Add notes: Kafka: The definitive guide

- Chapter 6: Kafka Internals

KafkaTheDefinitiveGuide/Chapter6/README.md

@@ -0,0 +1,25 @@
+# CHAPTER 6: Kafka Internals
+
+Kafka uses ZooKeeper’s ephemeral node feature to elect a controller and to notify the controller when nodes join and
+leave the cluster. The controller is responsible for electing leaders among the partitions and replicas whenever it
+notices nodes join and leave the cluster. The controller uses the epoch number to prevent a “split brain” scenario
+where two nodes believe each is the current controller.
+
+From the list of replicas for a partition displayed by the _kafka-topics.sh_ tool, the first replica in the list is
+always the preferred leader.
+
+This is true no matter who is the current leader and even if the replicas were reassigned to different brokers using
+the replica reassignment tool.
+
+When replicas are manually reassigned, the first replica specified will be the preferred one.
+
+Common types of client requests:
+
+- Produce requests: Sent by producers and contain messages the clients write to Kafka brokers
+- Fetch requests: Sent by consumers and follower replicas when they read messages from Kafka brokers
+- Admin requests: Sent by admin clients when performing metadata operations such as creating and deleting topics
+
+New brokers know how to handle old requests, but old brokers don't know how to handle new requests. Now, in release
+`0.10.0`, `ApiVersionRequest` was added, which allows clients to ask the broker which versions of each request are
+supported and to use the correct version accordingly. Clients that use this new capability correctly will be able
+to talk to older brokers by using a version of the protocol that is supported by the broker they are connecting to.

KafkaTheDefinitiveGuide/README.md

@@ -11,3 +11,5 @@ Main notes taken from the book [Kafka: The Definitive Guide: Real-Time Data and
 [CHAPTER 4: Kafka Consumers: Reading Data from Kafka](./Chapter4)
 
 [CHAPTER 5: Managing Apache Kafka Programmatically](./Chapter5)
+
+[CHAPTER 6: Kafka Internals](./Chapter6)