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We need a mechanism to distribute a clients quota across all the brokers in a cluster. Our proposal is to define the bandwidth on a per-broker basis. Each client can publish a maximum of X MBps (configurable) per broker before it gets throttled. This approach assumes that client requests are properly distributed among all the broker instances which may not always be the case. However, this is much better than having a fixed cluster wide bandwidth per client because that would require a mechanism to share current quota usage per-client among all the brokers. This can be very tricky to implement and is outside the scope of this proposal. 

Quota Actions

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How do the Kafka Server react when it detects a quota violation? In our proposal, the broker does not return an error rather it attempts to slow down the client exceeding it's quota. The behavior is subtly different for producer and consumer.

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Here's how the code might look inside the Broker (in KafkaApi's or helper class).

Metrics

Kafka server has decided to migrate it's metrics to Kafka Metrics (KM). For more details read this. Kafka metrics maintains a Sensor for each measured metric. This can be configured with a "Quota" which is a bound of the min and max value of a metric. If recording a value causes a bound to get exceeded, a QuotaViolationException is thrown.

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