org.apache.hudi.org.apache.hadoop.hbase.regionserver.wal.FSHLog Maven / Gradle / Ivy
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* 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.hadoop.hbase.regionserver.wal;
import static org.apache.hadoop.hbase.regionserver.wal.WALActionsListener.RollRequestReason.ERROR;
import static org.apache.hadoop.hbase.regionserver.wal.WALActionsListener.RollRequestReason.LOW_REPLICATION;
import static org.apache.hadoop.hbase.regionserver.wal.WALActionsListener.RollRequestReason.SIZE;
import static org.apache.hadoop.hbase.regionserver.wal.WALActionsListener.RollRequestReason.SLOW_SYNC;
import com.lmax.disruptor.BlockingWaitStrategy;
import com.lmax.disruptor.EventHandler;
import com.lmax.disruptor.ExceptionHandler;
import com.lmax.disruptor.LifecycleAware;
import com.lmax.disruptor.TimeoutException;
import com.lmax.disruptor.dsl.Disruptor;
import com.lmax.disruptor.dsl.ProducerType;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hbase.Abortable;
import org.apache.hadoop.hbase.HConstants;
import org.apache.hadoop.hbase.client.RegionInfo;
import org.apache.hadoop.hbase.trace.TraceUtil;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.hbase.util.ClassSize;
import org.apache.hadoop.hbase.util.CommonFSUtils;
import org.apache.hadoop.hbase.util.Threads;
import org.apache.hadoop.hbase.wal.FSHLogProvider;
import org.apache.hadoop.hbase.wal.WALEdit;
import org.apache.hadoop.hbase.wal.WALKeyImpl;
import org.apache.hadoop.hbase.wal.WALProvider.Writer;
import org.apache.hadoop.hdfs.DFSOutputStream;
import org.apache.hadoop.hdfs.client.HdfsDataOutputStream;
import org.apache.hadoop.hdfs.protocol.DatanodeInfo;
import org.apache.htrace.core.TraceScope;
import org.apache.hbase.thirdparty.com.google.common.util.concurrent.ThreadFactoryBuilder;
import org.apache.yetus.audience.InterfaceAudience;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.hbase.thirdparty.com.google.common.util.concurrent.ThreadFactoryBuilder;
/**
* The original implementation of FSWAL.
*/
@InterfaceAudience.Private
public class FSHLog extends AbstractFSWAL {
// IMPLEMENTATION NOTES:
//
// At the core is a ring buffer. Our ring buffer is the LMAX Disruptor. It tries to
// minimize synchronizations and volatile writes when multiple contending threads as is the case
// here appending and syncing on a single WAL. The Disruptor is configured to handle multiple
// producers but it has one consumer only (the producers in HBase are IPC Handlers calling append
// and then sync). The single consumer/writer pulls the appends and syncs off the ring buffer.
// When a handler calls sync, it is given back a future. The producer 'blocks' on the future so
// it does not return until the sync completes. The future is passed over the ring buffer from
// the producer/handler to the consumer thread where it does its best to batch up the producer
// syncs so one WAL sync actually spans multiple producer sync invocations. How well the
// batching works depends on the write rate; i.e. we tend to batch more in times of
// high writes/syncs.
//
// Calls to append now also wait until the append has been done on the consumer side of the
// disruptor. We used to not wait but it makes the implementation easier to grok if we have
// the region edit/sequence id after the append returns.
//
// TODO: Handlers need to coordinate appending AND syncing. Can we have the threads contend
// once only? Probably hard given syncs take way longer than an append.
//
// The consumer threads pass the syncs off to multiple syncing threads in a round robin fashion
// to ensure we keep up back-to-back FS sync calls (FS sync calls are the long poll writing the
// WAL). The consumer thread passes the futures to the sync threads for it to complete
// the futures when done.
//
// The 'sequence' in the below is the sequence of the append/sync on the ringbuffer. It
// acts as a sort-of transaction id. It is always incrementing.
//
// The RingBufferEventHandler class hosts the ring buffer consuming code. The threads that
// do the actual FS sync are implementations of SyncRunner. SafePointZigZagLatch is a
// synchronization class used to halt the consumer at a safe point -- just after all outstanding
// syncs and appends have completed -- so the log roller can swap the WAL out under it.
