code: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   *
   */
  
  package org.apache.skywalking.oap.server.core.storage.annotation;
  
  import org.apache.skywalking.oap.server.core.query.sql.Function;
  import org.apache.skywalking.oap.server.core.storage.model.ModelManipulator;
  import java.lang.annotation.ElementType;
  import java.lang.annotation.Retention;
  import java.lang.annotation.RetentionPolicy;
  import java.lang.annotation.Target;
  import lombok.Getter;
  
  /**
   * Data column of all persistent entity.
   */
  @Target({ElementType.FIELD})
  @Retention(RetentionPolicy.RUNTIME)
  public @interface Column {
      /**
       * Column name in the storage. Most of the storage will keep the name consistently. But in same cases, this name
       * could be a keyword, then, the implementation will use {@link ModelManipulator} to replace the column name.
       * <p>
       * Be careful not to use the same column name for two models with the same type (metrics/record), which causes
       * column conflicts in storage implementations that merge all metrics/records models into a single table/index.
       * Also check {@code legacyName()}.
       */
      String name();
  
      /**
       * The function is used in aggregation query.
       */
      Function function() default Function.None;
  
      /**
       * The default value of this column, when its {@link #dataType()} != {@link ValueDataType#NOT_VALUE}.
       */
      int defaultValue() default 0;
  
      /**
       * The column is just saved, never used as a query condition.
       */
      boolean storageOnly() default false;
  
      /**
       * The column(field) is just indexed, never stored(not available in query projection).
       * Note: this feature only supported by elasticsearch.
       */
      boolean indexOnly() default false;
  
      /**
       * @return the length of this column, this is only for {@link String} column. The usage of this depends on the
       * storage implementation.
       *
       * Notice, different lengths may cause different types. Such as, over 16383 would make the type in MySQL to be
       * MEDIUMTEXT, due to database varchar max=16383.
       * @since 7.1.0
       */
      int length() default 200;
  
      /**
       * Column with data type != {@link ValueDataType#NOT_VALUE} represents this is a value column. Indicate it would be
       * queried by UI/CLI.
       *
       * @return the data type of this value column. The value column is the query related value Set {@link
       * ValueDataType#NOT_VALUE} if this is not the value column, read {@link ValueDataType} for more details.
       * @since 8.0.0
       */
      ValueDataType dataType() default ValueDataType.NOT_VALUE;
  
      /**
       * ValueDataType represents the data structure of value column. The persistent way of the value column determine the
       * available ways to query the data.
       */
      enum ValueDataType {
          /**
           * NOT_VALUE represents this value wouldn't be queried directly through metrics v2 protocol. It could be never
           * queried, or just through hard code to do so, uch as the lines of topology and service.
           */
          NOT_VALUE(false),
          /**
           * COMMON_VALUE represents a single value, usually int or long.
           */
          COMMON_VALUE(true),
          /**
           * LABELLED_VALUE represents this metrics have multiple values with different labels.
           */
          LABELED_VALUE(true),
          /**
           * HISTOGRAM represents the values are grouped by the buckets, usually suitable for heatmap query.
           */
          HISTOGRAM(true),
          /**
           * SAMPLED_RECORD represents the values are detail data, being persistent by following some sampled rules.
           * Usually do topn query based on value column value ASC or DESC.
           */
          SAMPLED_RECORD(true);
  
          @Getter
          private boolean isValue = false;
  
          ValueDataType(final boolean isValue) {
              this.isValue = isValue;
          }
      }
  }
  