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We would like to allow users to create topics and partitions even when the current available number of brokers is less than the number of requested replicas. It would still require at least enough brokers to satisfy the  "min.insync.replicas" configuration of the topic. This guarantees producers will be able to use the topic/partition regardless of their acks setting, ie the lack of replicas will be invisible to all users. Because it's possible to create a topic with less replicas than "min.insync.replicas", the actual requirement will be min(min.insync.replicas, replicas).

When handling a CreateTopics or CreatePartitions request, in case not enough brokers are available to satisfy the replication factor, placeholder replica ids (-1, -2, -3) will be inserted for the missing replicas. In case a topic or partition is created under-replicated, the error code will still be NONE in the CreateTopics and CreatePartition responses.

When a new broker joins the cluster, the controller will check if any partitions have placeholders replicas ids and if so assign this broker as a replica (only if this broker is not already a replica).
This may lead to some topics/partitions not optimally spread across all racks, but note that this may already happen (without this KIP) when topics/partitions are created while all brokers in a rack are offline (ie: an availability zone is offline). Tracking topics/partitions not optimally spread across all racks can be tackled in a follow up KIP.