//
// We use ring buffer sequence as txid of FSWALEntry and SyncFuture.
private static final Logger LOG = LoggerFactory.getLogger(FSHLog.class);
private static final String TOLERABLE_LOW_REPLICATION = "hbase.regionserver.hlog.tolerable.lowreplication";
private static final String LOW_REPLICATION_ROLL_LIMIT = "hbase.regionserver.hlog.lowreplication.rolllimit";
private static final int DEFAULT_LOW_REPLICATION_ROLL_LIMIT = 5;
private static final String ROLL_ERRORS_TOLERATED = "hbase.regionserver.logroll.errors.tolerated";
private static final int DEFAULT_ROLL_ERRORS_TOLERATED = 2;
private static final String SYNCER_COUNT = "hbase.regionserver.hlog.syncer.count";
private static final int DEFAULT_SYNCER_COUNT = 5;
private static final String MAX_BATCH_COUNT = "hbase.regionserver.wal.sync.batch.count";
private static final int DEFAULT_MAX_BATCH_COUNT = 200;
private static final String FSHLOG_WAIT_ON_SHUTDOWN_IN_SECONDS = "hbase.wal.fshlog.wait.on.shutdown.seconds";
private static final int DEFAULT_FSHLOG_WAIT_ON_SHUTDOWN_IN_SECONDS = 5;
/**
* The nexus at which all incoming handlers meet. Does appends and sync with an ordering. Appends
* and syncs are each put on the ring which means handlers need to smash up against the ring twice
* (can we make it once only? ... maybe not since time to append is so different from time to sync
* and sometimes we don't want to sync or we want to async the sync). The ring is where we make
* sure of our ordering and it is also where we do batching up of handler sync calls.
*/
private final Disruptor disruptor;
/**
* This fellow is run by the above appendExecutor service but it is all about batching up appends
* and syncs; it may shutdown without cleaning out the last few appends or syncs. To guard against
* this, keep a reference to this handler and do explicit close on way out to make sure all
* flushed out before we exit.
*/
private final RingBufferEventHandler ringBufferEventHandler;
/**
* FSDataOutputStream associated with the current SequenceFile.writer
*/
private FSDataOutputStream hdfs_out;
// All about log rolling if not enough replicas outstanding.
// Minimum tolerable replicas, if the actual value is lower than it, rollWriter will be triggered
private final int minTolerableReplication;
// If live datanode count is lower than the default replicas value,
// RollWriter will be triggered in each sync(So the RollWriter will be
// triggered one by one in a short time). Using it as a workaround to slow
// down the roll frequency triggered by checkLowReplication().
private final AtomicInteger consecutiveLogRolls = new AtomicInteger(0);
private final int lowReplicationRollLimit;
// If consecutiveLogRolls is larger than lowReplicationRollLimit,
// then disable the rolling in checkLowReplication().
// Enable it if the replications recover.
private volatile boolean lowReplicationRollEnabled = true;
/** Number of log close errors tolerated before we abort */
private final int closeErrorsTolerated;
private final AtomicInteger closeErrorCount = new AtomicInteger();
private final int waitOnShutdownInSeconds;
private final ExecutorService closeExecutor = Executors.newCachedThreadPool(
new ThreadFactoryBuilder().setDaemon(true).setNameFormat("Close-WAL-Writer-%d").build());
/**
* Exception handler to pass the disruptor ringbuffer. Same as native implementation only it logs
* using our logger instead of java native logger.
*/
static class RingBufferExceptionHandler implements ExceptionHandler {
@Override
public void handleEventException(Throwable ex, long sequence, RingBufferTruck event) {
LOG.error("Sequence=" + sequence + ", event=" + event, ex);
throw new RuntimeException(ex);
}
@Override
public void handleOnStartException(Throwable ex) {
LOG.error(ex.toString(), ex);
throw new RuntimeException(ex);
}
@Override
public void handleOnShutdownException(Throwable ex) {
LOG.error(ex.toString(), ex);
throw new RuntimeException(ex);
}
}
/**
* Constructor.
