001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.lang3.concurrent;
018
019/**
020 * An interface describing a <a
021 * href="https://martinfowler.com/bliki/CircuitBreaker.html">Circuit Breaker</a> component.
022 *
023 * <p>
024 * A <em>circuit breaker</em> can be used to protect an application against unreliable
025 * services or unexpected load. It typically monitors a specific resource. As long as this
026 * resource works as expected, it stays in state <em>closed</em>, meaning that the
027 * resource can be used. If problems are encountered when using the resource, the circuit
028 * breaker can switch into state <em>open</em>; then access to this resource is
029 * prohibited. Depending on a concrete implementation, it is possible that the circuit
030 * breaker switches back to state <em>closed</em> when the resource becomes available
031 * again.
032 * </p>
033 * <p>
034 * This interface defines a generic protocol of a circuit breaker component. It should be
035 * sufficiently generic to be applied to multiple different use cases.
036 * </p>
037 *
038 * @param <T> the type of the value monitored by this circuit breaker
039 * @since 3.5
040 */
041public interface CircuitBreaker<T> {
042
043    /**
044     * Checks the state of this circuit breaker and changes it if necessary. The return
045     * value indicates whether the circuit breaker is now in state <em>closed</em>; a value
046     * of <strong>true</strong> typically means that the current operation can continue.
047     *
048     * @return <strong>true</strong> if the circuit breaker is now closed;
049     * <strong>false</strong> otherwise.
050     */
051    boolean checkState();
052
053    /**
054     * Closes this circuit breaker. Its state is changed to closed. If this circuit
055     * breaker is already closed, this method has no effect.
056     */
057    void close();
058
059    /**
060     * Increments the monitored value and performs a check of the current state of this
061     * circuit breaker. This method works like {@link #checkState()}, but the monitored
062     * value is incremented before the state check is performed.
063     *
064     * @param increment value to increment in the monitored value of the circuit breaker
065     * @return <strong>true</strong> if the circuit breaker is now closed;
066     * <strong>false</strong> otherwise
067     */
068    boolean incrementAndCheckState(T increment);
069
070    /**
071     * Tests the current closed state of this circuit breaker. A return value of
072     * <strong>true</strong> means that the circuit breaker is currently closed. This
073     * means that everything is okay with the monitored subsystem.
074     *
075     * @return the current closed state of this circuit breaker.
076     */
077    boolean isClosed();
078
079    /**
080     * Tests the current open state of this circuit breaker. A return value of
081     * <strong>true</strong> means that the circuit breaker is currently open indicating a
082     * problem in the monitored subsystem.
083     *
084     * @return the current open state of this circuit breaker.
085     */
086    boolean isOpen();
087
088    /**
089     * Opens this circuit breaker. Its state is changed to open. Depending on a concrete
090     * implementation, it may close itself again if the monitored subsystem becomes
091     * available. If this circuit breaker is already open, this method has no effect.
092     */
093    void open();
094}