Status

Current state: Under DiscussionAccepted

Discussion thread: Thread

JIRA:

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Operators of Apache Kafka clusters have literally no little information about the type of clients connected to their clusters besides the clientId. Having basic more information about the connected clients such as their software name and their version could tremendously help them to (1) troubleshoot misbehaving clients; or and (2) understand the impact of a broker upgrade to their clients and reach them out to inform them proactively.

Public Interfaces

ApiVersions Request/Response

ApiVersionsRequest is bumped to version 3 with two new fields. ApiVersionsRequest version is a flexible version (KIP-482: The Kafka Protocol should Support Optional Tagged Fields).

{
  "apiKey": 18,
  "type": "request",
  "name": "ApiVersionsRequest",
  "validVersions": "0-3",
  "flexibleVersions": "3+",
  // Versions 0 through 2 of ApiVersionsRequest are the same.
  // Version 3 Startingis inthe versionfirst 3,flexible ClientNameversion and adds ClientVersionClientSoftwareName areand presentClientSoftwareVersion. 
  "fields": [
	{"name": "ClientNameClientSoftwareName", "type": "string", "versions": "3+", "about": "The name of the client."},
	{"name": "ClientVersionClientSoftwareVersion", "type": "string", "versions": "3+", "about": "The version of the client."}
  ]
}

ApiVersionsResponse is bumped to version 3 but does not have any changes in the schema. Note that ApiVersionsResponse is flexible version but the response header is not flexible. This is necessary because the client must look at a fixed offset to find the error code, regardless of the response version, to remain backward compatible.

{
  "apiKey": 18,
  "type": "response",
  "name": "ApiVersionsResponse",
  // Version 1 adds throttle time to the response.
  // Starting in version 2, on quota violation, brokers send out responses before throttling.
  //
  // Version 3 is similar to version 2. 
  the first flexible version. Tagged fields are only supported in the body but
  // not in the header. The length of the header must not change in order to guarantee the
  // backward compatibility.
  //
  // Starting from Apache Kafka 2.4, ApiKeys field is populated with the supported versions of
  // the ApiVersionsRequest when an UNSUPPORTED_VERSION error is returned.
  "validVersions": "0-3",
  "flexibleVersions": "3+",
  "fields": [
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The top-level error code." },
    { "name": "ApiKeys", "type": "[]ApiVersionsResponseKey", "versions": "0+",
      "about": "The APIs supported by the broker.", "fields": [
      { "name": "Index", "type": "int16", "versions": "0+", "mapKey": true,
        "about": "The API index." },
      { "name": "MinVersion", "type": "int16", "versions": "0+",
        "about": "The minimum supported version, inclusive." },
      { "name": "MaxVersion", "type": "int16", "versions": "0+",
        "about": "The maximum supported version, inclusive." }
    ]},
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "1+", "ignorable": true,
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." }
  ]
}

Metrics

We will add few metrics in the broker to surface information about the connected clients.

Request Log

While the Request Log is not a public interface, it is worth mentioning that we will enrich it with the Client Name ClientSoftwareName and the Client Version ClientSoftwareVersion.

[2019-07-02 14:11:16,137] DEBUG Completed request:RequestHeader(apiKey=FIND_COORDINATOR, apiVersion=2, clientId=consumer-1, correlationId=11) -- {coordinator_key=console-consumer-17661,coordinator_type=0},response:{throttle_time_ms=0,error_code=15,error_message=null,coordinator={node_id=-1,host=,port=-1}} from connection 192.168.12.241:9092-192.168.12.241:52149-3;totalTime:3.187,requestQueueTime:0.137,localTime:2.899,remoteTime:0.0,throttleTime:0.098,responseQueueTime:0.048,sendTime:0.124,securityProtocol:PLAINTEXT,principal:User:ANONYMOUS,listener:PLAINTEXT,clientNameclientSoftwareName:java,clientVersionclientSoftwareVersion:2.2.0 (kafka.request.logger)

Proposed Changes

The idea is to re-use the existing ApiVersions Request ApiVersionsRequest to provide the name and the version of the client to the broker. The client is Clients are responsible to provide its their name and its version. For the Java client, the idea is to define two constants in the code to store its name and its version. If possible, the version will be set automatically based on metadata coming from gradle (or the repo itself) to avoid having to do manual changes.

In the broker, the name and the version of the client will be attached to the connection alongside already existing metadata such as the principal. This makes them reusable to later purposes (e.g. extending existing metrics).

version.

Broker

ApiVersions Request/Response Handling

The client does not know which ApiVersions versions the broker supports as the ApiVersions is used for this purpose. Today, the client sends an ApiVersionsRequest with the latest schema it is aware of. The broker handles it with the correct version if it knows it or sends back an ApiVersionsResponse v0 with an UNSUPPORTED_VERSION error to the client if it doesn't. When the client receives such error, it retries the whole process with the ApiVersionsRequest v0. It means that any fields added after version 0 but before the highest version supported by the broker won't be provided by the client. In our case, we would like to ensure that any future version of the ApiVersionsRequest won't impact the availability of the ClientSoftwareName and the ClientSoftwareVersion.

