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Kafka currently supports quotas by data volume.  Clients that produce or fetch messages at a byte rate that exceeds their quota are throttled by delaying the response by an amount that brings the byte rate within the configured quota. However, if a client sends requests too quickly (e.g., a consumer with fetch.max.wait.ms=0), it can still overwhelm the broker even though individual request/response size may be small. It will be useful to additionally support throttling by request rate to ensure that broker resources are not monopolized by some users/clients.

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Scenarios Scenarios

Scenarios:

1. Clients send requests too quickly - eg. a consumer with fetch.max.wait.ms=0 that polls continuously. Fetch byte rate quotas are not sufficient here, either request count quotas or request processing time quotas are required.
2. DoS attack from clients that overload brokers with continuous authorized or unauthorized requests. Either request count quotas or request processing time quotas that limit all unauthorized requests and all non-broker (client) requests is required.
3. Client sends produce requests with compressed messages where decompression takes a long time, blocking the request handler thread. Request processing time quotas are required since neither produce byte rate quotas nor request count quotas will be sufficient to limit the broker resources allocated to users/clients in this case.
4. Consumer group starts with 10 instances and then increases to 20 instances. Number of requests may double, so request counts increase, even though the load on the broker doesn't double since the number of partitions per fetch request has halved. Quotas based request count per second may not be easy to configure in this case.
5. Some requests may use more of their quota on the network threads rather than the request handler threads (eg. disk read for fetches happen on the network threads). While quotas of processing time on the request handler thread limit the request rate in many cases above, for a complete request rate quota solution, network thread utilization also needs to be taken into account.

This KIP proposes to allocate request processing time quotas as a percentage of the total time available in a quota window across all the request handler threads. Only request handler thread utilization will be taken into account in this KIP. Network thread utilization (Scenario 5) will be addressed separately in a follow-on future KIP since that is a lot more complex.

Public Interfaces

Request rate quotas

The current produce and fetch quota limits are based on byte rate within a quota window. Request rates that specify number of requests per second are not sufficient to control request handler thread utilization since the time used by different requests can vary significantly. Request rate quotas will be configured as a percentage of time within a quota window that a client is allowed to use, The limits will be applied to the same quota window configuration (quota.window.size.seconds with 1 second default)  as existing produce/fetch quotas. This approach keeps the code consistent with the existing quota implementation, while making it easy for administrators to allocate a slice of each quota window to users/clients to control broker resource utilization. If a client/user exceeds the request processing time limit, responses will be delayed by an amount that brings the request rate within the limit. The maximum delay applied will be the quota window size.

Default quotas

By default, clients will not be throttled based on request processing time, but defaults can be configured using the dynamic default properties at <client-id>, <user> and <user, client-id> levels. Defaults as well as overrides are stored as dynamic configuration properties in Zookeeper alongside the other rate limits.

Requests that may be throttled

Requests that update cluster state will be throttled only if authorization for ClusterAction fails. These are infrequent requests for cluster management, typically not used by clients:

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Fetch and produce requests will continue to be throttled based on byte rates and may also be throttled based on request rates. Fetch requests used for replication will not be throttled based on request rates since it is possible to configure replica.fetch.wait.max.ms and use the existing replication byte rate quotas to limit replication rate.

Metrics and sensors

Two new metrics and corresponding sensors will be added to the broker for tracking request-time and throttle-time of each quota entity for the new quota type Request. These will be handled similar to the metrics and sensors for Produce/Fetch.

Clients will expose average and maximum request throttle time as JMX metrics similar to the current produce/fetch throttle time metrics.

Tools

kafka-configs.sh will be extended to support request quotas.  A new quota property will be added, which can be applied to <client-id>, <user> or <user, client-id>:

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bin/kafka-configs  --zookeeper localhost:2181 --alter --add-config 'request_time_percent=10.0' --entity-type users

Proposed Changes

Quota entity

Request quotas will be supported for <client-id>, <user> and <user, client-id> similar to the existing produce/fetch byte rate quotas.  In addition to produce and fetch rates, an additional quota property will be added for request rate throttling. As with produce/fetch quotas, request quotas will be per-broker. Defaults can be configured using the dynamic default properties at <client-id>, <user> and <user, client-id> levels.

Request rate quotas

Quotas for requests will be configured as a percentage of time within a quota window that a client is allowed to use. For example, with the default configuration of 1 second quota window size and 8 I/O threads handling requests, the total time a broker can spend processing requests is 8 seconds across all the threads. If user alice has a request quota of 1 percent, the total time all clients of alice can spend in the request handler in any one second window is 80 milliseconds. When this time is exceeded, a delay is added to the response to bring alice’s usage within the configured quota. The maximum delay added to any response will be the window size.  The calculation of delay will be the same as the current calculation used for throttling produce/fetch requests:

• If O is the observed usage and T is the target usage over a window of W, to bring O down to T, we need to add a delay of X to W such that: O * W / (W + X) = T.
• Solving for X, we get X = (O - T)/T * W.
• The response will be throttled by min((X, W)

Sample configuration in Zookeeper

// Quotas for user1
// Zookeeper persistence path /config/users/<encoded-user1>
{
    "version":1,
    "config": {
        "producer_byte_rate":"1024",
        "consumer_byte_rate":"2048",
		"request_time_percent" : "1.0"
    }
}

Co-existence of multiple quotas

Produce and fetch byte rate quotas will continue to be applied as they are today. Request rate throttling will be applied on top if necessary. For example, if a large number of small produce requests are sent followed by a very large one, both request time quota and produce byte rate quota may be violated by the same request. The produce byte rate delay is applied first. Request time quota is checked only after the produce delay is applied. The request time quota is perhaps no longer violated (or the delay may be lower due to the first delay already applied). The remaining delay if any is applied to the response.

Metrics and sensors

Two new metrics and corresponding sensors will be added to track request-time and throttle-time of each quota entity for quota type Request.  The request-time sensor will be configured with the quota for the user/client so that quota violations can be used to add delays to the response. Quota window configuration (quota.window.size.seconds) will be the same as the existing configuration for produce/fetch quotas: 1 second window with 11 samples retained in memory by default.

Metrics and sensors will be expired as they are today for Produce/Fetch quotas.

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Future Work

As described in Scenario 5, to control broker resource utilization allocated to users/clients, both network thread utilization and request thread utilization should be limited by quotas. This KIP only addresses quotas for request thread utilization. Controlling network thread utilization is more complex and will be addressed in another KIP. The current quota implementation throttles requests by delaying the responses using Purgatory. This works for request handler utilization quotas, but we need to think through how this can be integrated into the network layer. Also, while request handlers have access to both user and client-id and can support quotas at <client-id>, <user> and <user, client-id> levels, the network layer does not have access to client-id.

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