* @param fs filesystem handle
* @param root path for stored and archived wals
* @param logDir dir where wals are stored
* @param conf configuration to use
*/
public FSHLog(final FileSystem fs, final Path root, final String logDir, final Configuration conf)
throws IOException {
this(fs, root, logDir, HConstants.HREGION_OLDLOGDIR_NAME, conf, null, true, null, null);
}
public FSHLog(final FileSystem fs, Abortable abortable, final Path root, final String logDir,
final Configuration conf) throws IOException {
this(fs, abortable, root, logDir, HConstants.HREGION_OLDLOGDIR_NAME, conf, null, true, null,
null);
}
public FSHLog(final FileSystem fs, final Path rootDir, final String logDir,
final String archiveDir, final Configuration conf, final List listeners,
final boolean failIfWALExists, final String prefix, final String suffix) throws IOException {
this(fs, null, rootDir, logDir, archiveDir, conf, listeners, failIfWALExists, prefix, suffix);
}
/**
* Create an edit log at the given dir location. You should never have to load an
* existing log. If there is a log at startup, it should have already been processed and deleted
* by the time the WAL object is started up.
* @param fs filesystem handle
* @param rootDir path to where logs and oldlogs
* @param logDir dir where wals are stored
* @param archiveDir dir where wals are archived
* @param conf configuration to use
* @param listeners Listeners on WAL events. Listeners passed here will be registered before we do
* anything else; e.g. the Constructor {@link #rollWriter()}.
* @param failIfWALExists If true IOException will be thrown if files related to this wal already
* exist.
* @param prefix should always be hostname and port in distributed env and it will be URL encoded
* before being used. If prefix is null, "wal" will be used
* @param suffix will be url encoded. null is treated as empty. non-empty must start with
* {@link org.apache.hadoop.hbase.wal.AbstractFSWALProvider#WAL_FILE_NAME_DELIMITER}
*/
public FSHLog(final FileSystem fs, final Abortable abortable, final Path rootDir,
final String logDir, final String archiveDir, final Configuration conf,
final List listeners, final boolean failIfWALExists, final String prefix,
final String suffix) throws IOException {
super(fs, abortable, rootDir, logDir, archiveDir, conf, listeners, failIfWALExists, prefix,
suffix);
this.minTolerableReplication = conf.getInt(TOLERABLE_LOW_REPLICATION,
CommonFSUtils.getDefaultReplication(fs, this.walDir));
this.lowReplicationRollLimit = conf.getInt(LOW_REPLICATION_ROLL_LIMIT, DEFAULT_LOW_REPLICATION_ROLL_LIMIT);
this.closeErrorsTolerated = conf.getInt(ROLL_ERRORS_TOLERATED, DEFAULT_ROLL_ERRORS_TOLERATED);
this.waitOnShutdownInSeconds = conf.getInt(FSHLOG_WAIT_ON_SHUTDOWN_IN_SECONDS,
DEFAULT_FSHLOG_WAIT_ON_SHUTDOWN_IN_SECONDS);
// This is the 'writer' -- a single threaded executor. This single thread 'consumes' what is
// put on the ring buffer.
String hostingThreadName = Thread.currentThread().getName();
// Using BlockingWaitStrategy. Stuff that is going on here takes so long it makes no sense
// spinning as other strategies do.
this.disruptor = new Disruptor<>(RingBufferTruck::new, getPreallocatedEventCount(),
new ThreadFactoryBuilder().setNameFormat(hostingThreadName + ".append-pool-%d")
.setDaemon(true).setUncaughtExceptionHandler(Threads.LOGGING_EXCEPTION_HANDLER).build(),
ProducerType.MULTI, new BlockingWaitStrategy());
// Advance the ring buffer sequence so that it starts from 1 instead of 0,
// because SyncFuture.NOT_DONE = 0.
this.disruptor.getRingBuffer().next();
int syncerCount = conf.getInt(SYNCER_COUNT, DEFAULT_SYNCER_COUNT);
int maxBatchCount = conf.getInt(MAX_BATCH_COUNT,
conf.getInt(HConstants.REGION_SERVER_HANDLER_COUNT, DEFAULT_MAX_BATCH_COUNT));
this.ringBufferEventHandler = new RingBufferEventHandler(syncerCount, maxBatchCount);
this.disruptor.setDefaultExceptionHandler(new RingBufferExceptionHandler());
this.disruptor.handleEventsWith(new RingBufferEventHandler[] { this.ringBufferEventHandler });
// Starting up threads in constructor is a no no; Interface should have an init call.
this.disruptor.start();
}
/**
* Currently, we need to expose the writer's OutputStream to tests so that they can manipulate the
* default behavior (such as setting the maxRecoveryErrorCount value). This is
* done using reflection on the underlying HDFS OutputStream. NOTE: This could be removed once Hadoop1 support is
* removed.
