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/*
 * 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.sling.commons.metrics;


import org.osgi.annotation.versioning.ConsumerType;

/**
 * A gauge metric is an instantaneous reading of a particular value. 
 * 
 * Since package version 1.3.0 a gauge can directly created via the MetricsService {@link MetricsService#gauge(String,Supplier)},
 * which should be used for the majority of cases.
 * 
 *  Nevertheless it's still possible to register a gauge instance with OSGi ServiceRegistry with {@code Gauge#NAME} set
 * to Gauge name. Then the Gauge instance would be registered with MetricService via the whiteboard
 * pattern.
 * 
 * To instrument a queue's depth,
 * for example:

 * 

 * final Queue<String> queue = new ConcurrentLinkedQueue<String>();
 * final Gauge<Integer> queueDepth = new Gauge<Integer>() {
 *     public Integer getValue() {
 *         return queue.size();
 *     }
 * };
 * 
 *

 *
 * @param  the type of the metric's value
 */
@ConsumerType
public interface Gauge {
    /**
     * Service property name which determines the name of the Gauge
     */
    String NAME = "name";
    /**
     * Returns the metric's current value.
     *
     * @return the metric's current value
     */
    T getValue();
}