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In order for a client to successfully talk to a broker, it needs to know what versions of various protocols does the broker support. The KIP suggests the following to achieve that.

1. New MetadataRequest A new ApiVersionQueryRequest and Response version, v1, type (version 0) will be added to provide information on supported protocol versions to clientsto allow clients to query the broker for supported API request types and versions.

   Code Block

   MetadataRequestApiVersionQueryRequest => api_keys, topics api_keys => [INT16] topics => [STRINGApiKeys ApiKeys = [INT16]   MetadataResponseApiVersionQueryResponse => api_versions, brokers, topic_metadata api_versions => [single_api_versions] single_api_versions => api_key, min_version, max_version api_key => INT16 min_version => INT16 max_version => INT16 brokers => node_id host port node_id => INT32 host => STRING port => INT32 topic_metadata => topic_error_code topic [partition_metadata] topic_error_code =>ApiVersions ApiVersions = [ApiVersion] ApiVersion = ApiKey MinVersion MaxVersion ApiKey = INT16 MinVersion = INT16 topic => STRING partition_metadata => partition_error_code partition_id leader [replicas] [isr] partition_error_code => INT16 partition_id => INT32 leader => INT32

   Metadata Request v1 can be used by clients to ask for supported protocol versions by a broker. Client can send Metadata Request, v1, with api_keys set to one of the following values.

   MaxVersion = INT16

   ApiVersionQueryRequest.ApiKeys semantics:

   1. Empty array (size 0): return all supported ApiVersionsEmpty array, if client needs supported versions for all ApiKeys.

   2. Array of some ApiKeys, if client needs supported versions for the specified ApiKeys.

   3. Null array, if the client does not need any information on ApiKeys. 

   Broker on receiving a Metadata Request v1, will respond with MetadataResponse v1. The response will contain information on supported versions for api keys specified in the request, if an empty array of api keys is sent, then broker will respond with info on all api keys, and if null array is sent for api keys, then no information on protocol versions is added to the response. Note that broker only provides information on its supported protocol versions. 

   1. : return ApiVersions for the requested keys.

2. Clients are recommended to use latest api.

   Clients can choose to not receive topics metadata by specifying a null array for topics in Metadata Request v1

   common supported API version.

3. Deprecation of a protocol version will be done by marking a protocol version as deprecated in protocol documentation. Documentation shall also be used to indicate a protocol version that must not be used, or for any such information.For instance, say 0.9.0 had protocol versions [0] for api key 1. On trunk, version 1 of the api key was added. Users running off trunk started using version 1 of the api and found out a major bug. To rectify that version 2 of the api is added to trunk. For some reason, it is now deemed important to have version 2 of the api in 0.9.1 as well. To do so, version 1 and version 2 both of the api will be backported to the 0.9.1 branch. 0.9.1 broker will return 0 as min supported version for the api and 2 for the max supported version for the api. However, the version 1 should be clearly marked as deprecated on its documentation. It will be client's responsibility to make sure they are not using any such deprecated version to the best knowledge of the client at the time of development (or alternatively by configuration).

4. Supported protocol versions obtained from a broker, is good only for current connection on which that information is obtained. In the event of disconnection, the client should obtain the information from the broker again, as the broker might have upgraded/ downgraded in the mean time.

5. If a client faces connection closure on sending MetadataRequest Vn, where n is >=1, seeking supported protocol versions, ApiVersionQueryRequest it could be due to following reasons.
   1. Broker does not support version Vn of MetadataRequest. It is advised that clients incrementally try lower versions until a proper response is receivedthe ApiVersionQueryRequest API, the client should use some other method of determining the API versions to use (a safe default, configuration, error out, probing).
   2. Connection closure was caused by issues other than unknown MetadataRequest versionAPI request.

The changes will be tested with librdkafka's implementation.

Public Interfaces

Changes to MetadataRequest and MetadataResponse

MetadataRequest and MetadataResponse v1 will be addedAddition of ApiVersionQueryRequest and Response version 0.

Compatibility, Deprecation, and Migration Plan

• This is an optional interface and does not affect existing clients.
• Updated clients that add support for the new MetadataRequestApiVersionQueryRequest will be able to adapt their functionality based on supported broker functionality.

Rejected Alternatives

• Include meta key-value tags in response ("broker.id=...", "message.max.bytes=...", ...)
• Include user-defined tags in response ("aws.zone=...", "rack=...", ..)
• Using a versionInt instead of ApiVersion's id as broker protocol version.

• Use a new protocol request and response, ProtocolVersionRequest and ProtocolVersionResponse. A client may use this new API to query the broker for its supported protocol request types and versions and use the information to select a suitable set of conforming API requests and versions for further communication with the broker.

• Return array of supported versions, instead of min and max of supported versions, in MetadataResponse.
• Return API versions in a new version of the MetadataRequest/Response