Phobos
Simplifying Kafka for Ruby apps!
Phobos is a micro framework and library for applications dealing with Apache Kafka.
- It wraps common behaviors needed by consumers and producers in an easy and convenient API
- It uses ruby-kafka as its Kafka client and core component
- It provides a CLI for starting and stopping a standalone application ready to be used for production purposes
Why Phobos? Why not ruby-kafka
directly? Well, ruby-kafka
is just a client. You still need to write a lot of code to manage proper consuming and producing of messages. You need to do proper message routing, error handling, retrying, backing off and maybe logging/instrumenting the message management process. You also need to worry about setting up a platform independent test environment that works on CI as well as any local machine, and even on your deployment pipeline. Finally, you also need to consider how to deploy your app and how to start it.
With Phobos by your side, all this becomes smooth sailing.
Table of Contents
- Installation
- Usage
- Standalone apps
- Consuming messages from Kafka
- Producing messages to Kafka
- As library in another app
- Configuration file
- Instrumentation
- Plugins
- Development
- Test
Installation
Add this line to your application's Gemfile:
gem 'phobos'
And then execute:
$ bundle
Or install it yourself as:
$ gem install phobos
Usage
Phobos can be used in two ways: as a standalone application or to support Kafka features in your existing project - including Rails apps. It provides a CLI tool to run it.
Standalone apps
Standalone apps have benefits such as individual deploys and smaller code bases. If consuming from Kafka is your version of microservices, Phobos can be of great help.
Setup
To create an application with Phobos you need two things:
- A configuration file (more details in the Configuration file section)
- A
phobos_boot.rb
(or the name of your choice) to properly load your code into Phobos executor
Use the Phobos CLI command init to bootstrap your application. Example:
# call this command inside your app folder
$ phobos init
create config/phobos.yml
create phobos_boot.rb
phobos.yml
is the configuration file and phobos_boot.rb
is the place to load your code.
Consumers (listeners and handlers)
In Phobos apps listeners are configured against Kafka - they are our consumers. A listener requires a handler (a ruby class where you should process incoming messages), a Kafka topic, and a Kafka group_id. Consumer groups are used to coordinate the listeners across machines. We write the handlers and Phobos makes sure to run them for us. An example of a handler is:
class MyHandler
include Phobos::Handler
def consume(payload, metadata)
# payload - This is the content of your Kafka message, Phobos does not attempt to
# parse this content, it is delivered raw to you
# metadata - A hash with useful information about this event, it contains: The event key,
# partition number, offset, retry_count, topic, group_id, and listener_id
end
end
Writing a handler is all you need to allow Phobos to work - it will take care of execution, retries and concurrency.
To start Phobos the start command is used, example:
$ phobos start
[2016-08-13T17:29:59:218+0200Z] INFO -- Phobos : <Hash> {:message=>"Phobos configured", :env=>"development"}
______ _ _
| ___ \ | | |
| |_/ / |__ ___ | |__ ___ ___
| __/| '_ \ / _ \| '_ \ / _ \/ __|
| | | | | | (_) | |_) | (_) \__ \
\_| |_| |_|\___/|_.__/ \___/|___/
phobos_boot.rb - find this file at ~/Projects/example/phobos_boot.rb
[2016-08-13T17:29:59:272+0200Z] INFO -- Phobos : <Hash> {:message=>"Listener started", :listener_id=>"6d5d2c", :group_id=>"test-1", :topic=>"test"}
By default, the start command will look for the configuration file at config/phobos.yml
and it will load the file phobos_boot.rb
if it exists. In the example above all example files generated by the init command are used as is. It is possible to change both files, use -c
for the configuration file and -b
for the boot file. Example:
$ phobos start -c /var/configs/my.yml -b /opt/apps/boot.rb
You may also choose to configure phobos with a hash from within your boot file.
In this case, disable loading the config file with the --skip-config
option:
$ phobos start -b /opt/apps/boot.rb --skip-config
Consuming messages from Kafka
Messages from Kafka are consumed using handlers. You can use Phobos executors or include it in your own project as a library, but handlers will always be used. To create a handler class, simply include the module Phobos::Handler
. This module allows Phobos to manage the life cycle of your handler.
A handler is required to implement the method #consume(payload, metadata)
.
