TLDR: Apache Kafka doesn’t need fsyncs to be safe because it includes recovery in its replication protocol. It is a real-world distributed system that uses asynchronous log writing + recovery with some additional extra safety built-in. Asynchronous log writing allows it to provide robust performance on a variety of hardware and with a wide variety of workloads.

Now that the TLDR is done, let’s dive into it.

The fact that by default Apache Kafka doesn’t flush writes to disk is sometimes used as ammunition against it. The argument is that if Kafka doesn’t flush data before acknowledging produce requests then surely the cluster can lose acknowledged data due to crashes and reboots. It sounds plausible and so people may believe it - but I’m here writing this today to explain why that isn’t the case.