# Messages Messages and Batches ``` The unit of data within Kafka is called a message A message is simply an array of bytes, as far as Kafka is concerned, so the data contained within it does not have a specific format or meaning to Kafka Messages can have an optional bit of metadata which is referred to as a key. The key is also a byte array, and as with the message, has no specific meaning to Kafka Keys are used when messages are to be written to partitions in a more controlled manner. The simplest such scheme is to treat partitions as a hash ring, and assure that messages with the same key are always written to the same partition. messages are written into Kafka in batches. A batch is just a collection of messages, all of which are being produced to the same topic and partition. n individual round trip across the network for each message would result in excessive overhead, and collecting messages together into a batch reduces this presents a tradeoff between latency and throughput: he larger the batches, the more messages that can be handled per unit of time, but the longer it takes an individual message to propagate. Batches are also typically compressed, which provides for more efficient data transfer and storage at the cost of some processing power. ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://nag-9-s.gitbook.io/apache-kafka-notes/messages.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.