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MapDB provides concurrent Maps, Sets and Queues backed by disk storage or off-heap memory. It is a fast, scalable and easy to use embedded Java database.
/*
* Copyright (c) 2012 Jan Kotek
*
* Licensed 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.mapdb;
import java.lang.ref.WeakReference;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* {@link Engine} wrapper which provides asynchronous serialization and asynchronous write.
* This class takes an object instance, passes it to background writer thread (using Write Cache)
* where it is serialized and written to disk. Async write does not affect commit durability,
* write cache is flushed into disk on each commit. Modified records are held in small instance cache,
* until they are written into disk.
*
* This feature is disabled by default and can be enabled by calling {@link DBMaker#asyncWriteEnable()}.
* Write Cache is flushed in regular intervals or when it becomes full. Flush interval is 100 ms by default and
* can be controlled by {@link DBMaker#asyncWriteFlushDelay(int)}. Increasing this interval may improve performance
* in scenarios where frequently modified items should be cached, typically {@link BTreeMap} import where keys
* are presorted.
*
* Asynchronous write does not affect commit durability. Write Cache is flushed during each commit, rollback and close call.
* You may also flush Write Cache manually by using {@link org.mapdb.AsyncWriteEngine#clearCache()} method.
* There is global lock which prevents record being updated while commit is in progress.
*
* This wrapper starts one threads named `MapDB writer #N` (where N is static counter).
* Async Writer takes modified records from Write Cache and writes them into store.
* It also preallocates new recids, as finding empty `recids` takes time so small stash is pre-allocated.
* It runs as `daemon`, so it does not prevent JVM to exit.
*
* Asynchronous Writes have several advantages (especially for single threaded user). But there are two things
* user should be aware of:
*
* * Because data are serialized on back-ground thread, they need to be thread safe or better immutable.
* When you insert record into MapDB and modify it latter, this modification may happen before item
* was serialized and you may not be sure what version was persisted
*
* * Asynchronous writes have some overhead and introduce single bottle-neck. This usually not issue for
* single or two threads, but in multi-threaded environment it may decrease performance.
* So in truly concurrent environments with many updates (network servers, parallel computing )
* you should disable Asynchronous Writes.
*
*
* @see Engine
* @see EngineWrapper
*
* @author Jan Kotek
*
*
*
*/
public class AsyncWriteEngine extends EngineWrapper implements Engine {
/** ensures thread name is followed by number */
protected static final AtomicLong threadCounter = new AtomicLong();
/** used to signal that object was deleted*/
protected static final Object TOMBSTONE = new Object();
protected final int maxSize;
protected final AtomicInteger size = new AtomicInteger();
protected final long[] newRecids = new long[CC.ASYNC_RECID_PREALLOC_QUEUE_SIZE];
protected int newRecidsPos = 0;
protected final ReentrantLock newRecidsLock = new ReentrantLock(CC.FAIR_LOCKS);
/** Associates `recid` from Write Queue with record data and serializer. */
protected final LongConcurrentHashMap> writeCache
= new LongConcurrentHashMap>();
/** Each insert to Write Queue must hold read lock.
* Commit, rollback and close operations must hold write lock
*/
protected final ReentrantReadWriteLock commitLock = new ReentrantReadWriteLock(CC.FAIR_LOCKS);
/** number of active threads running, used to await thread termination on close */
protected final CountDownLatch activeThreadsCount = new CountDownLatch(1);
/** If background thread fails with exception, it is stored here, and rethrown to all callers.*/
protected volatile Throwable threadFailedException = null;
/** indicates that `close()` was called and background threads are being terminated*/
protected volatile boolean closeInProgress = false;
/** flush Write Queue every N milliseconds */
protected final int asyncFlushDelay;
protected final AtomicReference action = new AtomicReference(null);
/**
* Construct new class and starts background threads.
* User may provide executor in which background tasks will be executed,
* otherwise MapDB starts two daemon threads.
*
* @param engine which stores data.
