MSK Serverless removes broker choices: you create a cluster and topics, and AWS 24 moves capacity behind one endpoint. Clients of Producers and Delivery and Consumers and Queues work unchanged, but every one must authenticate with IAM (no Kafka 129 ACLs; authorization is IAM policy on topics and groups), in one of fourteen regions. The per-cluster quotas are the real design constraints: 200 MBps in and 400 MBps out, 5 and 10 MBps per partition, 2,400 partitions (120 if compacted), 8 MiB messages, 3,000 connections and 500 consumer groups, with unlimited retention.
Pricing has five meters, shown here with AWS's US East example rates: $0.75 per cluster-hour, $0.0015 per partition-hour, $0.10 per GB written, $0.05 per GB read and $0.10 per GB-month stored. The cluster-hour alone is $558 for a 31-day month; BookNest's nine partitions add about $10 and its traffic rounds to nothing, so a serverless BookNest costs about $570 a month, more than three provisioned brokers. Serverless pays off for spiky traffic of tens of MBps that you would otherwise provision for at its peak.
Two practical notes. You may set only seven topic properties (retention.ms and retention.bytes, which default to 7 days and 250 GiB, max.message.bytes, compression.type, the two timestamp settings, and cleanup.policy at creation time); segment.bytes is fixed, which trips Kafka Streams 129 applications that create internal topics with their own value. And IAM-only means every client, Kafka UI and the command-line tools included, needs AWS credentials and the signer of MSK Provisioned Clusters.