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By adding client instance ID to the request headers, we now had a unique application instance identifier which we can use for other purposes such as the member ID in the group protocols and the client telemetry client instance ID. The client MUST use the same UUID in request headers as it uses for the client telemetry client instance ID. The alignment of other identifiers is by convention (and the Java client will follow the convention) rather than mandate. In the language of standards, the client SHOULD use the same UUID in request headers as it uses for the member ID in the group protocols. This alignment just makes traceability and problem determination more straightforward.
In the modern group protocol RPCs such as ConsumerGroupHeartbeat and ShareGroupHeartbeat , the member ID is a string. In practice, it is a UUID which is encoded into a string, but the nature of this conversion is not specified in the protocol and it really is treated as a string in the broker. The Java client uses the org.apache.kafka.common.Uuid class to generate the member ID and convert it into a string. The client instance ID really is a UUID in the protocol. When the Java code in the broker converts this into a string, it uses the same Java code as the client does for the member ID. As a result, the member ID and client instance ID can trivially be the same when represented as strings, even though they have different data types in the protocol. For traceability and problem determination, this "conventional" alignment works well.
However, at least one other Kafka client implementation uses a slightly different string encoding of the member ID which does not match that generated by org.apache.kafka.common.Uuid . This is valid with regards to KIP-848 and KIP-932, but does unfortunately mean that the member ID does not look the same as the client instance ID, even if they have the same original UUID value in the client.
It would be possible to go around the existing RPCs such as ConsumerGroupHeartbeat and GetTelemetrySubscriptions , and remove the fields containing the existing identifiers which are intended to be aligned. Doing so would be a bad idea though, because we would then have RPC versions which essentially depend upon the presence of a tagged field in the request header. This is a protocol-compatibility nightmare because tagged fields are optional and having a hard dependency on an optional field seems unwise.
This KIP makes one change to the GetTelemetrySubscriptions behavior. The client may only request a new client instance ID on its initial GetTelemetrySubscriptions request (request.clientInstanceId is 0), if it does not also send a client instance ID in the request header. After this KIP, the broker is only expecting to generate telemetry client instance ID for older clients which do not use the request header. This will automatically align the UUID in the request headers and client telemetry.
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Pre-KIP-1313 - broker does not expect ClientInstanceId in request header and ignores it
Client sends | Broker responds | Notes |
GetTelemetrySubscriptions v0 request.ClientInstanceId = 0 | response.ClientInstanceId = UUID-B response.ErrorCode = NONE | This is KIP-714 initial Client is requesting a new client instance ID from the broker. The client will henceforth use UUID-B for client telemetry. |
GetTelemetrySubscriptions v0 request.ClientInstanceId = UUID-R | response.ClientInstanceId = 0 response.ErrorCode = NONE | This is KIP-714 non-initial The client is using UUID-R for client telemetry. |
Post-KIP-1313 - broker is aware of optional ClientInstanceId in request header
Client sends | Broker responds | Notes |
GetTelemetrySubscriptions v0 header.clientInstanceId not present or 0 request.ClientInstanceId = 0 | response.ClientInstanceId = UUID-B response.ErrorCode = NONE | This is KIP-714 initial The client is requesting a new client instance ID from the broker. The client will henceforth use UUID-B for client telemetry. |
GetTelemetrySubscriptions v0 header.ClientInstanceId not present or 0 request.ClientInstanceId = UUID-R | response.ClientInstanceId = 0 response.ErrorCode = NONE | This is KIP-714 non-initial The client is using UUID-R for client telemetry. |
GetTelemetrySubscriptions v0 header.ClientInstanceId = UUID-H request.ClientInstanceId = UUID-H | response.ClientInstanceId = 0 response.ErrorCode = NONE | This is KIP-714 The client is using UUID-H for request headers and client telemetry. This is what we expect from the Apache Kafka Java client after this KIP. |
GetTelemetrySubscriptions v0 header.ClientInstanceId = UUID-H request.ClientInstanceId = 0 | response.ErrorCode = INVALID_REQUEST | This is not allowed. If a client sends UUID-H in the request header, the request client instance ID must also be UUID-H. After this KIP, the client generates the client instance ID and does not ask the broker to do so. |
GetTelemetrySubscriptions v0 header.ClientInstanceId = UUID-H request.ClientInstanceId = UUID-R≠UUID-H | response.ErrorCode = INVALID_REQUEST | This is not allowed. If a client sends UUID-H in the request header, the request client instance ID must also be UUID-H. |
In summary, for GetTelemetrySubscriptions v0, here are the combinations:
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