The data team at Cloudflare uses Kafka to process tens of petabytes a day. All this data is moved using the 2 foundational Kafka api calls: Produce (api key 0) and Fetch (api key 1). Understanding the structure of these calls (and of the underlying RecordSet structure) is key to building high throughput clients. The talk describes the basics of the Kafka wire protocol (api keys, correlation id), and the structure of the Produce and Fetch calls. It shows how the asynchronous nature of the wire protocol can combine with the structure of the Produce and Fetch calls to increase latency and reduce client throughput; a solution is offered through use of synchronous single-partition calls. The RecordSet structure, which is used to encode and store sets (batches) of records is described, and its implications on Fetch requests are discussed. The relationship between Fetch api calls and ""consume"" operations is discussed, as is the impact of offset alignment to RecordSet boundaries.