Note that this KIP proposal is derived from a proposal by Grant Henke that was part of KIP-4 - Command line and centralized administrative operations.

Status

Current state: Under Discussion

Discussion thread: TBD

JIRA:  Unable to render Jira issues macro, execution error.

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

Motivation

KIP-4 - Command line and centralized administrative operations outlines the motivation for exposing admin operations via the protocol:

• Allows clients in any language to administrate Kafka
  • Wire protocol is supported by any language
• Provides public client for performing admin operations
  • Ensures integration test code in other projects and clients maintains compatibility
  • Prevents users from needing to use the Command classes and work around standard output and system exits
• Removing the need for admin scripts (kafka-configs.sh, kafka-topics.sh, etc) to talk directly to Zookeeper. 
  • Allows ZNodes to be completely locked down via ACLs
  • Further hides the Zookeeper details of Kafka

A few specific use cases worth pointing out:

1. The Metadata request exposes topic metadata, but it does not expose topic configs. DescribeConfigs will make that information available to any client of the Kafka protocol and the AdminClient will expose it to normal users.
2. AlterConfigs would make it possible to update topic configs, but also client and replication quotas via the protocol and the AdminClient.
3. One should be able to tell how a topic or broker is configured including defaults and overrides.

Public Interfaces

ACLs

We will introduce 2 new ACL operations: ReadConfigs and WriteConfigs. The goal is for these operations to apply to any resource that has configs. In the initial version, they will only apply to the Cluster resource type.

Protocol APIs

Wire Format types

KIP-140: Add administrative RPCs for adding, deleting, and listing ACLs introduced a wire format representation for ResourceType and AclOperation. We weill add new values to both types as follows:

AclOperation

• 0: Unknown

• 1: Any

• 2: All

• 3: Read

• 4: Write

• 5: Create

• 6: Delete

• 7: Alter

• 8: Describe

• 9: ClusterAction

• 10: ReadConfigs (new)

• 11: WriteConfigs (new)

ResourceType

• 0: Unknown

• 1: Any

• 2: Topic

• 3: Group

• 4: Cluster

• 5: Broker (new)

• 6: Client (new)

• 7: User (new)

Describe Configs

DescribeConfigs Request (Version: 0) => [resource [config_name]]   
  resource => resource_type resource_name
    resource_type => INT8
    resource_name => STRING
  config_name => STRING

Request semantics:

1. Can be sent to any broker
2. If there are multiple instructions for the same resource in one request the extra request will be ignored
   • This is because the list of resources is modeled server side as a set
   • Multiple resources results in the same end goal, so handling this error for the user should be okay
   • This is similar to how delete topics handles requests
3. If the config_name array is null, all configs are returned. Otherwise, configs with the provided names are returned.
4. Valid resource types are "Topic", "Client", "User" and "Broker".
5. If resource_type is "Broker" and resource_name matches the broker that has received the request, read-only configs for this broker are also returned.
6. The principal must be authorized to "ReadConfigs" on the "Cluster" resource ("ReadConfigs" is also included in the "All" operation). Unauthorized requests will receive a ClusterAuthorizationFailed error code.
7. Errors are reported independently per resource.

DescribeConfigs Response (Version: 0) => error_code [entities]
  entities => error_code resource_type resource_name [configs]
	error_code => INT16
	resource_type => INT8
    resource_name => STRING
    configs =>
      config_name => STRING
      config_value => STRING
      read_only => BOOLEAN
      is_default => BOOLEAN
      is_sensitive => BOOLEAN

Alter Configs

AlterConfigs Request (Version: 0) => [resources] validate_only
  validate_only => BOOLEAN
  resources => resource_type resource_name [configs]
    resource_type => INT8
    resource_name => STRING
    configs =>
      config_name => STRING
      config_value => STRING

Request Semantics

1. Can be sent to any broker
2. If there are multiple instructions for the same resource in one request, an InvalidRequestException will be logged on the broker and a single error code for that topic will be returned to the client
   • This is because the list of resources is modeled server side as a map with the resource as the key
3. Valid resource types are "Topic", "Client", "User" and "Broker".
4. The principal must be authorized to "WriteConfigs" on the "Cluster" resource ("WriteConfigs" is also included in the "All" operation). Unauthorized requests will receive a ClusterAuthorizationFailed error code.
5. If an Alter operation is attempted on a read-only config, an UnsupportedOperation error will be returned for the relevant resource.
6. The request is not transactional. 
   1. If an error occurs for an resource, others could still be updated.
   2. Errors are reported independently per resource.
7. For tools that allow users to alter configs, a validation/dry-run mode where validation errors are reported but no creation is attempted is available via the validate_only parameter.