Instances of your handler will be created for every message, so keep a constructor without arguments. If consume
raises an exception, Phobos will retry the message indefinitely, applying the back off configuration presented in the configuration file. The metadata
hash will contain a key called retry_count
with the current number of retries for this message. To skip a message, simply return from #consume
.
When the listener starts, the class method .start
will be called with the kafka_client
used by the listener. Use this hook as a chance to setup necessary code for your handler. The class method .stop
will be called during listener shutdown.
class MyHandler
include Phobos::Handler
def self.start(kafka_client)
# setup handler
end
def self.stop
# teardown
end
def consume(payload, metadata)
# consume or skip message
end
end
It is also possible to control the execution of #consume
with the method #around_consume(payload, metadata)
. This method receives the payload and metadata, and then invokes #consume
method by means of a block; example:
class MyHandler
include Phobos::Handler
def around_consume(payload, metadata)
Phobos.logger.info "consuming..."
output = yield
Phobos.logger.info "done, output: #{output}"
end
def consume(payload, metadata)
# consume or skip message
end
end
Note: around_consume
was previously defined as a class method. The current code supports both implementations, giving precendence to the class method, but future versions will no longer support .around_consume
.
class MyHandler
include Phobos::Handler
def self.around_consume(payload, metadata)
Phobos.logger.info "consuming..."
output = yield
Phobos.logger.info "done, output: #{output}"
end
def consume(payload, metadata)
# consume or skip message
end
end
Finally, it is also possible to preprocess the message payload before consuming it using the before_consume
hook which is invoked before #around_consume
and #consume
. The result of this operation will be assigned to payload, so it is important to return the modified payload. This can be very useful, for example if you want a single point of decoding Avro messages and want the payload as a hash instead of a binary.
class MyHandler
include Phobos::Handler
def before_consume(payload, metadata)
# optionally preprocess payload
payload
end
end
Take a look at the examples folder for some ideas.
The hander life cycle can be illustrated as:
.start
-> #consume
-> .stop
or optionally,
.start
-> #before_consume
-> #around_consume
[ #consume
] -> .stop
Batch Consumption
In addition to the regular handler, Phobos provides a BatchHandler
. The
basic ideas are identical, except that instead of being passed a single message
at a time, the BatchHandler
is passed a batch of messages. All methods
follow the same pattern as the regular handler except that they each
end in _batch
and are passed an array of Phobos::BatchMessage
s instead
of a single payload.
To enable handling of batches on the consumer side, you must specify
a delivery method of inline_batch
in phobos.yml,
and your handler must include BatchHandler
. Using a delivery method of batch
assumes that you are still processing the messages one at a time and should
use Handler
.
class MyBatchHandler
includes Phobos::BatchHandler
def before_consume_batch(payloads, metadata)
payloads.each do |p|
p.payload[:timestamp] = Time.zone.now
end
end
def around_consume_batch(payloads, metadata)
yield
end
def consume_batch(payloads, metadata)
payloads.each do |p|
logger.info("Got payload #{p.payload}, #{p.partition}, #{p.offset}, #{p.key}, #{p.payload[:timestamp]}")
end
end
end
Note that retry logic will happen on the batch level in this case. If you are
processing messages individually and an error happens in the middle, Phobos's
retry logic will retry the entire batch. If this is not the behavior you want,
consider using batch
instead of inline_batch
.
Producing messages to Kafka
ruby-kafka
provides several options for publishing messages, Phobos offers them through the module Phobos::Producer
. It is possible to turn any ruby class into a producer (including your handlers), just include the producer module, example:
class MyProducer
include Phobos::Producer
end
Phobos is designed for multi threading, thus the producer is always bound to the current thread. It is possible to publish messages from objects and classes, pick the option that suits your code better.
The producer module doesn't pollute your classes with a thousand methods, it includes a single method the class and in the instance level: producer
.
my = MyProducer.new
my.producer.publish('topic', 'message-payload', 'partition and message key')
# The code above has the same effect of this code:
MyProducer.producer.publish('topic', 'message-payload', 'partition and message key')
It is also possible to publish several messages at once:
MyProducer
.producer
.publish_list([
{ topic: 'A', payload: 'message-1', key: '1' },
{ topic: 'B', payload: 'message-2', key: '2' },
{ topic: 'B', payload: 'message-3', key: '3' }
])
There are two flavors of producers: regular producers and async producers.