* @return null if underlying stream is not ready.
*/
OutputStream getOutputStream() {
FSDataOutputStream fsdos = this.hdfs_out;
return fsdos != null ? fsdos.getWrappedStream() : null;
}
/**
* Run a sync after opening to set up the pipeline.
*/
private void preemptiveSync(final ProtobufLogWriter nextWriter) {
long startTimeNanos = System.nanoTime();
try {
nextWriter.sync(useHsync);
postSync(System.nanoTime() - startTimeNanos, 0);
} catch (IOException e) {
// optimization failed, no need to abort here.
LOG.warn("pre-sync failed but an optimization so keep going", e);
}
}
/**
* This method allows subclasses to inject different writers without having to extend other
* methods like rollWriter().
* @return Writer instance
*/
@Override
protected Writer createWriterInstance(final Path path) throws IOException {
Writer writer = FSHLogProvider.createWriter(conf, fs, path, false, this.blocksize);
if (writer instanceof ProtobufLogWriter) {
preemptiveSync((ProtobufLogWriter) writer);
}
return writer;
}
/**
* Used to manufacture race condition reliably. For testing only.
* @see #beforeWaitOnSafePoint()
*/
protected void afterCreatingZigZagLatch() {
}
/**
* @see #afterCreatingZigZagLatch()
*/
protected void beforeWaitOnSafePoint() {
};
@Override
protected void doAppend(Writer writer, FSWALEntry entry) throws IOException {
writer.append(entry);
}
@Override
protected void doReplaceWriter(Path oldPath, Path newPath, Writer nextWriter) throws IOException {
// Ask the ring buffer writer to pause at a safe point. Once we do this, the writer
// thread will eventually pause. An error hereafter needs to release the writer thread
// regardless -- hence the finally block below. Note, this method is called from the FSHLog
// constructor BEFORE the ring buffer is set running so it is null on first time through
// here; allow for that.
SyncFuture syncFuture = null;
SafePointZigZagLatch zigzagLatch = null;
long sequence = -1L;
if (this.writer != null && this.ringBufferEventHandler != null) {
// Get sequence first to avoid dead lock when ring buffer is full
// Considering below sequence
// 1. replaceWriter is called and zigzagLatch is initialized
// 2. ringBufferEventHandler#onEvent is called and arrives at #attainSafePoint(long) then wait
// on safePointReleasedLatch
// 3. Since ring buffer is full, if we get sequence when publish sync, the replaceWriter
// thread will wait for the ring buffer to be consumed, but the only consumer is waiting
// replaceWriter thread to release safePointReleasedLatch, which causes a deadlock
sequence = getSequenceOnRingBuffer();
zigzagLatch = this.ringBufferEventHandler.attainSafePoint();
}
afterCreatingZigZagLatch();
try {
// Wait on the safe point to be achieved. Send in a sync in case nothing has hit the
// ring buffer between the above notification of writer that we want it to go to
// 'safe point' and then here where we are waiting on it to attain safe point. Use
// 'sendSync' instead of 'sync' because we do not want this thread to block waiting on it
// to come back. Cleanup this syncFuture down below after we are ready to run again.
try {
if (zigzagLatch != null) {
// use assert to make sure no change breaks the logic that
// sequence and zigzagLatch will be set together
assert sequence > 0L : "Failed to get sequence from ring buffer";
TraceUtil.addTimelineAnnotation("awaiting safepoint");
syncFuture = zigzagLatch.waitSafePoint(publishSyncOnRingBuffer(sequence, false));
}
} catch (FailedSyncBeforeLogCloseException e) {
// If unflushed/unsynced entries on close, it is reason to abort.
