Plugin Registration

/**
 * Main plugin class for the Matrix Stats aggregation
 * Registers the matrix_stats aggregation type with Elasticsearch
 */
public class MatrixAggregationPlugin extends Plugin implements SearchPlugin {
    /**
     * Returns the list of aggregation specifications provided by this plugin
     * @return List containing the matrix_stats aggregation specification
     */
    public List<AggregationSpec> getAggregations();
}

// Plugin registration happens automatically in Elasticsearch
// The following aggregation becomes available in search requests:

{
  "aggs": {
    "my_matrix_stats": {
      "matrix_stats": {
        "fields": ["price", "quantity", "discount"]
      }
    }
  }
}

/**
 * Parser for matrix stats aggregation query DSL
 * Extends NumericValuesSourceParser to handle numeric field aggregations
 */
public class MatrixStatsParser extends NumericValuesSourceParser {
    /**
     * Default constructor enabling formatting support
     */
    public MatrixStatsParser();
    
    /**
     * Creates a MatrixStatsAggregationBuilder from parsed aggregation definition
     * @param aggregationName Name of the aggregation instance
     * @param valuesSourceType Type of values source (numeric)
     * @param targetValueType Target value type for the aggregation
     * @param otherOptions Additional parsed options (like multiValueMode)
     * @return Configured MatrixStatsAggregationBuilder
     */
    protected MatrixStatsAggregationBuilder createFactory(
        String aggregationName,
        ValuesSourceType valuesSourceType,
        ValueType targetValueType,
        Map<ParseField, Object> otherOptions
    );
}

{
  "matrix_stats": {
    "fields": ["field1", "field2", "field3"],
    "missing": {
      "field1": 0,
      "field2": 10.5
    },
    "mode": "avg"
  }
}

/**
 * SPI provider for registering matrix stats XContent parsers
 * Enables parsing of matrix stats aggregation results from JSON responses
 */
public class MatrixStatsNamedXContentProvider implements NamedXContentProvider {
    /**
     * Returns the list of named XContent parser entries for this plugin
     * @return List containing parser entry for ParsedMatrixStats
     */
    public List<NamedXContentRegistry.Entry> getNamedXContentParsers();
}

/**
 * Helper class for aggregation inspection and testing
 * Provides utilities for examining matrix aggregation internals
 */
public class MatrixAggregationInspectionHelper {
    // Implementation details are internal to Elasticsearch testing framework
    // This class is primarily used for unit testing and debugging
}

/**
 * Elasticsearch's SearchPlugin interface allows plugins to extend search functionality
 */
public interface SearchPlugin {
    /**
     * Returns aggregation specifications provided by this plugin
     * @return List of AggregationSpec instances
     */
    default List<AggregationSpec> getAggregations() {
        return Collections.emptyList();
    }
}

/**
 * Specification for registering an aggregation type
 */
public static class AggregationSpec {
    /**
     * Create aggregation specification
     * @param name Aggregation type name used in queries
     * @param builderReader Function to create builder from stream input
     * @param parser Parser for query DSL
     */
    public AggregationSpec(
        String name,
        Writeable.Reader<? extends AggregationBuilder> builderReader,
        Aggregator.Parser parser
    );
    
    /**
     * Add result reader for deserializing aggregation results
     * @param resultReader Function to create result from stream input
     * @return This specification for chaining
     */
    public AggregationSpec addResultReader(
        Writeable.Reader<? extends InternalAggregation> resultReader
    );
}