    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}