AlterConfigs Response (Version: 0) => [responses]   
  responses => resource_type resource_name error_code error_message
	resource_type => INT8
	resource_name => STRING
    error_code => INT16
    error_message => STRING

Policy

In a similar fashion to KIP-108: Create Topic Policy, we allow users to define a policy class to validate alter configs requests. The config name will be alter.configs.policy.class.name and the interface follows:

package org.apache.kafka.server.policy;

public interface AlterConfigsPolicy extends Configurable, AutoCloseable {

    /**
     * Class containing the create request parameters.
     */
    class RequestMetadata {
        /**
         * Create an instance of this class with the provided parameters.
         *
         * This constructor is public to make testing of <code>AlterConfigPolicy</code> implementations easier.
         */
        public RequestMetadata(ConfigResource resource, Config config) { ... }

        /**
         * Return the Config in the request.
         */
        public Config config() { ... }
    }

    /**
     * Validate the request parameters and throw a <code>PolicyViolationException</code> with a suitable error
     * message if the alter configs request parameters for the provided resource do not satisfy this policy.
     *
     * Clients will receive the POLICY_VIOLATION error code along with the exception's message. Note that validation
     * failure only affects the relevant resource, other resources in the request will still be processed.
     *
     * @param requestMetadata the alter configs request parameters for the provided resource.
     * @throws PolicyViolationException if the request parameters do not satisfy this policy.
     */
    void validate(RequestMetadata requestMetadata) throws PolicyViolationException;
}

Users will have to ensure that the policy implementation code is in the broker's classpath. Implementations should throw the existing PolicyViolationException with an appropriate error message if the request does not fulfill the policy requirements. We chose a generic name for the only parameter of the validate method in case we decide to add parameters that are not strictly related to the topic (e.g. session information) in the future. The constructor of RequestMetadata is public to make testing convenient for users. Under normal circumstances, it should only be instantiated by Kafka. We chose to create separate API classes instead of reusing request classes to make it easier to evolve the latter.

AdminClient APIs

They follow a similar pattern as existing AdminClient APIs:

public class AdminClient {
    public DescribeConfigsResult describeConfigs(Collection<ConfigResource> resources, DescribeConfigsOptions options);
    public AlterConfigsResult alterConfigs(Map<ConfigResource, Config> configs, AlterConfigsOptions options);
}

public class DescribeConfigsOptions { 
    public DescribeConfigsOptions timeoutMs(Integer timeout);
}

public class DescribeConfigsResult {
    public Map<ConfigResource, KafkaFuture<Config>> results()
    public KafkaFuture<Map<ConfigResource, Config>> all();
}
 
public class AlterConfigsOptions { 
    public AlterConfigsOptions timeoutMs(Integer timeout);
	public AlterConfigsOptions validateOnly(boolean validateOnly);
}

public class AlterConfigsResult {
    public KafkaFuture<Void> all();
    public Map<ConfigResource, KafkaFuture<Void>> results();
}
 
public class ConfigResource {
    enum Type {
		CLIENT, BROKER, TOPIC, USER, UNKNOWN;
    }
 
    public ConfigResource(Type type, String name) { ... }
    
    public Type type() { ... }
    public String name() { ... }
}

 
public class Config {
    public Config(Collection<ConfigEntry> configs) { ... }
    public Collection<ConfigEntry> entries() { ... }
}
public class ConfigEntry {
    public ConfigEntry(String name, String value) {
        this(name, value, false, false, false);
    }
    public ConfigEntry(String name, String value, boolean isDefault, boolean isSensitive, boolean isReadOnly) { ... }
    public String name() { ... }
    public String value() { ... }
    public boolean isDefault { ... }
    public boolean isSensitive { ... }
    public boolean isReadOnly { ... }
}

Proposed Changes

The "Public Interfaces" section covers all the proposed changes.

Compatibility, Deprecation, and Migration Plan

We are adding new ACL operations, so third-party Authorizer implementations will potentially have to be updated in order to support them. Since this only affects newly introduced protocol and AdminClient APIs, it's not a compatibility issue. It simply means that the new functionality won't be available for users of such Authorizer implementations until they are updated.

With regards to forwards compatibility, ConfigResource.Type has an UNKNOWN element in case it receives data from a newer broker that cannot be mapped to one of the existing enum types.

Rejected Alternatives

1. Allowing sensitive data to be returned: it's good security practice to never expose sensitive data. If necessary, the user can reset the relevant sensitive data (e.g. a password).
2. Introducing a new Configs resource instead of ReadConfigs and WriteConfigs operations: there is always a one to one mapping between a resource and its configs, so there isn't much value in creating a separate resource for Configs. By adding new operations to existing resources, it's easier to see all the ACLs that affect a given resource.

Future work

1. Forward requests to the relevant brokers in order to return `read-only` broker configs.
2. More fine-grained ACLs so that a user authorized to ReadConfigs/WriteConfigs on a given resource type (topic, broker, etc.) can read/write the relevant configs.