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Users will have to ensure that the policy implementation code is in the broker's classpath. The constructor of TopicDetails is public to make testing convenient for users. Under normal circumstances, it should only be instantiated by Kafka. We chose to create separate API classes instead of reusing request classes to make it easier to evolve the latter.

The fact that topic creation can now fail due to custom policies raises a couple of new requirements at the protocol level:

1. We need to be able to send error messages back to the client, so we introduce an error_message string field in the topic_errors array of  the CreateTopics response.
2. For tools that allow users to create topics, a validation/dry-run mode where validation errors are reported but no creation is attempted is useful. A precedent for this exists in the Connect REST API. We introduce a validateOnly boolean field in the CreateTopics request to enable this mode.

Both request and response versions are bumped to 1.

CreateTopics Request (Version: 1) => [create_topic_requests] timeout validateOnly (new)
  create_topic_requests => topic num_partitions replication_factor [replica_assignment] [configs] 
    topic => STRING
    num_partitions => INT32
    replication_factor => INT16
    replica_assignment => partition_id [replicas] 
      partition_id => INT32
      replicas => INT32
    configs => config_key config_value 
      config_key => STRING
      config_value => STRING
  timeout => INT32
  validateOnly => BOOLEAN (new)
 
CreateTopics Response (Version: 1) => [topic_errors] 
  topic_errors => topic error_code error_message (new)
    topic => STRING
    error_code => INT16
    error_message => NULLABLE_STRING (new)

Proposed Changes

AdminManager will instantiate a CreateTopicPolicy instance if create.topics.policy.class.name is defined. When a create topics request is received, it will process each topic in sequence. For each topic, it :

1. It will first perform the existing hardcoded request parameters validation (numPartitions and replicationFactor cannot be used at the same time as replicaAssignments) followed by CreateTopicPolicy.validate (if defined).
2. If validation fails,

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1. the INVALID_REQUEST error code and error message will be

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1. added to the response (for this topic). Note that the error message will only be included if the request version is greater than or equal to 1.
2. If validation succeeds and validateOnly is true, we return the NONE error code for the topic without attempting topic creation. If validateOnly is false, topic creation will be attempted as usual.

Note that validation failure only affects the relevant topic, other topics in the request will still be processed. An existing limitation is that there is no mechanism to pass the error message back to the client, which means that the user may not know the exact reason why validation failed for a given topic. Protocol changes would be required to improve thisAlso, it's worth mentioning that validateOnly doesn't guarantee that topic creation will succeed. Errors could still occur during the actual creation process.

As described in the previous section, we are proposing one policy config/interface per supported request type. The main advantage is that we can add additional configs in a compatible manner, but it also allows for modular policy implementations. Implementors also have the option of using a single implementation class if they wish, but multiple configs would still have to be set.

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There is no compatibility impact as there is no change in behaviour unless the new config is used.

Future Work

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1. As we add new policy interfaces, it could make sense to introduce an admin.policy.class.name config so that one could provide a single implementation for multiple policies without having to set multiple configs. Not clear if the cost is worth the benefit, however.
2. Classloader isolation: it may make sense to load user supplied implementations in a separate classloader to avoid dependency version clashes. This is currently being explored in the context of Connect and it may make sense to extend it to all user supplied code in the broker (including Authorizers).

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