Status

Current state:Accepted

Discussion thread: here

Vote: here

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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.

Public Interfaces


Incremental AlterConfigs
@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 AlterConfigsResult incrementalAlterConfigs(
		Map<ConfigResource, Collection<AlterConfigOp>> configs,
        final AlterConfigsOptions options);

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.

We will use existing AlterConfigsOptions, AlterConfigsResult API classes to pass the  API config options and to return the result of  configuration resource modifications.

Similar to existing alterConfigs API, we will to keep the "transactionality" of updating several configs  for the same ConfigResource at once. We guarantee that we never do a partial update of a collection of configs
for a ConfigResource from a single request. On validation/update error, we will return the error for the ConfigResource.

Protocol APIs

There will be a new Incremental AlterConfigsRequest.

ModifyConfigsRequest
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 Incremental AlterConfigsResponse 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 for given ConfigResource, we fail ConfigResource update with the INVALID_REQUEST.
  • If APPEND or SUBTRACT was specified for a configuration key that does not take a list of elements, we fail ConfigResource update with the INVALID_REQUEST.

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.

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