Consuming messages
Consumers should inherit from the ApplicationConsumer. You need to define a #consume
method that will execute your business logic code against a batch of messages.
Karafka fetches and consumes messages in batches by default.
Consuming messages
Karafka framework has a long-running server process responsible for fetching and consuming messages.
To start the Karafka server process, use the following CLI command:
bundle exec karafka server
In batches
Data fetched from Kafka is accessible using the #messages
method. The returned object is an enumerable containing received data and additional information that can be useful during the processing.
To access the payload of your messages, you can use the #payload
method available for each received message:
class EventsConsumer < ApplicationConsumer
def consume
# Print all the payloads one after another
messages.each do |message|
puts message.payload
end
end
end
You can also access all the payloads together to elevate things like batch DB operations available for some of the ORMs:
class EventsConsumer < ApplicationConsumer
def consume
# Insert all the events at once with a single query
Event.insert_all messages.payloads
end
end
One at a time
While we encourage you to process data in batches to elevate in-memory computation and many DBs batch APIs, you may want to process messages one at a time.
You can achieve this by defining a base consumer with such a capability:
class SingleMessageBaseConsumer < Karafka::BaseConsumer
attr_reader :message
def consume
messages.each do |message|
@message = message
consume_one
mark_as_consumed(message)
end
end
end
class Consumer < SingleMessageBaseConsumer
def consume_one
puts "I received following message: #{message.payload}"
end
end
Accessing topic details
If, in any case, your logic is dependent on some routing details, you can access them from the consumer using the #topic
method. You could use it, for example, in case you want to perform a different logic within a single consumer based on the topic from which your messages come:
class UsersConsumer < ApplicationConsumer
def consume
send(:"topic_#{topic.name}")
end
def topic_a
# do something
end
def topic_b
# do something else if it's a "b" topic
end
end
If you're interested in all the details that are stored in the topic, you can extract all of them at once, by using the #to_h
method:
class UsersConsumer < ApplicationConsumer
def consume
puts topic.to_h #=> { name: 'x', ... }
end
end
Consuming from earliest or latest offset
Karafka, by default, will start consuming messages from the earliest it can reach. You can, however configure it to start consuming from the latest message by setting the initial_offset
value:
# This will start from the earliest (default)
class KarafkaApp < Karafka::App
setup do |config|
config.initial_offset = 'earliest'
end
end
# This will make Karafka start consuming from the latest message on a given topic
class KarafkaApp < Karafka::App
setup do |config|
config.initial_offset = 'latest'
end
end
Note: This setting applies only to the first execution of a Karafka process. All following executions will pick up from the last offset where the process ended previously.
Detecting revocation midway
Karafka will invoke a consumer method called #revoked
(if defined) each time a partition is revoked. It may happen, though, that the revocation occurs during the processing.
Both #mark_as_consumed
and #mark_as_consumed!
return a boolean result that indicates whether the given topic partition is still owned by the consumer and set its revocation state. You can use this result to terminate processing early midway.
Once you mark the message as consumed, you can also use the #revoked?
to check the revocation state.
def consume
messages.each do |message|
Message.create!(message)
mark_as_consumed(message)
return if revoked?
end
end
In most cases, especially if you do not use Long-Running Jobs, the Karafka default offset management strategy should be more than enough. It ensures that after batch processing as well as upon rebalances, before partition reassignment, all the offsets are committed. In a healthy system with stable deployment procedures and without frequent short-lived consumer generations, the number of re-processings should be close to zero.
Note: You need to mark the message as consumed for the #revoked?
method result to change.
Note: When using the Long-Running Jobs feature, #revoked?
result changes independently from marking messages.
Consumer persistence
Karafka consumer instances are persistent by default. This means that a single consumer instance will "live" as long as a given process instance consumes a given topic partition. This means you can elevate in-memory processing and buffering to achieve better performance.
Karafka consumer instance for a given topic partition will be re-created in case a given partition is lost and re-assigned.
Note: if you decide to utilize such techniques, you may be better with manual offset management.
# A consumer that will buffer messages in memory until it reaches 1000 of them. Then it will flush
# and commit the offset.
class EventsConsumer < ApplicationConsumer
# Flush every 1000 messages
MAX_BUFFER_SIZE = 1_000
def initialize
@buffer = []
end
def consume
# Print all the payloads one after another
@buffer += messages.payloads
return if @buffer.size < MAX_BUFFER_SIZE
flush
end
private
def flush
Event.insert_all @buffer
mark_as_consumed @buffer.last
@buffer.clear!
end
end
Shutdown and partition revocation handlers
Karafka consumer, aside from the #consume
method, allows you to define two additional methods that you can use to free any resources that you may be using upon certain events. Those are:
#revoked
- will be executed when there is a rebalance resulting in the given partition being revoked from the current process.#shutdown
- will be executed when the Karafka process is being shutdown.
class LogsConsumer < ApplicationConsumer
def initialize
@log = File.open('log.txt', 'a')
end
def consume
messages.each do |message|
@log << message.raw_payload
end
end
def shutdown
@log.close
end
def revoked
@log.close
end
end