org.apache.jena.tdb.sys.DatasetControlMRSW Maven / Gradle / Ivy

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TDB is a storage subsystem for Jena and ARQ, it is a native triple store providing persistent storage of triples/quads.
There is a newer version: 4.10.0
/*
 * 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.jena.tdb.sys;

import static java.lang.String.format ;

import java.util.ConcurrentModificationException ;
import java.util.Iterator ;
import java.util.NoSuchElementException ;
import java.util.concurrent.atomic.AtomicLong ;

import org.apache.jena.atlas.iterator.Iter ;
import org.apache.jena.atlas.lib.Closeable ;

/** A policy that checks, but does not enforce, single writer or multiple writer locking policy */ 
public class DatasetControlMRSW implements DatasetControl
{
    private final boolean concurrencyChecking = true ;
    // Update counters, used to check iterators. No need to start at 0.
    private final AtomicLong epoch = new AtomicLong(5) ;                
    private final AtomicLong readCounter = new AtomicLong(0) ;
    private final AtomicLong writeCounter = new AtomicLong(0) ;
    
    public DatasetControlMRSW() {}

    @Override
    public void startRead() {
        readCounter.getAndIncrement();
        checkConcurrency();
    }

    @Override
    public void finishRead() {
        readCounter.decrementAndGet();
    }

    @Override
    public void startUpdate() {
        epoch.getAndIncrement();
        writeCounter.getAndIncrement();
        checkConcurrency();
    }

    @Override
    public void finishUpdate() {
        writeCounter.decrementAndGet();
    }

    private void checkConcurrency() {
        long R, W;
        synchronized (this) {
            R = readCounter.get();
            W = writeCounter.get();
        }

        if ( R > 0 && W > 0 )
            policyError(R, W);
        if ( W > 1 )
            policyError(R, W);
    }
    
    @Override
    public  Iterator iteratorControl(Iterator iter) {
        return new IteratorCheckNotConcurrent<>(iter, epoch);
    }

    private static class IteratorCheckNotConcurrent implements Iterator, Closeable {
        private Iterator iter;
        private AtomicLong  eCount;
        private boolean     finished = false;
        private long        startEpoch;

        IteratorCheckNotConcurrent(Iterator iter, AtomicLong eCount) {
            // Assumes correct locking to set up, i.e. eCount not changing (writer on
            // separate thread).
            this.iter = iter;
            this.eCount = eCount;
            this.startEpoch = eCount.get();
        }

        private void checkIterConcurrentModification() {
            if ( finished )
                return;

            long now = eCount.get();
            if ( now != startEpoch ) {
                policyError(format("Iterator: started at %d, now %d", startEpoch, now));

            }
        }

        @Override
        public boolean hasNext() {
            checkIterConcurrentModification();
            boolean b = iter.hasNext();
            if ( !b )
                close();
            return b;
        }

        @Override
        public T next() {
            checkIterConcurrentModification();
            try {
                return iter.next();
            }
            catch (NoSuchElementException ex) {
                close();
                throw ex;
            }
        }

        @Override
        public void remove() {
            checkIterConcurrentModification();
            iter.remove();
        }

        @Override
        public void close() {
            finished = true;
            Iter.close(iter);
        }
    }

    private static void policyError(long R, long W) {
        policyError(format("Reader = %d, Writer = %d", R, W));
    }

    private static void policyError(String message) {
        throw new ConcurrentModificationException(message);
    }
}