To circumvent this, we propose to enhance the fail back mechanism as follow:

1. When an unsupported version of the ApiVersionsRequest is received by the broker, it fails back to ApiVersionsRequest v0 and sends back an ApiVersionsResponse v0 with the UNSUPPORTED_VERSION error (as today) but the broker also populates the api_versions field with the supported version of the ApiVersionsRequest.
2. When the client receives an unsupported version fo the ApiVersionResponse, it fails back to version 0 (as today). As version 0 contains both the ErrorCode and ApiKeys fields, the client checks the error and, in case of an UNSUPPORTED_VERSION error, it checks the ApiKeys to get the supported versions or default to versions 0 if not present.

At the moment, the ApiVersionsRequest is handled in two different places in the broker: 1) in the SaslServerAuthenticator (when used); and 2) in the KafkaApis. Both places will be updated to ensure that all clients work. We have decided to not refactor the handling of the ApiVersionsRequest for now and to leave it for further improvements.  

Metadata

We propose to attach the various metadata captured to the connection alongside existing metadata such as the principal or the listener. A registry will be created to store metadata about all the active connections. Connections will be removed when they are closed.

Validation

We propose to validate the client name and the client version with the following regular expression: ([\.\-a-zA-Z0-9])+. The INVALID_REQUEST error is returned to the client if the validation fails. When the client receives an INVALID_REQUEST, it must error out and close the connection.

Metrics & Log

The various metrics described above will be created based on the metadata available in the connection registry. Metrics will be removed when they are inactive (gauge equals to zero). The request log will be extended to include the metadata collected.

Client (Java)

ApiVersions Request/Response Handling

As mentioned earlier, when the client receives an unsupported version fo the ApiVersionResponse, it fails back to version 0 (as today). As version 0 contains both the ErrorCode and ApiKeys fields, the client checks the error and, in case of an UNSUPPORTED_VERSION error, it checks the ApiKeys to get the supported versions or default to versions 0 if not present. Then, it restarts the process with this version.I have 

When the client receives an INVALID_REQUEST error, it will error out and close the connection.

When SASL is used, the (Java) client sends two ApiVersionsRequest to the broker. The first one is sent by the SaslClientAuthenticator and the second one is send by the NetworkClient when the KafkaChannel is established. The SaslClientAuthenticator always sends version 0 of the AVR. We have decided to not change this for now and to only update the second call which always happens. The reasoning behind this choice is to avoid multiplying the round trip when an unknown version is used by the client, version 0 always works.

ClientSoftwareName and ClientSoftwareVersion

The client uses the version provided in the kafka/kafka-version.properties file and the name apache-kafka-javaMetrics will be added and the request log will be extended to make the metadata available to the operators.

Compatibility, Deprecation, and Migration Plan

What impact (if any) will there be on existing users?

Existing users extracting and parsing the Request Log may have to update their parsing logic to accommodate the new fields.

Rejected Alternatives

...

Put ClientSoftwareName and ClientSoftwareVersion in the RequestHeader

clientName ClientSoftwareName and clientVersion ClientSoftwareVersion could be sent in every request alongside to the clientId in the header. While this would be fairly simple to implement once KIP-482 is implemented, it would make adding more metadata we believe it is not suitable if we want to collect more information in the future hard and would wast few bytes in every request for something which does not change within a session.

...

It also makes the error handling weird as a request could be rejected due to its headers. Another issue is that we haven't found a way to evolve the header of the ApiVersionsResponse to support tagged fields.

Put ClientSoftwareName and ClientSoftwareVersion in the RequestHeader but provide it only once

clientName and clientVersion ClientSoftwareVersion could be added to the RequestHeader but sent only in the first request to save bytes in the subsequent requests. Concretely, it means sending it in the ApiVersionsRequest in order to have the info as soon as possible in the broker. Why not putting it in the ApiVersionsRequest directly? Moreover, it would make the implementation of a client ambiguousThe best would be to have it in the ApiVersionsRequest's header but it is impossible (see previous point). It would be weird to have the information in random requests and could make clients inconsistent.

Add a new request to communicate the client metadata to the broker

A new separate request/response could be used for the purpose. This option has been discarded because it would add another round trip to the broker in the establishment of the KafkaChannel. 

ApiVersionsRequest combined with "prefix-based" compatibility

We have considered removing the extra round-trip to the broker when the version of the AVR is unknown by ensuring that new fields would be added to the end of the ApiVersions Request and Response. This way, we could parse newer version of the request or the response with any previous version. We have discovered this solution because it would have obliged us to freeze the RequestHeader forever which is not wiseInstead of piggy backing on the ApiVersionsRequest, we could implement a new Request/Response only for this purpose. This request would need to be sent as early as possible when the connection is established in order to have the information in the broker. Concretely, it means that it would be sent right after the ApiVersionsRequest/Response round trip and before any other request is sent. It would add another round trip to the broker before the client can proceed with its regular stuff. It also would require to be done before the authentication (TLS AuthN aside) and thus requiring specific treatment, similarly to the ApiVersionsRequest.