConsumer do
  it "creates a Contact for each email address in the topic" do
    message = double("message", value: "john@example.com")
    consumer = CreateContactsConsumer.new
    
    consumer.process(message)
    
    expect(Contact.where(email: "john@example.com")).to exist
  end
end
```

## 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/zendesk/racecar.

## Copyright and license

Copyright 2015 Zendesk

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License.

You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.