Consumer Internals
This page serves to document the broader design of the internals of Sarama's consumer.
There are three primary structs: Consumer, PartitionConsumer, and brokerConsumer.
The Consumer is a simple structure that does nothing but store state that has to be shared between the many instances of the other two structs. It has no goroutines. The primary public method is ConsumePartition which spawns a new PartitionConsumer.
PartitionConsumers are spawned by a Consumer and are responsible for consuming from a single topic/partition. They store state specific to that partition, and manage two goroutines (dispatcher and responseFeeder).
The dispatcher is only responsible for associating the PartitionConsumer with a broker, so it is idle in normal operation. It only activates when leadership changes in the cluster and the PartitionConsumer must change brokers.
The responseFeeder is responsible for taking FetchResponses returned from the broker and feeding the messages out to the user.
brokerConsumerss work consuming from a single broker, and manage the set of PartitionConsumers that are currently led by that broker.
They manage two goroutines: subscriptionConsumer which loops fetching from the broker, and subscriptionManager which serves to collect any new PartitionConsumers while subscriptionConsumer is blocked in network IO.