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Current state: Draft

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread] 

JIRA: here [Change the link from KAFKA-1 to your own ticket]

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Currently when handling topics or partitions creation requests, Kafka 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 alive replicas 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, as long as the expected size of the cluster under normal conditions is large enough. As soon as brokers re-join the cluster, the missing replicas will be created.

To allow that, we propose to keep track of brokers that are part of the cluster in zookeeper even after they disconnect ("observed brokers"), and use that information to determine an assignment (or to determine if a given assignment is feasible) on handling a topic creation request or partition creation request.

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.

Because this feature can make partitions appear under replicated, it is disabled by default and can be enabled by a broker configuration: enable.under.replicated.topic.When the number of available brokers is sufficient and no assignment is specified, creating an under-replicated topic/partition may be preferred: e.g. replication factor 3, in a 3-zones region with 2 brokers per region, but a zone is down.
In such case the create request could make use of all observed brokers rather than the currently online ones. A topic will only be created if there are at least "min.insync.replicas" brokers available, ie a topic can be created under-replicated but it will always be at least at min.insync at creation.

Public Interfaces

Broker Configuration

Administrators can disable (false), or enable (true) creations of topics/partitions without the full replication factor.

Zookeeper changes:

New non-ephemeral node under /brokers called observed_ids.
Upon starting brokers will create/update the non ephemeral node for their current id containing the same information as the existing ephemeral brokers/ids node. 

The data in these new nodes (including rack if available) will be used on Topic/Partition creation when the request number of replicas (or the requested broker ids if assignment is specified) are not currently available.

Defaulting to false for compatibility.

Compatibility, Deprecation, and Migration Plan

• 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.
  
• Updates to the Decommissioning brokers section in the documentation
  
• will mention that if a broker id is never to be reused then its corresponding node in zookeeper /brokers/observed_ids will need to be removed
  

Rejected Alternatives

Rejected Alternatives

• 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 administrators to delete znodes when decommisioning brokers. We cannot tell people to modify ZK directly for thisAsync replica assignment: Instead of relying on previously known brokers, we considered creating replicas on the available brokers at topic/partition create time and saving in zookeeper some information to asynchronously create missing replicas on the next suitable brokers that would join the cluster. Such a process would involve the controller checking if missing replicas existed when a broker joins the cluster. This alternative felt too complicated. Also the next joining broker may not be on the preferred rack assignment.
• Update CreateTopicsRequest and CreatePartitionsRequest to allow users to disable creation of under replicated topics. This approach felt too complicatedoption adds an extra configuration to think about to end users and it's unclear how many people would want to take 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 We think the error message associated with the new error code can suffice.