Apache Kafka follower replicas are often described as both synchronous and asynchronous, causing confusion. The distinction lies in two separate questions: how replication physically works (pull-based, followers can lag — asynchronous) versus when a write is considered committed (ISR followers must acknowledge before acks=all succeeds — synchronous). In Confluent Platform Multi-Region Clusters, this distinction is made explicit: normal replicas in the ISR are synchronous replicas, while observers are asynchronous replicas that don't participate in the ISR-based acknowledgement path. The min.insync.replicas configuration controls the minimum ISR size required for a write to succeed with acks=all, making it critical for durability guarantees in stretched and 2.5 DC architectures.