Regular producers will deliver the messages synchronously and disconnect, it doesn't matter if you use publish
or publish_list
after the messages get delivered the producer will disconnect.
Async producers will accept your messages without blocking, use the methods async_publish
and async_publish_list
to use async producers.
An example of using handlers to publish messages:
class MyHandler
include Phobos::Handler
include Phobos::Producer
PUBLISH_TO = 'topic2'
def consume(payload, metadata)
producer.async_publish(PUBLISH_TO, {key: 'value'}.to_json)
end
end
Note about configuring producers
Since the handler life cycle is managed by the Listener, it will make sure the producer is properly closed before it stops. When calling the producer outside a handler remember, you need to shutdown them manually before you close the application. Use the class method async_producer_shutdown
to safely shutdown the producer.
Without configuring the Kafka client, the producers will create a new one when needed (once per thread). To disconnect from kafka call kafka_client.close
.
# This method will block until everything is safely closed
MyProducer
.producer
.async_producer_shutdown
MyProducer
.producer
.kafka_client
.close
Phobos as a library in an existing project
When running as a standalone service, Phobos sets up a Listener
and Executor
for you. When you use Phobos as a library in your own project, you need to set these components up yourself.
First, call the method configure
with the path of your configuration file or with configuration settings hash.
Phobos.configure('config/phobos.yml')
or
Phobos.configure(kafka: { client_id: 'phobos' }, logger: { file: 'log/phobos.log' })
Listener connects to Kafka and acts as your consumer. To create a listener you need a handler class, a topic, and a group id.
listener = Phobos::Listener.new(
handler: Phobos::EchoHandler,
group_id: 'group1',
topic: 'test'
)
# start method blocks
Thread.new { listener.start }
listener.id # 6d5d2c (all listeners have an id)
listener.stop # stop doesn't block
This is all you need to consume from Kafka with back off retries.
An executor is the supervisor of all listeners. It loads all listeners configured in phobos.yml
. The executor keeps the listeners running and restarts them when needed.
executor = Phobos::Executor.new
# start doesn't block
executor.start
# stop will block until all listers are properly stopped
executor.stop
When using Phobos executors you don't care about how listeners are created, just provide the configuration under the listeners
section in the configuration file and you are good to go.
Configuration file
The configuration file is organized in 6 sections. Take a look at the example file, config/phobos.yml.example.
The file will be parsed through ERB so ERB syntax/file extension is supported beside the YML format.
logger configures the logger for all Phobos components. It automatically
outputs to STDOUT
and it saves the log in the configured file.
kafka provides configurations for every Kafka::Client
created over the application.
All options supported by ruby-kafka
can be provided.
producer provides configurations for all producers created over the application,
the options are the same for regular and async producers.
All options supported by ruby-kafka
can be provided.
consumer provides configurations for all consumer groups created over the application.
All options supported by ruby-kafka
can be provided.
backoff Phobos provides automatic retries for your handlers. If an exception is raised, the listener will retry following the back off configured here. Backoff can also be configured per listener.
listeners is the list of listeners configured. Each listener represents a consumer group.
Additional listener configuration
In some cases it's useful to share most of the configuration between
multiple phobos processes, but have each process run different listeners. In
that case, a separate yaml file can be created and loaded with the -l
flag.
Example:
$ phobos start -c /var/configs/my.yml -l /var/configs/additional_listeners.yml
Note that the config file must still specify a listeners section, though it can be empty.
Custom configuration/logging
Phobos can be configured using a hash rather than the config file directly. This can be useful if you want to do some pre-processing before sending the file to Phobos. One particularly useful aspect is the ability to provide Phobos with a custom logger, e.g. by reusing the Rails logger:
Phobos.configure(
custom_logger: Rails.logger,
custom_kafka_logger: Rails.logger
)
If these keys are given, they will override the logger
keys in the Phobos
config file.
Instrumentation
Some operations are instrumented using Active Support Notifications.
In order to receive notifications you can use the module Phobos::Instrumentation
, example:
Phobos::Instrumentation.subscribe('listener.start') do |event|
puts(event.payload)
end
Phobos::Instrumentation
is a convenience module around ActiveSupport::Notifications
, feel free to use it or not. All Phobos events are in the phobos
namespace. Phobos::Instrumentation
will always look at phobos.
events.
