org.apache.jackrabbit.oak.spi.state.Clusterable Maven / Gradle / Ivy

/*
 * 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.jackrabbit.oak.spi.state;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

/**
 * Interface for bearing cluster node specific information.
 */
public interface Clusterable {
    /**
     * 
     * Will return a unique number per instance across the cluster. It will only make its best
     * effort to preserve the same number across restarts but it must be unique across the cluster.
     * 
     * 
     * @return Cannot be null or empty.
     */
    @Nonnull
    String getInstanceId();

    /**
     * Returns the visibility token of the underlying NodeStore. A 'visibility
     * token' is an opaque String that can be used to verify if changes done on
     * one NodeStore are visible on another NodeStore of the same cluster. This
     * can be achieved by generating such a visibility token on the source
     * NodeStore, passing it on to the target NodeStore (by whatever means) and
     * checking for visibility on that target NodeStore.
     *
     * The visibility check returns true if the target NodeStore sees at least
     * all the changes that the source NodeStore saw at time of visibility token
     * generation. Once a visibility token is visible on a particular NodeStore
     * it will always return true ever after. This also implies that the
     * visibility check can only state whether at least all source changes are
     * visible on the target and that it is independent of any further
     * modifications.
     *
     * When source and target NodeStore are identical, the visibility check is
     * expected to return true, immediately. This is based on the assumption
     * that with a session.refresh() on that NodeStore you'll always get the
     * latest changes applied by any other session locally.
     *
     * Visibility tokens are meant to be lightweight and are not expected to be
     * persisted by the implementor. Nevertheless they should survive their
     * validity in the case of crashes of the source and/or the target instance.
     */
    @Nullable
    String getVisibilityToken();

    /**
     * Checks if the underlying NodeStore sees at least the changes that were
     * visible at the time the visibility token was created on potentially
     * another instance if in a clustered NodeStore setup.
     *
     * If the visibility token was created on the underlying NodeStore this
     * check always returns true, immediately.
     * 
     * @param visibilityToken
     *            the visibility token that was created on another instance in a
     *            clustered NodeStore setup. Providing null is not supported and
     *            might throw a RuntimeException
     * @param maxWaitMillis
     *            if >-1 waits (at max this many milliseconds if >0,
     *            forever if ==0) until the underlying NodeStore sees at least
     *            the changes represented by the provided visibility token. if
     *            < 0 the method does not wait
     * @return true if the underlying NodeStore sees at least the changes that
     *         were visible at the time the visibility token was created
     * @throws InterruptedException
     *             (optionally) thrown if interrupted while waiting
     */
    boolean isVisible(@Nonnull String visibilityToken, long maxWaitMillis) throws InterruptedException;
}