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Java B+Tree Key-Value Store Library
There is a newer version: 3.3.0
/**
 * Copyright © 2011-2012 Akiban Technologies, Inc.  All rights reserved.
 * 
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Public License v1.0 which
 * accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 * 
 * This program may also be available under different license terms.
 * For more information, see www.akiban.com or contact [email protected].
 * 
 * Contributors:
 * Akiban Technologies, Inc.
 */

package com.persistit;

import java.util.concurrent.atomic.AtomicLong;

import com.persistit.exception.PersistitInterruptedException;
import com.persistit.util.Util;

class TimestampAllocator {

    private final static int CHECKPOINT_TIMESTAMP_MARKER_INTERVAL = 100;

    private final static long UNAVAILABLE_CHECKPOINT_TIMESTAMP = -1;
    /**
     * Default interval in nanoseconds between checkpoints - two minutes.
     */
    private final AtomicLong _timestamp = new AtomicLong();

    private volatile long _checkpointTimestamp;

    public long updateTimestamp() {
        return _timestamp.incrementAndGet();
    }

    long bumpTimestamp(final long delta) {
        return _timestamp.addAndGet(delta);
    }

    public long updateTimestamp(final long timestamp) {
        _timestamp.incrementAndGet();
        while (true) {
            final long expected = _timestamp.get();
            if (expected < timestamp) {
                if (_timestamp.compareAndSet(expected, timestamp)) {
                    return timestamp;
                }
            } else {
                return expected;
            }
        }
    }

    /**
     * 
     * Atomically allocate a new checkpoint timestamp. This method temporarily
     * marks the checkpoint timestamp field as "unavailable" until a new value
     * is computed. This prevents another thread executing
     * {@link Buffer#writePageOnCheckpoint(long)} from trusting the old
     * checkpoint timestamp while a new one is being allocated.
     * 
     * 
     * Here's the execution sequence to avoid:
     * 
     * Let the initial checkpoint timestamp be c0.
     * 
Thread A (the checkpoint thread invoking this method) gets an updated
     * timestamp c1 which will become the new checkpoint timestamp.
     * Thread B (a thread attempting to modify a page that is already dirty
     * with timestamp t0) gets an updated timestamp t1 greater than c1
     * Thread B in writePageOnCheckpoint determines that t0 is larger than
     * the current checkpoint c0 (because the checkpoint timestamp field has not
     * been updated yet) and therefore continues on to further modify the page
     * without writing its state at t0.
     * Thread A now updates the checkpoint timestamp field to contain c1.
     *  In this scenario the version of the page at t0 was never written to
     * the journal even though t0 < c1 and t1 > c1. This violates the recovery
     * safety requirement.
     * 
     * 
     * The solution is to stall the {@link #getProposedCheckpointTimestamp()}
     * method called by writePageOnCheckpoint before c1 is allocated until after
     * the checkpointTimestamp field containing c1 is visible.
     * 
     * 
     * @return the allocated timestamp
     */
    long allocateCheckpointTimestamp() {
        /*
         * Add a gap to the timestamp counter - this is useful only for humans
         * trying to decipher timestamps in the journal - is not necessary for
         * correct function.
         */
        bumpTimestamp(CHECKPOINT_TIMESTAMP_MARKER_INTERVAL);
        /*
         * Prevents Buffer#writePageOnCheckpoint from seeing the old checkpoint
         * while we are assigning the new one. The
         * getProposedCheckpointTimestamp method below spins until this has been
         * settled.
         */
        _checkpointTimestamp = UNAVAILABLE_CHECKPOINT_TIMESTAMP;
        final long timestamp = updateTimestamp();
        _checkpointTimestamp = timestamp;
        return timestamp;
    }

    public long getCurrentTimestamp() {
        return _timestamp.get();
    }

    long getProposedCheckpointTimestamp() throws PersistitInterruptedException {
        long timestamp;
        /*
         * Spin until stable. This value must not be observed until set.
         */
        for (int iterations = 0; (timestamp = _checkpointTimestamp) == UNAVAILABLE_CHECKPOINT_TIMESTAMP; iterations++) {
            if (iterations > 10) {
                Util.spinSleep();
            }
        }

        return timestamp;
    }

}