Kafka Broker, Kafka Topic, Consumer and Record Flow in Kafka
By: Kajal Rawal
Tags: Streaming, Stream, Kafka Streams, Kafka, Data

Kafka Topic

A Topic is a category/feed name to which records are stored and published.

As said before, all Kafka records are organized into topics. Producer applications write data to topics and consumer applications read from topics. Records published to the cluster stay in the cluster until a configurable retention period has passed by.

Kafka retains records in the log, making the consumers responsible for tracking the position in the log, known as the “offset”. Typically, a consumer advances the offset in a linear manner as messages are read. However, the position is actually controlled by the consumer, which can consume messages in any order. For example, a consumer can reset to an older offset when reprocessing records.

Kafka topic partition

Kafka topics are divided into a number of partitions, which contain records in an unchangeable sequence. Each record in a partition is assigned and identified by its unique offset. A topic can also have multiple partition logs. This allows multiple consumers to read from a topic in parallel.

Partitions allow topics to be parallelized by splitting the data into a particular topic across multiple brokers.

In Kafka, replication is implemented at the partition level. The redundant unit of a topic partition is called a replica. Each partition usually has one or more replicas meaning that partitions contain messages that are replicated over a few Kafka brokers in the cluster.

Every partition (replica) has one server acting as a leader and the rest of them as followers. The leader replica handles all read-write requests for the specific partition and the followers replicate the leader. If the lead server fails, one of the follower servers becomes the leader by default. You should strive to have a good balance of leaders so each broker is a leader of an equal amount of partitions to distribute the load.

When a producer publishes a record to a topic, it is published to its leader. The leader appends the record to its commit log and increments its record offset. Kafka only exposes a record to a consumer after it has been committed and each piece of data that comes in will be stacked on the cluster.

A producer must know which partition to write to, this is not up to the broker. It’s possible for the producer to attach a key to the record dictating the partition the record should go to. All records with the same key will arrive at the same partition. Before a producer can send any records, it has to request metadata about the cluster from the broker. The metadata contains information on which broker is the leader for each partition and a producer always writes to the partition leader. The producer then uses the key to know which partition to write to, the default implementation is to use the hash of the key to calculate partition, you can also skip this step and specify partition yourself.

A common error when publishing records is setting the same key or null key for all records, which results in all records ending up in the same partition and you get an unbalanced topic.

Consumers and consumer groups

Consumers can read messages starting from a specific offset and are allowed to read from any offset point they choose. This allows consumers to join the cluster at any point in time.

Low-level consumers

There are two types of consumers in Kafka. First, the low-level consumer, where topics and partitions are specified as is the offset from which to read, either fixed position, at the beginning or at the end. This can, of course, be cumbersome to keep track of which offsets are consumed so the same records aren’t read more than once. So Kafka added another easier way of consuming with:

High-level consumer

The high-level consumer (more known as consumer groups) consists of one or more consumers. Here a consumer group is created by adding the property “group.id” to a consumer. Giving the same group id to another consumer means it will join the same group.

The broker will distribute according to which consumer should read from which partitions and it also keeps track of which offset the group is at for each partition. It tracks this by having all consumers committing which offset they have handled.

Every time a consumer is added or removed from a group the consumption is rebalanced between the group. All consumers are stopped on every rebalance, so clients that time out or are restarted often will decrease the throughput. Make the consumers stateless since the consumer might get different partitions assigned on a rebalance.

Consumers pull messages from topic partitions. Different consumers can be responsible for different partitions. Kafka can support a large number of consumers and retain large amounts of data with very little overhead. By using consumer groups, consumers can be parallelized so that multiple consumers can read from multiple partitions on a topic, allowing a very high message processing throughput. The number of partitions impacts the maximum parallelism of consumers as there cannot be more consumers than partitions.

Records are never pushed out to consumers, the consumer will ask for messages when the consumer is ready to handle the message.

The consumers will never overload themselves with lots of data or lose any data since all records are being queued up in Kafka. If the consumer is behind during message processing, it has the option to eventually catch up and get back to handle data in real-time.

Record flow in Apache Kafka

Now we have been looking at the producer and the consumer, and we will check at how the broker receives and stores records coming in the broker.

We have an example, where we have a broker with three topics, where each topic has 8 partitions.

The producer sends a record to partition 1 in topic 1 and since the partition is empty the record ends up at offset 0.

Next record is added to partition 1 will and up at offset 1, and the next record at offset 2 and so on.

This is what is referred to as a commit log, each record is appended to the log and there is no way to change the existing records in the log. This is also the same offset that the consumer uses to specify where to start reading.

Steps to follow when setting up a connection and publishing a message/consuming a message.