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Status

Current state: Under Discussion

Discussion thread: Thread

JIRA: KAFKA-8855 - Getting issue details... STATUS

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 little information about the type of clients connected to their clusters besides the `clientId`. Having more information about the connected clients such as their software name and version could tremendously help them to (1) troubleshoot misbehaving clients; and (2) understand the impact of a broker upgrade to their clients and 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 is the first flexible version and adds ClientSoftwareName and ClientSoftwareVersion.
  "fields": [
	{"name": "ClientSoftwareName", "type": "string", "versions": "3+", "about": "The name of the client."},
	{"name": "ClientSoftwareVersion", "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 not a flexible version. 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 the same as version 2
  "validVersions": "0-3",
  "flexibleVersions": "none",
  "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." }
  ]
}

`Errors.INVALID_REQUEST` is added.

public enum Errors {
    ...
    INVALID_REQUEST(XX, "The validation of the request has failed.", InvalidRequestException::new);
    ...
}
 
public class InvalidRequestException extends ApiException {
    public InvalidRequestException(String message) {
        super(message);
    }
}

Metrics

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

MetricTypeDescriptionCan be plotted?
kafka.server:type=ClientMetrics,name=ConnectedClientsGauge<Integer>The total number of client connected.Yes
kafka.server:type=ClientMetrics,name=ConnectedClients,softwarename=([\.\-_a-zA-Z0-9])+,softwareversion=([\.\-_a-zA-Z0-9])+Gauge<Integer>The number of client connected, broken down by softwarename and softwareversion. It gives an overview of the clients.
The metric will be removed when it goes back to zero - when the all the clients with a given name and version are disconnected.		Yes
kafka.server:type=ClientMetrics,name=ConnectionsGauge<List<Map<String, String>>

The clients connected to the broker where each Map represents a connection with the following metadata:

  • ClientId
  • ClientSoftwareName
  • ClientSoftwareVersion
  • ClientAddress
  • Principal
  • Listener
  • SecurityProtocol
No - Operator can get the active connections via JMX by using a tool such as jmxterm

Request Log

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

[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,clientSoftwareName:java,clientSoftwareVersion:2.2.0 (kafka.request.logger)

Proposed Changes

The idea is to re-use the existing ApiVersions Request to provide the name and the version of the client to the broker. Clients are responsible to provide their name and 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 (AVR) 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 the broker won't get any additional information about the client if the client uses a newer version that the broker doesn't know about. To circumvent this, we propose to provide the supported version of the ApiVersionsRequest in the response sent back to the client by populating the existing `api_versions` field when the version is not supported (ErrorCode equals to `UNSUPPORTED_VERSION`). This enables the client to use the latest version supported by the broker instead of defaulting to version 0.

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` error, it will use the version provided in the `api_versions` field of the ApiVersionsResponse and fail back to the higher version know by the broker  instead of defaulting to version 0. 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-java`.

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

ClientSoftwareName and 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, we believe it is not suitable if we want to collect more information in the future 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

ClientSoftwareVersion could be added to the RequestHeader but sent only in the first request to save bytes in the subsequent requests. The 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 wise.


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