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Neo4j kernel is a lightweight, embedded Java database designed to store data structured as graphs rather than tables. For more information, see http://neo4j.org.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.kernel.availability;

/**
 * The availability guard ensures that the database will only take calls when it is in an ok state.
 * It tracks a set of requirements (added via {@link #require(AvailabilityRequirement)}) that must all be marked
 * as fulfilled (using {@link #fulfill(AvailabilityRequirement)}) before the database is considered available again.
 * Consumers determine if it is ok to call the database using {@link #isAvailable()},
 * or await availability using {@link #isAvailable(long)}.
 */
public interface AvailabilityGuard {
    AvailabilityGuard NOT_AVAILABLE_GUARD = new NotAvailableGuard();

    /**
     * Indicate a requirement that must be fulfilled before the database is considered available.
     *
     * @param requirement the requirement object
     */
    void require(AvailabilityRequirement requirement);

    /**
     * Indicate that a requirement has been fulfilled.
     *
     * @param requirement the requirement object
     */
    void fulfill(AvailabilityRequirement requirement);

    /**
     * Check if the database is available for transactions to use.
     *
     * @return true if there are no requirements waiting to be fulfilled and the guard has not been shutdown
     */
    boolean isAvailable();

    /**
     * Check if the database has been shut down.
     */
    boolean isShutdown();

    /**
     * Check if the database is available for transactions to use.
     *
     * @param millis to wait for availability
     * @return true if there are no requirements waiting to be fulfilled and the guard has not been shutdown
     */
    boolean isAvailable(long millis);

    /**
     * Await the database becoming available.
     *
     * @param millis to wait for availability
     * @throws UnavailableException thrown when the timeout has been exceeded or the guard has been shutdown
     */
    void await(long millis) throws UnavailableException;

    /**
     * Add a listener for changes to availability.
     *
     * @param listener the listener to receive callbacks when availability changes
     */
    void addListener(AvailabilityListener listener);

    /**
     * Remove a listener for changes to availability.
     *
     * @param listener the listener to remove
     */
    void removeListener(AvailabilityListener listener);

    /**
     * @return a textual representation of what components, if any, are blocking
     */
    String describe();

    class NotAvailableGuard implements AvailabilityGuard {
        private static final String ALWAYS_NOT_AVAILABLE_GUARD = "Always not available guard";

        private NotAvailableGuard() {}

        @Override
        public void require(AvailabilityRequirement requirement) {}

        @Override
        public void fulfill(AvailabilityRequirement requirement) {}

        @Override
        public boolean isAvailable() {
            return false;
        }

        @Override
        public boolean isShutdown() {
            return false;
        }

        @Override
        public boolean isAvailable(long millis) {
            return false;
        }

        @Override
        public void await(long millis) throws UnavailableException {}

        @Override
        public void addListener(AvailabilityListener listener) {}

        @Override
        public void removeListener(AvailabilityListener listener) {}

        @Override
        public String describe() {
            return ALWAYS_NOT_AVAILABLE_GUARD;
        }
    }
}