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Currently when handling topics or partitions creation requests, Kafka currently enforces all replicas to be created in order to fulfill the request. While all other functionalities (produce/consume) are fault tolerant and can handle some brokers down, topics and partitions creations creation stop working as soon as there are no enough replicas available. In small clusters, when one node is unavailable, for example when a broker is being restarted, it's possible that there are not enough online brokers to satisfy topic/partition creation even though there are enough brokers to satisfy the "min.insync.replicas" configuration of the topic.

This is obvious in a few scenarios:

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mostly impacting clusters with 3 brokers for which topics with replication factor 3

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can't be created while a rolling restart is happening. This makes this cluster size unsuitable for environments requiring maximum availability and forces administrators to deploy at least 4 brokers. In case of a stretch cluster spanning 3 availability zones, which is now a relatively common deployment, this forces administrators to have 6 brokers.
 

This features will allow small clusters to stay fully available over rolling restarts and basic maintenance operationsThe same scenarios also apply to adding partitions to existing topics (CreatePartitions API).

Proposed Changes

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. Placeholder replica ids (-1, -2, -3) will be inserted for the missing replicas.

When a broker joins the cluster, the controller will check if any partitions have placeholders replicas ids and if so assign this broker as a replica (obviosuly only if this broker is not already a replica).

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• For compatibility the default behavior remains unaltered: a number of active online brokers greater or equals (or greater) to the full replication factor is required for successful creation.
  
  
• No migration is necessary.

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• Store "observed brokers" in Zookeeper and assign replicas to them even if they are offline. Because broker registration is implicit, this alternative required storing extra data in Zookeeper and also required administrator administrators to delete znodes when decommisioning brokers. We cannot tell people to modify ZK directly for this.
• Update CreateTopicsRequest and CreatePartitionsRequest to allow users to disable creation of under replicated topics. This option adds an extra configuration to think abobut about to end users and it's unclear how many people would want to take advantageo advantage of such a feature.
  
• Update schema of CreateTopicsResponse and CreatePartitionsResponse to contain the actual replication factor at creation. Like assignments, it's not data the client can act on.
  
• Create a new error code when a topic/partition is created under replicated. As this is not an error case, it's best to keep returning NONE to avoid breaking existing logic.

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