* @param _asyncFlushDelay flush Write Queue every N milliseconds
* @param executor optional executor to run tasks. If null daemon threads will be created
*/
public AsyncWriteEngine(Engine engine, int _asyncFlushDelay, int queueSize, Executor executor) {
super(engine);
this.asyncFlushDelay = _asyncFlushDelay;
this.maxSize = queueSize;
startThreads(executor);
}
public AsyncWriteEngine(Engine engine) {
this(engine, CC.ASYNC_WRITE_FLUSH_DELAY, CC.ASYNC_WRITE_QUEUE_SIZE, null);
}
protected static final class WriterRunnable implements Runnable{
protected final WeakReference engineRef;
protected final long asyncFlushDelay;
protected final AtomicInteger size;
protected final int maxParkSize;
private final ReentrantReadWriteLock commitLock;
public WriterRunnable(AsyncWriteEngine engine) {
this.engineRef = new WeakReference(engine);
this.asyncFlushDelay = engine.asyncFlushDelay;
this.commitLock = engine.commitLock;
this.size = engine.size;
this.maxParkSize = engine.maxSize/4;
}
@Override public void run() {
try{
//run in loop
for(;;){
//if conditions are right, slow down writes a bit
if(asyncFlushDelay!=0 && !commitLock.isWriteLocked() && size.get()> iter = writeCache.longMapIterator();
while(iter.moveToNext()){
//usual write
final long recid = iter.key();
Fun.Tuple2 item = iter.value();
if(item == null) continue; //item was already written
if(item.a==TOMBSTONE){
//item was not updated, but deleted
AsyncWriteEngine.super.delete(recid, item.b);
}else{
//call update as usual
AsyncWriteEngine.super.update(recid, item.a, item.b);
}
//record has been written to underlying Engine, so remove it from cache with CAS
if(writeCache.remove(recid, item))
size.decrementAndGet();
if(((++counter)&(63))==0){ //it is like modulo 64, but faster
preallocateRefill();
}
}
}while(latch!=null && !writeCache.isEmpty());
preallocateRefill();
//operations such as commit,close, compact or close needs to be executed in Writer Thread
//for this case CountDownLatch is used, it also signals when operations has been completed
//CountDownLatch is used as special case to signalise special operation
if(latch!=null){
assert(writeCache.isEmpty());
final long count = latch.getCount();
if(count == 0){ //close operation
return false;
}else if(count == 1){ //commit operation
AsyncWriteEngine.super.commit();
latch.countDown();
}else if(count==2){ //rollback operation
AsyncWriteEngine.super.rollback();
preallocateRollback();
latch.countDown();
latch.countDown();
}else if(count==3){ //compact operation
AsyncWriteEngine.super.compact();
latch.countDown();
latch.countDown();
latch.countDown();
}else{throw new AssertionError();}
}
return true;
}
protected void preallocateRollback(){
newRecidsLock.lock();
try{
getWrappedEngine().preallocate(newRecids);
newRecidsPos = newRecids.length;
}finally {
newRecidsLock.unlock();
}
}
@Override
public long preallocate() {
commitLock.readLock().lock();
try{
return preallocateNoCommitLock();
}finally{
commitLock.readLock().unlock();
}
}
@Override
public void preallocate(long[] recids) {
commitLock.readLock().lock();
try{
for(int i=0;i put: User calls put method
wri-> put: takes preallocated recid
put -> wri: forward record into Write Queue
put -> user: return recid to user
deactivate put
note over wri: eventually\n writes record\n before commit
*/
@Override
public long put(A value, Serializer serializer) {
int size2 = 0;
long recid =0;
commitLock.readLock().lock();
try{
checkState();
recid = preallocateNoCommitLock();
if(writeCache.put(recid, new Fun.Tuple2(value, serializer))==null)
size2 = size.incrementAndGet();
}finally{
commitLock.readLock().unlock();
}
if(size2>maxSize)
clearCache();
return recid;
}
/**
* {@inheritDoc}
*
* This method first looks up into Write Cache if record is not currently being written.
* If not it continues as usually
*
*
*/
@Override
public A get(long recid, Serializer serializer) {
commitLock.readLock().lock();
try{
checkState();
Fun.Tuple2 item = writeCache.get(recid);
if(item!=null){
if(item.a == TOMBSTONE) return null;
return (A) item.a;
}
return super.get(recid, serializer);
}finally{
commitLock.readLock().unlock();
}
}
/**
* {@inheritDoc}
*
* This methods forwards record into Writer Thread and returns asynchronously.
