Introduction and Overview of Apache Kafka, TriHUG July 23, 2013

Apache Kafka
//TODO Insert Logo

About Me
David Arthur
http://mumrah.github.io/
● Software Engineer at LucidWorks
● Open source contributor
● Gardener
● Dad

TOC
● Project overview
● Architecture
● Implementation
● Miscellany

Project info
● http://kafka.apache.org
● Written in Scala
● Open sourced by LinkedIn SNA in 2011
● Soon after entered Apache Incubator
● Not just an idle "open source donation"
● Active development, 0.8 out now
● Apache TLP late 2012, 13 committers
● Still no logo

12 word overview
Apache Kafka is publish-subscribe messaging
rethought as a distributed commit log

Ok...
Kafka is a persistent, distributed, replicated
pub/sub messaging system. Publishers send
messages to a cluster of brokers. The brokers
persist the messages to disk.
Consumers then request a range of messages
using an (offset, length) style API. Use of NIO
FileChannel allows for very fast transfer of
data in and out of the system.

Highlights
● ZooKeeper for Broker coordination
● Configurable Producer acks
● Consumer Groups
● TTL persistence
● Sync/Async producer API
● Durable
● Scalable
● Fast
● <Additional buzzwords>

Motivation
● Activity stream processing (user interactions)
● Batch latency was too high
● Existing queues handle large volumes of
(unconsumed) data poorly - durability is
expensive
● Need something fast and durable

Key design choices
● Pub/sub messaging pattern
● Messages are persistent
● Everything is distributed - producers,
brokers, consumers, the queue itself
● Consumers maintain their own state (i.e.,
"dumb" brokers)
● Throughput is key

Brokers
● Receive messages from Producers (push),
deliver messages to Consumers (pull)
● Responsible for persisting the messages for
some time
● Relatively lightweight - mostly just handling
TCP connections and keeping open file
handles to the queue files

Log-based queue
Messages are persisted to append-only log
files by the broker. Producers are appending to
these log files (sequential write), and
consumers are reading a range of these files
(sequential reads).

Log-based queue
Message 0
Message 1
Message 2
Message 3
Producer
Message 4
Message 5
Message 6
Consumer A read 0-4 Consumer Bread 1-3
Broker

Topics
Topics are queues. They are logical collections
of partitions (the physical files). A broker
contains some of the partitions for a topic
Partition 0 Partition 1
Topic A
Topic B
Broker 1
Topic A
Topic B
Broker 2

Replication
Partitions of a topic are replicated. One broker
is the "leader" of a partition. All writes and
reads must go to the leader. Replicas exist for
fault-tolerance, not scalability.
When writing, messages can be synchronously
written to N replicas (depending on the
producer's ACKiness)

Producers
Producers are responsible for load balancing
messages to the various brokers. They can
discover all brokers from any one broker.
In 0.7, producers are fire-and-forget. In 0.8,
there are 3 ack levels:
● No ack (0)
● Ack from N replicas (1..N)
● Ack from all replicas (-1)

Consumers
Consumers request a range of messages from
a Broker. They are responsible for their own
state
Default implementation uses ZooKeeper to
manage state. In 0.8.1, Brokers will expose an
API for offset management to remove direct
communication between consumers and
ZooKeeper (a good thing).

Result?
● Brokers keep very little state, mostly just
open file pointers and connections
● Can scale to thousands of producers and
consumers*
● Stable performance, good scalability
● In 0.7, 50MB/s producer throughput,
100MB/s consumer throughput
● No numbers yet from 0.8, but the same
comparable (with acks=0)

High-level producer
API
Producer#send(KeyedMessage<K,V> datum)
KeyedMessage<K,V>(String topic, K key,
V value)
The Producer class determines where a
message is sent based on the routing key. If a
null key is given, the message is sent to a
random partition

Message routing
Topic A
Broker 1
Topic A
Broker 2
hash("foo") % 4
Partitioner
Producer.send("A", "foo",
messages)
Producer

Message routing
● Producers can be configured with a custom
routing function (implementing the Partitioner
interface)
● Default is hash-mod
● One side effect of routing is that there is no
total ordering for a topic, but there is within a
partition (this is actually really useful)

(Partially) Ordered
Messages
Consider a system that is processing updates.
If you partition the messages based on the
primary key you guarantee all messages for a
given key end up in the same partition. Since
Kafka guarantees ordering of the messages at
the partition level, your updates will be
processed in the correct sequence.