Executor notifications
executor.retry_listener_error
is sent when the listener crashes and the executor wait for a restart. It includes the following payload:- listener_id
- retry_count
- waiting_time
- exception_class
- exception_message
- backtrace
executor.stop
is sent when executor stops
Listener notifications
listener.start_handler
is sent when invokinghandler.start(kafka_client)
. It includes the following payload:- listener_id
- group_id
- topic
- handler
listener.start
is sent when listener starts. It includes the following payload:- listener_id
- group_id
- topic
- handler
listener.process_batch
is sent after process a batch. It includes the following payload:- listener_id
- group_id
- topic
- handler
- batch_size
- partition
- offset_lag
- highwater_mark_offset
listener.process_message
is sent after processing a message. It includes the following payload:- listener_id
- group_id
- topic
- handler
- key
- partition
- offset
- retry_count
listener.process_batch_inline
is sent after processing a batch withbatch_inline
mode. It includes the following payload:- listener_id
- group_id
- topic
- handler
- batch_size
- partition
- offset_lag
- retry_count
listener.retry_handler_error
is sent after waiting forhandler#consume
retry. It includes the following payload:- listener_id
- group_id
- topic
- handler
- key
- partition
- offset
- retry_count
- waiting_time
- exception_class
- exception_message
- backtrace
listener.retry_handler_error_batch
is sent after waiting forhandler#consume_batch
retry. It includes the following payload:- listener_id
- group_id
- topic
- handler
- batch_size
- partition
- offset_lag
- retry_count
- waiting_time
- exception_class
- exception_message
- backtrace
listener.retry_aborted
is sent after waiting for a retry but the listener was stopped before the retry happened. It includes the following payload:- listener_id
- group_id
- topic
- handler
listener.stopping
is sent when the listener receives signal to stop.- listener_id
- group_id
- topic
- handler
listener.stop_handler
is sent after stopping the handler.- listener_id
- group_id
- topic
- handler
listener.stop
is send after stopping the listener.- listener_id
- group_id
- topic
- handler
Plugins
List of gems that enhance Phobos:
-
Phobos DB Checkpoint is drop in replacement to Phobos::Handler, extending it with the following features:
- Persists your Kafka events to an active record compatible database
- Ensures that your handler will consume messages only once
- Allows your system to quickly reprocess events in case of failures
-
Phobos Checkpoint UI gives your Phobos DB Checkpoint powered app a web gui with the features below. Maintaining a Kafka consumer app has never been smoother:
- Search events and inspect payload
- See failures and retry / delete them
-
Phobos Prometheus adds prometheus metrics to your phobos consumer.
- Measures total messages and batches processed
- Measures total duration needed to process each message (and batch)
- Adds
/metrics
endpoit to scrape data
Development
After checking out the repo:
- make sure
docker
is installed and running (for windows and mac this also includesdocker-compose
). - Linux: make sure
docker-compose
is installed and running. - run
bin/setup
to install dependencies - run
docker-compose up
to start the required kafka containers in a window - run
rspec
to run the tests in another window
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.
Test
Phobos exports a spec helper that can help you test your consumer. The Phobos lifecycle will conveniently be activated for you with minimal setup required.
process_message(handler:, payload:, metadata: {}, encoding: nil)
- Invokes your handler with payload and metadata, using a dummy listener (encoding and metadata are optional).
require 'spec_helper'
describe MyConsumer do
let(:payload) { 'foo' }
let(:metadata) { Hash(foo: 'bar') }
it 'consumes my message' do
expect_any_instance_of(described_class).to receive(:around_consume).with(payload, metadata).once.and_call_original
expect_any_instance_of(described_class).to receive(:before_consume).with(payload, metadata).once.and_call_original
expect_any_instance_of(described_class).to receive(:consume).with(payload, metadata).once.and_call_original
process_message(handler: described_class, payload: payload, metadata: metadata)
end
end
Contributing
Bug reports and pull requests are welcome on GitHub at https://github.com/klarna/phobos.
Linting
Phobos projects Rubocop to lint the code, and in addition all projects use Rubocop Rules to maintain a shared rubocop configuration. Updates to the shared configurations are done in phobos/shared repo, where you can also find instructions on how to apply the new settings to the Phobos projects.
Acknowledgements
Thanks to Sebastian Norde for the awesome logo!
License
Copyright 2016 Klarna
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.