if (isUnflushedEntries()) {
throw e;
}
LOG.warn(
"Failed sync-before-close but no outstanding appends; closing WAL" + e.getMessage());
}
long oldFileLen = 0L;
// It is at the safe point. Swap out writer from under the blocked writer thread.
if (this.writer != null) {
oldFileLen = this.writer.getLength();
// In case of having unflushed entries or we already reached the
// closeErrorsTolerated count, call the closeWriter inline rather than in async
// way so that in case of an IOE we will throw it back and abort RS.
inflightWALClosures.put(oldPath.getName(), writer);
if (isUnflushedEntries() || closeErrorCount.get() >= this.closeErrorsTolerated) {
closeWriter(this.writer, oldPath, true);
} else {
Writer localWriter = this.writer;
closeExecutor.execute(() -> {
try {
closeWriter(localWriter, oldPath, false);
} catch (IOException e) {
// We will never reach here.
}
});
}
}
logRollAndSetupWalProps(oldPath, newPath, oldFileLen);
this.writer = nextWriter;
if (nextWriter != null && nextWriter instanceof ProtobufLogWriter) {
this.hdfs_out = ((ProtobufLogWriter) nextWriter).getStream();
} else {
this.hdfs_out = null;
}
} catch (InterruptedException ie) {
// Perpetuate the interrupt
Thread.currentThread().interrupt();
} catch (IOException e) {
long count = getUnflushedEntriesCount();
LOG.error("Failed close of WAL writer " + oldPath + ", unflushedEntries=" + count, e);
throw new FailedLogCloseException(oldPath + ", unflushedEntries=" + count, e);
} finally {
// Let the writer thread go regardless, whether error or not.
if (zigzagLatch != null) {
// Reset rollRequested status
rollRequested.set(false);
zigzagLatch.releaseSafePoint();
// syncFuture will be null if we failed our wait on safe point above. Otherwise, if
// latch was obtained successfully, the sync we threw in either trigger the latch or it
// got stamped with an exception because the WAL was damaged and we could not sync. Now
// the write pipeline has been opened up again by releasing the safe point, process the
// syncFuture we got above. This is probably a noop but it may be stale exception from
// when old WAL was in place. Catch it if so.
if (syncFuture != null) {
try {
blockOnSync(syncFuture);
} catch (IOException ioe) {
if (LOG.isTraceEnabled()) {
LOG.trace("Stale sync exception", ioe);
}
}
}
}
}
}
private void closeWriter(Writer writer, Path path, boolean syncCloseCall) throws IOException {
try {
TraceUtil.addTimelineAnnotation("closing writer");
writer.close();
TraceUtil.addTimelineAnnotation("writer closed");
} catch (IOException ioe) {
int errors = closeErrorCount.incrementAndGet();
boolean hasUnflushedEntries = isUnflushedEntries();
if (syncCloseCall && (hasUnflushedEntries || (errors > this.closeErrorsTolerated))) {
LOG.error("Close of WAL " + path + " failed. Cause=\"" + ioe.getMessage() + "\", errors="
+ errors + ", hasUnflushedEntries=" + hasUnflushedEntries);
throw ioe;
}
LOG.warn("Riding over failed WAL close of " + path
+ "; THIS FILE WAS NOT CLOSED BUT ALL EDITS SYNCED SO SHOULD BE OK", ioe);
} finally {
inflightWALClosures.remove(path.getName());
}
}
@Override
protected void doShutdown() throws IOException {
// Shutdown the disruptor. Will stop after all entries have been processed. Make sure we
// have stopped incoming appends before calling this else it will not shutdown. We are
// conservative below waiting a long time and if not elapsed, then halting.
if (this.disruptor != null) {
long timeoutms = conf.getLong("hbase.wal.disruptor.shutdown.timeout.ms", 60000);
try {
this.disruptor.shutdown(timeoutms, TimeUnit.MILLISECONDS);
} catch (TimeoutException e) {
LOG.warn("Timed out bringing down disruptor after " + timeoutms + "ms; forcing halt "
+ "(It is a problem if this is NOT an ABORT! -- DATALOSS!!!!)");
this.disruptor.halt();
this.disruptor.shutdown();
}
}
if (LOG.isDebugEnabled()) {
LOG.debug("Closing WAL writer in " + CommonFSUtils.getPath(walDir));
}
if (this.writer != null) {
this.writer.close();
this.writer = null;
}
closeExecutor.shutdown();
try {
if (!closeExecutor.awaitTermination(waitOnShutdownInSeconds, TimeUnit.SECONDS)) {
LOG.error("We have waited {} seconds but the close of writer(s) doesn't complete."