Persistent Messages
● MessageSets received by the broker and
flushed to append-only log files
● Simple log format (next slide)
● Zero-copy (i.e., sendfile) for file to socket
transfer
● Log files are not kept forever

Log Format
append
Log file
MessageSet
MessageSet
MessageSet
MessageSet
/tmp/kafka-logs/00000000000000000000.kafka
append
append
appendBroker
ByteBufferMessageSet#writeTo(FileChannel)
Index
0: 0
1: 1024
2: 2048
Message offset
index

Message sets
● To maximize throughput, the same binary
format for messages is used throughout the
system (producer API, consumer API, and
log file format)
● Broker only decodes far enough to read the
checksum and validate it, then writes it to the
disk

Compressed message sets
or, message batching
● The value of a Message can be a
compressed MessageSet
Message(value=gzip(MessageSet(...)))
● Useful for increasing throughput (especially
if you are buffering messages in the
producer)
● Can also be used as a way to atomically
write multiple messages

Zero-copy
Reading data out of Kafka is super fast thanks
to java.nio.channels.FileChannel#transferTo.
This method uses the "sendfile" system call
which allows for very efficient transfer of data
from a file to another file (including sockets).
This optimization is what allows us to pull
~100MB/s out of a single broker

High-level Consumer
API
Map topicMap = Collections.singletonMap("topic", 2);
Map<String,List<KafkaStream>> streams =
Consumer.createJavaConsumerConnector(topicMap);
● KafkaStream is an iterable
● Blocking or non-blocking behavior
● Auto-commit offset, or manual commit
● Participate in a "consumer group"

Consumer Groups
Multiple high-level consumers can participate in
a single "consumer group". A consumer group
is coordinated using ZooKeeper, so it can span
multiple machines. In a group, each partition
will be consumed by exactly one consumer (i.
e., KafkaStream).
This allows for broadcast or pub/sub type of
messaging pattern

Consumer Groups
P0 P1
Topic "foo"
Broker 1
Consumer Group A Consumer Group B
P2 P3
Topic "foo"
Broker 2

But, I already have a MQ
Many people with distributed systems already
have something like JMS, RabbitMQ, Kestrel,
or ØMQ in place.
These are all different systems with different
implementation details and semantics. Decide
what features you need, and then chose a MQ -
not the other way around :)
RabbitMQ discussion: http://bit.ly/140ZMOx

Caveats
● Not designed for large payloads. Decoding is
done for a whole message (no streaming
decoding).
● Rebalancing can screw things up if you're
doing any aggregation in the consumer by
the partition key
● Number of partitions cannot be easily
changed (chose wisely)
● Lots of topics can hurt I/O performance

Clients
● Many exist, some more complete than
others
● No official clients (other than Java/Scala)
● Community maintained
● Most lack support for high-level consumer
due to ZooKeeper dependency (it's tricky)
● Excellent Python client: https://github.
com/mumrah/kafka-python (yes, I wrote it)
● Only Python, C, and Clojure support the 0.8
protocol

Hadoop InputFormat
● InputFormat/OutputFormat implementations
● Uses low-level consumer, no offsets in ZK
(they are stored on HDFS instead)
● Useful for long term storage of messages
● We use this!
● LinkedIn released Camus, a Kafka to HDFS
pipeline

Integration
A few good integration points. I expect more to
show up as Kafka gains adoption
● log4j appender (in Kafka itself)
● storm-kafka (in storm-contrib)
● camel-kafka
● LogStash (loges)
More listed on Kafka wiki http://bit.ly/1btZz8p

Applications
Log Aggregation
Many applications running on many machines.
You want centralized logging, but don't want to
install an agent (Flume, etc).
Kafka includes a log4j appender
<appender class="kafka.producer.KafkaLog4jAppender" name="kafka-solr">
<param name="zkConnect" value="localhost:2181"/>
<param name="topic" value="solr-logs"/>
<layout class="org.apache.log4j.PatternLayout">
<param value="%d{ISO8601} %p %c %m" name="ConversionPattern"/>
</layout>
</appender>

Applications
Notifications
Store data into data store A, need to sync it to
data store B. Suppose you can send a
message to Kafka when an update happens in
A
A
Kafka
Brecord X changed
get updated record X

Applications
Stream Processing
Using a stream processing tool like Storm or
Apache Camel, Kafka provides an excellent
backbone.
Data in
Kafka
topic A
Process A
Kafka
topic B
Process B

Bonus Slides
Stats
LinkedIn stats:
● Peak writes per second: 460k
● Average writes per day: 28 billion
● Average reads per second: 2.3 million
● ~700 topics
● Thousands of producers
● ~1000 consumers

Bonus Slides
Logos!
Being heatedly debated in JIRA as we speak!

Links!
● Slides: http://bit.ly/kafka-trihug
● Kafka Project: http://kafka.apache.org/
● Kafka Code: https://github.com/apache/kafka
● LinkedIn Camus: https://github.com/linkedin/camus
● Zero-Copy IBM article: http://ibm.co/gsETNm
● LinkedIn Kafka paper: http://bit.ly/mC8TLS
● LinkedIn blog post on replication: http://linkd.in/YAWslH

Introduction and Overview of Apache Kafka, TriHUG July 23, 2013

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