|Developer(s)||Apache Software Foundation|
|Initial release||January 2011|
1.1.0 / March 28, 2018
|Written in||Scala, Java|
|Type||Stream processing, Message broker|
|License||Apache License 2.0|
Apache Kafka is an open-source stream-processing software platform developed by the Apache Software Foundation, written in Scala and Java. The project aims to provide a unified, high-throughput, low-latency platform for handling real-time data feeds. Its storage layer is essentially a "massively scalable pub/sub message queue architected as a distributed transaction log," making it highly valuable for enterprise infrastructures to process streaming data. Additionally, Kafka connects to external systems (for data import/export) via Kafka Connect and provides Kafka Streams, a Java stream processing library.
Apache Kafka was originally developed by LinkedIn, and was subsequently open sourced in early 2011. Graduation from the Apache Incubator occurred on 23 October 2012. In November 2014, Jun Rao, Jay Kreps, and Neha Narkhede, who had worked on Kafka at LinkedIn, created a new company named Confluent with a focus on Kafka. According to a Quora post from 2014, Kreps seems to have named the software after the author Franz Kafka. Kreps chose to name the system after an author because it is "a system optimized for writing", and he liked Kafka's work.
Apache Kafka is based on the commit log, and it allows users to subscribe to it and publish data to any number of systems or real-time applications. Example applications include managing passenger and driver matching at Uber, providing real-time analytics and predictive maintenance for British Gas' smart home, and performing numerous real-time services across all of LinkedIn.
Kafka stores key-value messages which come from arbitrarily many processes called "producers". The data can thereby be partitioned in different "partitions" within different "topics". Within a partition, messages are strictly ordered by their offsets (the position of a message within a partition), and indexed and stored together with a timestamp. Other processes called "consumers" can read messages from partitions. For stream processing, Kafka offers the Streams API that allows writing Java applications that consume data from Kafka and write results back to Kafka. Apache Kafka also works with external stream processing systems such as Apache Apex, Apache Flink, Apache Spark, and Apache Storm.
Kafka runs on a cluster of one or more servers (called brokers), and the partitions of all topics are distributed across the cluster nodes. Additionally, partitions are replicated to multiple brokers. This architecture allows Kafka to deliver massive streams of messages in a fault-tolerant fashion and has made it replace some of the conventional messaging systems like JMS, AMQP, etc. Since the 0.11.0.0 release, Kafka offers "transactional writes", which allow for exactly-once stream processing using the Streams API.
Kafka supports two types of topics: Regular and compacted. Regular topics can be configured with a retention time or space bound. If there are records that are older than the specified retention time or the space bound is exceeded for a partition, Kafka is allowed to delete old data to free storage space. By default, topics are configured with a retention time of 7 days but it's also possible to store data indefinitely. For compacted topics, records don't expire based on time or space bounds. Instead, Kafka treats later messages as updates to older message with the same key and guarantees to never delete the latest message per key. Users can delete messages entirely by writing a so-called tombstone message with null-value for a specific key.
There are four major APIs in Kafka:
The consumer and producer APIs build on top of the Kafka messaging protocol and offer a reference implementation for Kafka consumers and producers clients in Java. The underlying messaging protocol is a binary protocol that allows developers to write their own consumer or producer clients in any programming language. This unlocks Kafka from the Java Virtual Machine (JVM) based eco-system. A list of available non-Java clients is maintained in the Apache Kafka wiki.
Kafka Connect (or Connect API) is a framework to import/export data from/to other systems. It was added in the Kafka 0.9.0.0 release and uses the Producer and Consumer API internally. The Connect framework itself executes so-called "connectors" that implement the actual logic to read/write data from other system. The Connect API defines the programming interface that must be implemented to build a custom connector. Many open source and commercial connectors for popular data systems are available already. However, Apache Kafka itself does not include production ready connectors.
Kafka Streams (or Streams API) is a stream-processing library written in Java. It was added in the Kafka 0.10.0.0 release. The library allows for the development of stateful stream-processing applications that are scalable, elastic, and fully fault-tolerant. The main API is a stream-processing DSL that offers high-level operators like filter, map, grouping, windowing, aggregation, joins, and the notion of tables. Additionally, the Processor API can be used to implement custom operators for a more low-level development approach. The DSL and Processor API can be mixed, too. For stateful stream processing, Kafka Streams uses RocksDB to maintain local operator state. Because RocksDB can write to disk, the maintained state can be larger than available main memory. For fault-tolerance, all updates to local state stores are also written into a topic in the Kafka cluster. This allows recreating state by reading those topics and feed all data into RocksDB.
Up to version 0.9.x, Kafka brokers are backward compatible with older clients only. Since Kafka 0.10.0.0, brokers are also forward compatible with newer clients. If a newer client connects to an older broker, it can only use the features the broker supports. For the Streams API, full compatibility starts with version 0.10.1.0: a 0.10.1.0 Kafka Streams application is not compatible with 0.10.0 or older brokers.
Due to its widespread integration into enterprise-level infrastructures, monitoring Kafka performance at scale has become an increasingly important issue. Monitoring end-to-end performance requires tracking metrics from brokers, consumer, and producers, in addition to monitoring ZooKeeper which is used by Kafka for coordination among consumers. There are currently several monitoring platforms to track Kafka performance, either open-source, like LinkedIn's Burrow, or paid, like Datadog. In addition to these platforms, collecting Kafka data can also be performed using tools commonly bundled with Java, including JConsole.
The following is a list of notable enterprises that have used or are using Kafka:
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