+ "Please check the status of underlying filesystem"
+ " or increase the wait time by the config \"{}\"", this.waitOnShutdownInSeconds,
FSHLOG_WAIT_ON_SHUTDOWN_IN_SECONDS);
}
} catch (InterruptedException e) {
LOG.error("The wait for termination of FSHLog writer(s) is interrupted");
Thread.currentThread().interrupt();
}
}
@Override
protected long append(final RegionInfo hri, final WALKeyImpl key, final WALEdit edits,
final boolean inMemstore) throws IOException {
return stampSequenceIdAndPublishToRingBuffer(hri, key, edits, inMemstore,
disruptor.getRingBuffer());
}
/**
* Thread to runs the hdfs sync call. This call takes a while to complete. This is the longest
* pole adding edits to the WAL and this must complete to be sure all edits persisted. We run
* multiple threads sync'ng rather than one that just syncs in series so we have better latencies;
* otherwise, an edit that arrived just after a sync started, might have to wait almost the length
* of two sync invocations before it is marked done.
*
* When the sync completes, it marks all the passed in futures done. On the other end of the sync
* future is a blocked thread, usually a regionserver Handler. There may be more than one future
* passed in the case where a few threads arrive at about the same time and all invoke 'sync'. In
* this case we'll batch up the invocations and run one filesystem sync only for a batch of
* Handler sync invocations. Do not confuse these Handler SyncFutures with the futures an
* ExecutorService returns when you call submit. We have no use for these in this model. These
* SyncFutures are 'artificial', something to hold the Handler until the filesystem sync
* completes.
*/
private class SyncRunner extends Thread {
private volatile long sequence;
// Keep around last exception thrown. Clear on successful sync.
private final BlockingQueue syncFutures;
private volatile SyncFuture takeSyncFuture = null;
SyncRunner(final String name, final int maxHandlersCount) {
super(name);
// LinkedBlockingQueue because of
// http://www.javacodegeeks.com/2010/09/java-best-practices-queue-battle-and.html
// Could use other blockingqueues here or concurrent queues.
//
// We could let the capacity be 'open' but bound it so we get alerted in pathological case
// where we cannot sync and we have a bunch of threads all backed up waiting on their syncs
// to come in. LinkedBlockingQueue actually shrinks when you remove elements so Q should
// stay neat and tidy in usual case. Let the max size be three times the maximum handlers.
// The passed in maxHandlerCount is the user-level handlers which is what we put up most of
// but HBase has other handlers running too -- opening region handlers which want to write
// the meta table when succesful (i.e. sync), closing handlers -- etc. These are usually
// much fewer in number than the user-space handlers so Q-size should be user handlers plus
// some space for these other handlers. Lets multiply by 3 for good-measure.
this.syncFutures = new LinkedBlockingQueue<>(maxHandlersCount * 3);
}
void offer(final long sequence, final SyncFuture[] syncFutures, final int syncFutureCount) {
// Set sequence first because the add to the queue will wake the thread if sleeping.
this.sequence = sequence;
for (int i = 0; i < syncFutureCount; ++i) {
this.syncFutures.add(syncFutures[i]);
}
}
/**
* Release the passed syncFuture
* @return Returns 1.
*/
private int releaseSyncFuture(final SyncFuture syncFuture, final long currentSequence,
final Throwable t) {
if (!syncFuture.done(currentSequence, t)) {
throw new IllegalStateException();
}
// This function releases one sync future only.
return 1;
}
/**
* Release all SyncFutures whose sequence is <= currentSequence.
* @param t May be non-null if we are processing SyncFutures because an exception was thrown.
* @return Count of SyncFutures we let go.
