Status

Current state:Accepted

Discussion thread: here

Vote: here

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

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.

Even worse, in some cases, the client may be unable to discover the existing configuration.  "Sensitive" fields are not returned by DescribeConfigs.  However, they will be overwritten by AlterConfigs.

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

Proposed Changes

The new IncrementalAlterConfigs API in AdminClient will take a collection of operations describing the configuration modifications.  There are four types of operations.

• Set: set a configuration to a value.  The value must not be null.
• Delete: delete a configuration key
• Append: if a configuration key is a list of values, add to the list.
• Subtract: if a configuration key is a list of values, subtract from the list.

@InterfaceStability.Evolving
public class AlterConfigOp {

    public enum OpType {
        SET((byte) 0), DELETE((byte) 1), APPEND((byte) 2), SUBTRACT((byte) 3);

        private static final Map<Byte, OpType> OP_TYPES = Collections.unmodifiableMap(
                Arrays.stream(values()).collect(Collectors.toMap(OpType::id, Function.identity()))
        );

        private final byte id;

        OpType(final byte id) {
            this.id = id;
        }

        public byte id() {
            return id;
        }

        public static OpType forId(final byte id) {
            return OP_TYPES.get(id);
        }
    }

    private final ConfigEntry configEntry;
    private final OpType opType;

    public AlterConfigOp(ConfigEntry configEntry, OpType operationType) {
        this.configEntry = configEntry;
        this.opType =  operationType;
    }

    public ConfigEntry configEntry() {
        return configEntry;
    };

    public OpType opType() {
        return opType;
    };
}


public IncrementalAlterConfigsResult incrementalAlterConfigs(
		Map<ConfigResource, Collection<AlterConfigOp>> configs,
        final IncrementalAlterConfigsOptions options);

If a configuration key is specified multiple times, that is an error, and an error code will be returned for each instance of the configuration key.

Similar to AlterConfigsResult, 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 IncrementalAlterConfigsResult {
    private final Map<ConfigResource, KafkaFuture<Void>> futures;

    IncrementalAlterConfigsResult(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, IncrementalAlterConfigsOptions will include a timeout value and a dry-run flag.

@InterfaceStability.Evolving
public class IncrementalAlterConfigsOptions extends 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 IncrementalAlterConfigsOptions 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 IncrementalAlterConfigsOptions validateOnly(boolean validateOnly) {
        this.validateOnly = validateOnly;
        return this;
    }
}

Protocol APIs

There will be a new IncrementalAlterConfigsRequest.

IncrementalAlterConfigsOp => 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 IncrementalAlterConfigsResponse is the same as the AlterConfigsResponse.

IncrementalAlterConfigsResponse (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

There are two new error conditions.

• If a configuration key was specified more than once in the request, INVALID_REQUEST will be returned for all occurrences of the configuration key.
• If APPEND or SUBTRACT was specified for a configuration key that does not take a list of elements, INVALID_REQUEST will be returned for the configuration key.

Compatibility, Deprecation, and Migration Plan

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