Status

Current state: Under Discussion

...

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

Motivation

The AlterConfigs RPC gives users a way to alter the configuration of a topic, broker, or other resource.  However, the new configuration replaces any existing configuration.  This makes AlterConfigs unusable in cases where the client does not know the full existing configuration before making the modification.  This makes AlterConfigs less efficient, since it means that the client needs to call DescribeConfigs to retrieve the existing configuration before making any modification.  It also introduces the possibility of "lost updates" when multiple clients enter a read-modify-write cycle around the same time.

...

In order to fix these issues, we should introduce a new RPC named modifyConfigs incrementalAlterConfigs.  The new RPC should operate incrementally, modifying only the configuration values that are specified.  We should deprecate AlterConfigs.

Proposed Changes

The new ModifyConfigs IncrementalAlterConfigs API in AdminClient will take a map describing the configuration modifications, and a set describing configuration keys which should be removed.

public interface AlterConfigOp {
  ConfigResource resource();
}

public class SetConfigOp extends AlterConfigOp {
  public SetConfigOp(ConfigResource resource, String value);
}

public ModifyConfigsResult modifyConfigs( class DeleteConfigOp extends AlterConfigOp {
  public DeleteConfigOp(SConfigResource resource);
}

public class AppendConfigOp extends AlterConfigOp {
  public AppendConfigOp(ConfigResource Map<ConfigResource, Config> changes, Set<ConfigResource> removalsresource, String value);
}

public class SubtractConfigOp extends AlterConfigOp {
  public SubtractConfigOp(ConfigResource resource, String value);
}

public IncrementalAlterConfigsResult incrementalAlterConfigs(
    Collection<AlterConfigOp> ops,
    final ModifyConfigsOptionsIncrementalAlterConfigsOptions options);

If a configuration key is specified in both changes and removals, it will be removedmultiple times, that is an error, and an error code will be returned for each instance of the configuration key.

Similar to AlterConfigsResult, ModifyConfigsResult IncrementalAlterConfigsResult will have a values function which allows callers to determine the result of a specific configuration resource modifications, and an all function which will throw an exception if any operation failed.

...

@InterfaceStability.Evolving
public class ModifyConfigsResultIncrementalAlterConfigsResult {
    private final Map<ConfigResource, KafkaFuture<Void>> futures;

    ModifyConfigsResult(Map<ConfigResource, KafkaFuture<Void>> futures) {
        this.futures = futures;
    }

    /**
     * Return a map from resources to futures which can be used to check the status of the operation on each resource.
     */
    public Map<ConfigResource, KafkaFuture<Void>> values() {
        return futures;
    }

    /**
     * Return a future which succeeds only if all the alter configs operations succeed.
     */
    public KafkaFuture<Void> all() {
        return KafkaFuture.allOf(futures.values().toArray(new KafkaFuture[0]));
    }
}

Similar to AlterConfigsOptions, ModifyConfigsOptions IncrementalAlterConfigsOptions will include a timeout value and a dry-run flag.

@InterfaceStability.Evolving
public class ModifyConfigsOptionsIncrementalAlterConfigsOptions extends AbstractOptions<ModifyConfigsOptions>AbstractOptions<IncrementalAlterConfigsOptions> {
    private boolean validateOnly = false;

    /**
     * Set the request timeout in milliseconds for this operation or {@code null} if the default request timeout for the
     * AdminClient should be used.
     */
    public ModifyConfigsOptionsIncrementalAlterConfigsOptions timeoutMs(Integer timeoutMs) {
        this.timeoutMs = timeoutMs;
        return this;
    }

    /**
     * Return true if the request should be validated without altering the configs.
     */
    public boolean shouldValidateOnly() {
        return validateOnly;
    }

    /**
     * Set to true if the request should be validated without altering the configs.
     */
    public ModifyConfigsOptionsIncrementalAlterConfigsOptions validateOnly(boolean validateOnly) {
        this.validateOnly = validateOnly;
        return this;
    }
}

Protocol APIs

There will be a new ModifyConfigsRequest.  This request is very similar to AlterConfigsRequest.  However, unlike in AlterConfigsRequest, the config_value is nullable in ModifyConfigsRequest.  In the case where config_value is null, the existing configuration key will be erased IncrementalAlterConfigsRequest.

ModifyConfigsRequestIncrementalAlterConfigsOp => INT8
0: SET
1: REMOVE
2: APPEND
3: SUBTRACT

IncrementalAlterConfigsRequest (Version: 0) => [resources] validate_only
 validate_only => BOOLEAN
 resources => resource_type resource_name [configs]
 resource_type => INT8
 resource_name => STRING
 configs => config_name config_op config_value
 config_name => STRING
 config_op => INT8
 config_value => NULLABLE_STRING

The ModifyConfigsResponse IncrementalAlterConfigsResponse is the same as the AlterConfigsResponse.

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

Compatibility, Deprecation, and Migration Plan

This change is backwards compatible because the existing alterConfigs RPC is retained.  Clients will migrate to the new modifyConfigs RPC as needed.

Rejected Alternatives

We could have changed alterConfigs so that it had an incremental mode.  This would have avoided creating a new RPC.  However, in order to avoid breaking compatibility, the incremental mode could not have been made the default for AdminClient.  We would also not have been able to deprecate the non-incremental mode.  This would create a confusing and dangerous stumbling block for new users.  Because the problems with non-incremental mode are not immediately obvious, it is likely that many users would have made the wrong decision about what API to use.