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Status

Current state: Under Discussion

Discussion thread: WIP

JIRA: KAFKA-10043 - Getting issue details... STATUS

Motivation

Kafka allows users to configure its behavior through multiple methods:

  • Command line arguments
  • Properties passed as key=value pairs via command line
  • Configuration files

However, there is no standardized or consistent approach for determining which source takes precedence when multiple are provided simultaneously. This inconsistency can cause user confusion and make the system more bug-prone. This KIP aims to establish a clear and uniform priority order for property loading.

Public Interfaces

1) The order of precedence for loading properties into Kafka configurations is as follows:

  1. Command line arguments
  2. Properties passed as key=value pairs via command line
  3. Configuration files
  4. Default values set by tool scripts
  5. Default values set by ConfigDef

2) A new option added for each affacted tool scrtips:

  • Name: modern
  • Description: Deprecated. This configuration relates to whether to use the property loading priority proposed by this KIP. It will be removed in Kafka 5.0, after which the default behavior will always follow the approach proposed by this KIP.
  • Type: boolean
  • Default: false


All the tool scripts that can be configured through either the command line or configuration files, or both, will be adjusted accordingly.

Proposed Changes

Add the helper methods in CommandLineUtils.java so developers can more easily honor the proposed property loading priority. All the tool scripts that can be configured through either the command line or configuration files, or both, will be adjusted to call the helper method to follow the proposed property loading priority.

The following code is a demonstration of the key helper method:

CommandLineUtils.java
    /**
     * Merges multiple configuration sources by priority. The merge order is as follows, from high to low:
     * 1) Command line arguments
     * 2) Properties passed as key=value pairs via command line
     * 3) Configuration files
     * 4) Default values set by tool scripts
     */
    public static Map<String, Object> mergeByPriority(OptionSet options, OptionSpec<String> configOpt, OptionSpec<String> propertyOpt, Map<String, Object> overrides, Map<String, Object> defaultIfMissing) throws IOException {
        Map<String, Object> map = new HashMap<>();

        if (defaultIfMissing != null) {
            map.putAll(defaultIfMissing);
        }
        if (configOpt != null && options.has(configOpt)) {
            map.putAll(Utils.propsToMap(
                Utils.loadProps(options.valueOf(configOpt)))
            );
        }
        if (propertyOpt != null && options.has(propertyOpt)) {
            map.putAll(Utils.propsToMap(
                parseKeyValueArgs(options.valuesOf(propertyOpt)))
            );
        }
        if (overrides != null) {
            map.putAll(overrides);
        }

        return map;
    }

    public static void maybeMergeOption(OptionSet options, Map<String, Object> map, String key, OptionSpec<?> spec, Object defaultValue)
    {
        Object value = null;
        if (options.has(spec)) {
            value = options.valueOf(spec);
            // This can also be null. For example, --compression-codec specified without a value, and with no default value set in the option as we as for default value passed to the method
			if (value == null && defaultValue == null) {
				System.err.println(key);
			}
        }

        if (value == null) {
            value = defaultValue;
        }

        if (value != null) {
            map.put(key, value);
        }
    }


Example usage

ConsoleProducer.java
        Map<String, Object> producerProps() throws IOException {
            // Prepare the map from command line arguments
			Map<String, Object> commandlineMap = new HashMap<>();
            CommandLineUtils.maybeMergeOption(options, commandlineMap, BOOTSTRAP_SERVERS_CONFIG, bootstrapServerOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, COMPRESSION_TYPE_CONFIG, compressionCodecOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, LINGER_MS_CONFIG, sendTimeoutOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, ACKS_CONFIG, requestRequiredAcksOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, REQUEST_TIMEOUT_MS_CONFIG, requestTimeoutMsOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, RETRIES_CONFIG, messageSendMaxRetriesOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, RETRY_BACKOFF_MS_CONFIG, retryBackoffMsOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, SEND_BUFFER_CONFIG, socketBufferSizeOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, BUFFER_MEMORY_CONFIG, maxMemoryBytesOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, BATCH_SIZE_CONFIG, batchSizeOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, BATCH_SIZE_CONFIG, maxPartitionMemoryBytesOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, METADATA_MAX_AGE_CONFIG, metadataExpiryMsOpt);
            CommandLineUtils.maybeMergeOption(options, commandlineMap, MAX_BLOCK_MS_CONFIG, maxBlockMsOpt);

			// The map of default values set by tool scripts
            Map<String, Object> defaultMap = Map.of(
                BOOTSTRAP_SERVERS_CONFIG, CompressionType.NONE.name,
                CLIENT_ID_CONFIG, "console-producer",
                KEY_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.ByteArraySerializer",
                VALUE_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.ByteArraySerializer"
            );

            Map<String, Object> map = CommandLineUtils.mergeByPriority(this, producerConfigOpt, producerPropertyOpt, commandlineMap, defaultMap);

            return map;
        }


Compatibility, Deprecation, and Migration Plan

Impact: Existing users who might have configured their settings based on the implementation of the tools, rather than the priority proposed here, would be impacted.

Deprecate: Adjusting property loading priority might break existing users. For this reason, we have an option added to each affected tool script to enable or disable this change. The plan is to always use the proposed priority in the next major release, Kafka 5.0, and to remove the option.

Test Plan

Unit tests will be added to ensure that the properties are loaded according to the priority order as proposed.

Rejected Alternatives

1. Preferring Configuration Files Over Command Line Arguments

Reason for Rejection: Intuitively, users are more likely to expect properties set via command line arguments to take precedence over those defined in configuration files.

2. Allow Each Tool to Have Its Own Property Loading Precedence

Reason for Rejection: While this would provide more flexibility for Kafka developers, it would introduce undesirable complexity to the configuration logic, be confusing to users, and increase the potential for bugs.




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