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This KIP proposes to allow Kafka producer and consumer clients to repeat the bootstrap process when fetching updating metadata if none of the known nodes are available. The bootstrap.serversbrokers are available. A broker is unavailable when the client doesn't have an established connection with it and cannot establish a connection (e.g. due to the reconnect backoff).

During the rebootstrap process, the client forgets the brokers it knows about and falls back on the bootstrap brokers (i.e. provided by bootstrap.servers provided by the client configuration) as if it had just been initialized which was originally provided via the client configuration is used for this.

The client will check the cluster ID returned by the broker during the rebootstrap process. If no cluster ID was known to the client (i.e. it was originally bootstrapped with an old broker version that doesn't support cluster IDs), any returned value will be considered valid. Otherwise, the client will fail if the returned cluster ID doesn't match the previously known one.

The admin client behaves differently and doesn't update metadata. Because of this, it is excluded from the scope of this KIP.

reconnect.backoff.max.ms can be configured so low that brokers that are truly unavailable will never be considered as such, i.e. always will be eligible for reconnect. This is a known limitation. Unfortunately, it's hard to find a good criteria when to ignore this and trigger rebootstrapping nevertheless. It was decided to keep this out of the scope of this KIP.

Since this changes the user-facing behavior, it’s proposed to make this configurable (see Public Interfaces), defaulting to the current behavior.

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Compatibility, Deprecation, and Migration Plan

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