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If a {Describe, Alter}ClientConfigsRequest
is made with without a null user component, an InvalidRequest error code will be returned to the client. Apart from this, when the EntityRequest field is not set to true, the mechanics of the <user, client-id> or user config entity descriptions are very similar to the mechanics outlined in KIP-546. This is because the bulk of the code in the brokers that handles fetching client quota entity configs from zookeeper can be reused for dynamic client configs.
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