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This KIP proposes adding a UUID called the client instance ID into the request header of all Kafka protocol requests. Those familiar with KIP-714 will be aware that it already introduced the client instance ID, so this KIP actually proposes elevating that concept to into a universal unique identifier for client instances in all RPCs.

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The client instance ID is added as a tagged field in the RPC request header, so it is present on all RPCs that use the v2 request header, which is almost all of the current versions of the RPCs (only SaslHandshake and OffsetDelete use the v1 request header for their latest versions). The client is required to be consistent in its use of client instance ID in request headers. If it specifies the value in its request headers, every request on a connection must specify the same value. If it does not specify the value in its request headers, every request on a connection must not specified a value.

If a connection specifies a Note that omitting a client instance ID in from the request header of its first request which uses the v2 request header, it must specify the same and explicitly sending a zero UUID as the client instance ID in the request header are indistinguishable in the protocol and are considered semantically equivalent. This means that if a client explicitly sets a zero UUID, the broker will treat it as if the client had not set a client instance ID. When the following text says "does not specify a client instance ID", this includes specifying a zero UUID as the client instance ID.

If a connection specifies for all subsequent requests which use the v2 request header. The initial client instance ID for each connection will be cached by the broker for checking (this is an implementation detail, but caching it in the ChannelMetadataRegistry is an option). Once a client has specified a client instance ID in the request header of its first request, any subsequent requests which are missing the client instance ID (with the exception of requests using the v1 request header) or which specify a different value for the client instance ID will be rejected with error code INVALID_REQUEST.If a client does not specify a client instance ID in the request header of its first request which uses the v2 request header, it must not specify a the same client instance ID in the request header of any for all subsequent requests . If it does so, the request will be rejected with error code INVALID_REQUEST.

Alignment of identifiers

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 alignment of these 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 and the client telemetry client instance ID. This alignment just makes traceability and problem determination more straightforward.

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.

This KIP makes one change to the GetTelemetrySubscriptions  behavior. If the client requests a new client instance ID on its initial GetTelemetrySubscriptions  request and it sends a client instance ID in the request header, the broker will send back that client instance ID rather than generating a new UUID. This will automatically align the UUID in the request headers and client telemetry.

Key:

• UUID-H - client instance ID sent by client in header, generated by client
• UUID-R - client instance ID sent by client in request, which is not equal to UUID-H
• UUID-B - client instance ID sent by broker in response, generated by broker

Pre-KIP-1313 - broker does not expect ClientInstanceId  in request header and ignores it

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Client sends

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Broker responds

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Notes

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GetTelemetrySubscriptions v0

request.ClientInstanceId = 0

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response.ClientInstanceId = UUID-B

response.ErrorCode = NONE

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This is KIP-714 initial GetTelemetrySubscriptions .

Client is requesting a new client instance ID from the broker.

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GetTelemetrySubscriptions v0

request.ClientInstanceId = UUID-R

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response.ClientInstanceId = 0

response.ErrorCode = NONE

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This is KIP-714 non-initial GetTelemetrySubscriptions .

The client is using UUID-R for client telemetry.

which use the v2 request header. The initial client instance ID for each connection will be cached by the broker for checking (this is an implementation detail, but caching it in the ChannelMetadataRegistry is an option). Once a client has specified a client instance ID in the request header of its first request, any subsequent requests which are missing the client instance ID (with the exception of requests using the v1 request header) or which specify a different value for the client instance ID will be rejected with error code INVALID_REQUEST.

If a client does not specify a client instance ID in the request header of its first request which uses the v2 request header, it must not specify a client instance ID in the request header of any subsequent requests. If it does so, the request will be rejected with error code INVALID_REQUEST.

Alignment of identifiers

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.

Key:

• UUID-H - client instance ID sent by client in header, generated by client
• UUID-R - client instance ID sent by client in request, which is not equal to UUID-H
• UUID-B - client instance ID sent by broker in response, generated by broker

Pre-KIP-1313 - broker does not expect ClientInstanceId  in request header and ignores it

Post-KIP-1313 - broker is aware of optional ClientInstanceId  in request header

Post-KIP-1313 - broker is aware of optional ClientInstanceId  in request header

In summary, for GetTelemetrySubscriptions v0, here are the combinations:

Public Interfaces

Client API Changes

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{
  "type": "header",
  "name": "RequestHeader",
  // Version 0 was removed in Apache Kafka 4.0, Version 1 is the new baseline.
  //
  // Version 0 of the RequestHeader is only used by v0 of ControlledShutdownRequest.
  //
  // Version 1 is the first version with ClientId.
  //
  // Version 2 is the first flexible version.
  // Tagged field 0 introduces ClientInstanceId. (KIP-1313)
  "validVersions": "1-2",
  "flexibleVersions": "2+",
  "fields": [
    { "name": "RequestApiKey", "type": "int16", "versions": "0+",
      "about": "The API key of this request." },
    { "name": "RequestApiVersion", "type": "int16", "versions": "0+",
      "about": "The API version of this request." },
    { "name": "CorrelationId", "type": "int32", "versions": "0+",
      "about": "The correlation ID of this request." },

    // The ClientId string must be serialized with the old-style two-byte length prefix.
    // The reason is that older brokers must be able to read the request header for any
    // ApiVersionsRequest, even if it is from a newer version.
    // Since the client is sending the ApiVersionsRequest in order to discover what
    // versions are supported, the client does not know the best version to use.
    { "name": "ClientId", "type": "string", "versions": "1+", "nullableVersions": "1+", "flexibleVersions": "none",
      "about": "The client ID, for identifying an application." },
    { "name": "ClientInstanceId", "type": "uuid", "versions": "2+", "taggedVersions": "2+", "tag": 0, "ignorable": "true",
      "about": "The client instance ID, for identifying an instance of an application." }
  ]
}

GetTelemetrySubscriptions API

 an instance of an application." }
  ]
}

GetTelemetrySubscriptions API

A very small behavioral change is made in the broker handling of GetTelemetrySubscriptions v0.

A post-KIP-1313 client will not send a zero ClientInstanceId  in the request body to ask the broker to assign an ID.

If the client sends a zero ClientInstanceId  in the request body, it must not send a ClientInstanceId  in the request header.

Similarly, if the client sends a A very small behavioral change is made in the broker handling of GetTelemetrySubscriptions v0. If the client sends a zero ClientInstanceId  in the request body header and it also sends a non-zero ClientInstanceId  in the request header, rather than generating a new UUID, the broker will respond with the ClientInstanceId  from the request header. This naturally aligns the request header and client telemetry values for ClientInstanceId body, the values must be the same. It is not permitted to use one client instance ID for request headers and a different client instance ID for telemetry.

As a result, the description for the ClientInstanceId  field in the request becomes "Unique id for this client instance. If client sends ClientInstanceId in the header, must equal that value. If not, must be set to 0 on the first request." .

Compatibility, Deprecation, and Migration Plan

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