*
* 
* @uml async-update.png
* actor user
* participant "update method" as upd
* participant "Writer Thread" as wri
* activate upd
* user -> upd: User calls update method
* upd -> wri: forward record into Write Queue
* upd -> user: returns
* deactivate upd
* note over wri: eventually\n writes record\n before commit
*/
@Override
public void update(long recid, A value, Serializer serializer) {
int size2 = 0;
commitLock.readLock().lock();
try{
checkState();
if(writeCache.put(recid, new Fun.Tuple2(value, serializer))==null)
size2 = size.incrementAndGet();
}finally{
commitLock.readLock().unlock();
}
if(size2>maxSize)
clearCache();
}
/**
* {@inheritDoc}
*
* This method first looks up Write Cache if record is not currently being written.
* Successful modifications are forwarded to Write Thread and method returns asynchronously.
* Asynchronicity does not affect atomicity.
*/
@Override
public boolean compareAndSwap(long recid, A expectedOldValue, A newValue, Serializer serializer) {
int size2 = 0;
boolean ret;
commitLock.writeLock().lock();
try{
checkState();
Fun.Tuple2 existing = writeCache.get(recid);
A oldValue = existing!=null? (A) existing.a : super.get(recid, serializer);
if(oldValue == expectedOldValue || (oldValue!=null && oldValue.equals(expectedOldValue))){
if(writeCache.put(recid, new Fun.Tuple2(newValue, serializer))==null)
size2 = size.incrementAndGet();
ret = true;
}else{
ret = false;
}
}finally{
commitLock.writeLock().unlock();
}
if(size2>maxSize)
clearCache();
return ret;
}
/**
* {@inheritDoc}
*
* This method places 'tombstone' into Write Queue so record is eventually
* deleted asynchronously. However record is visible as deleted immediately.
*/
@Override
public void delete(long recid, Serializer serializer) {
update(recid, (A) TOMBSTONE, serializer);
}
/**
* {@inheritDoc}
*
* This method blocks until Write Queue is flushed and Writer Thread writes all records and finishes.
* When this method was called `closeInProgress` is set and no record can be modified.
*/
@Override
public void close() {
commitLock.writeLock().lock();
try {
if(closeInProgress) return;
checkState();
closeInProgress = true;
//notify background threads
if(!action.compareAndSet(null,new CountDownLatch(0)))
throw new AssertionError();
//wait for background threads to shutdown
activeThreadsCount.await(1000,TimeUnit.MILLISECONDS);
//put preallocated recids back to store
newRecidsLock.lock();
try{
while(newRecidsPos>0){
super.delete(newRecids[--newRecidsPos],Serializer.ILLEGAL_ACCESS);
}
}finally{
newRecidsLock.unlock();
}
AsyncWriteEngine.super.close();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}finally {
commitLock.writeLock().unlock();
}
}
protected void waitForAction(int actionNumber) {
commitLock.writeLock().lock();
try{
checkState();
//notify background threads
CountDownLatch msg = new CountDownLatch(actionNumber);
if(!action.compareAndSet(null,msg))
throw new AssertionError();
//wait for response from writer thread
while(!msg.await(100, TimeUnit.MILLISECONDS)){
checkState();
}
} catch (InterruptedException e) {
throw new RuntimeException(e);
}finally {
commitLock.writeLock().unlock();
}
}
/**
* {@inheritDoc}
*
* This method blocks until Write Queue is flushed.
* All put/update/delete methods are blocked while commit is in progress (via global ReadWrite Commit Lock).
* After this method returns, commit lock is released and other operations may continue
*/
@Override
public void commit() {
waitForAction(1);
}
/**
* {@inheritDoc}
*
* This method blocks until Write Queue is cleared.
* All put/update/delete methods are blocked while rollback is in progress (via global ReadWrite Commit Lock).
* After this method returns, commit lock is released and other operations may continue
*/
@Override
public void rollback() {
waitForAction(2);
}
/**
* {@inheritDoc}
*
* This method blocks all put/update/delete operations until it finishes (via global ReadWrite Commit Lock).
*
*/
@Override
public void compact() {
waitForAction(3);
}
/**
* {@inheritDoc}
*
* This method blocks until Write Queue is empty (written into disk).
* It also blocks any put/update/delete operations until it finishes (via global ReadWrite Commit Lock).
*/
@Override
public void clearCache() {
commitLock.writeLock().lock();
try{
checkState();
//wait for response from writer thread
while(!writeCache.isEmpty()){
checkState();
Thread.sleep(100);
}
} catch (InterruptedException e) {
throw new RuntimeException(e);
}finally {
commitLock.writeLock().unlock();
}
super.clearCache();
}
}
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