*/
private int releaseSyncFutures(final long currentSequence, final Throwable t) {
int syncCount = 0;
for (SyncFuture syncFuture; (syncFuture = this.syncFutures.peek()) != null;) {
if (syncFuture.getTxid() > currentSequence) {
break;
}
releaseSyncFuture(syncFuture, currentSequence, t);
if (!this.syncFutures.remove(syncFuture)) {
throw new IllegalStateException(syncFuture.toString());
}
syncCount++;
}
return syncCount;
}
/**
* @param sequence The sequence we ran the filesystem sync against.
* @return Current highest synced sequence.
*/
private long updateHighestSyncedSequence(long sequence) {
long currentHighestSyncedSequence;
// Set the highestSyncedSequence IFF our current sequence id is the 'highest'.
do {
currentHighestSyncedSequence = highestSyncedTxid.get();
if (currentHighestSyncedSequence >= sequence) {
// Set the sync number to current highwater mark; might be able to let go more
// queued sync futures
sequence = currentHighestSyncedSequence;
break;
}
} while (!highestSyncedTxid.compareAndSet(currentHighestSyncedSequence, sequence));
return sequence;
}
boolean areSyncFuturesReleased() {
// check whether there is no sync futures offered, and no in-flight sync futures that is being
// processed.
return syncFutures.size() <= 0
&& takeSyncFuture == null;
}
@Override
public void run() {
long currentSequence;
while (!isInterrupted()) {
int syncCount = 0;
try {
// Make a local copy of takeSyncFuture after we get it. We've been running into NPEs
// 2020-03-22 16:54:32,180 WARN [sync.1] wal.FSHLog$SyncRunner(589): UNEXPECTED
// java.lang.NullPointerException
// at org.apache.hadoop.hbase.regionserver.wal.FSHLog$SyncRunner.run(FSHLog.java:582)
// at java.lang.Thread.run(Thread.java:748)
SyncFuture sf;
while (true) {
takeSyncFuture = null;
// We have to process what we 'take' from the queue
takeSyncFuture = this.syncFutures.take();
// Make local copy.
sf = takeSyncFuture;
currentSequence = this.sequence;
long syncFutureSequence = sf.getTxid();
if (syncFutureSequence > currentSequence) {
throw new IllegalStateException("currentSequence=" + currentSequence
+ ", syncFutureSequence=" + syncFutureSequence);
}
// See if we can process any syncfutures BEFORE we go sync.
long currentHighestSyncedSequence = highestSyncedTxid.get();
if (currentSequence < currentHighestSyncedSequence) {
syncCount += releaseSyncFuture(sf, currentHighestSyncedSequence, null);
// Done with the 'take'. Go around again and do a new 'take'.
continue;
}
break;
}
// I got something. Lets run. Save off current sequence number in case it changes
// while we run.
//TODO handle htrace API change, see HBASE-18895
//TraceScope scope = Trace.continueSpan(takeSyncFuture.getSpan());
long start = System.nanoTime();
Throwable lastException = null;
try {
TraceUtil.addTimelineAnnotation("syncing writer");
long unSyncedFlushSeq = highestUnsyncedTxid;
writer.sync(sf.isForceSync());
TraceUtil.addTimelineAnnotation("writer synced");
if (unSyncedFlushSeq > currentSequence) {
currentSequence = unSyncedFlushSeq;
}
currentSequence = updateHighestSyncedSequence(currentSequence);
} catch (IOException e) {
LOG.error("Error syncing, request close of WAL", e);
lastException = e;
} catch (Exception e) {
LOG.warn("UNEXPECTED", e);
lastException = e;
} finally {
// reattach the span to the future before releasing.
//TODO handle htrace API change, see HBASE-18895
// takeSyncFuture.setSpan(scope.getSpan());
// First release what we 'took' from the queue.
syncCount += releaseSyncFuture(takeSyncFuture, currentSequence, lastException);
// Can we release other syncs?
syncCount += releaseSyncFutures(currentSequence, lastException);
if (lastException != null) {
requestLogRoll(ERROR);
} else {
checkLogRoll();
}
}
postSync(System.nanoTime() - start, syncCount);
} catch (InterruptedException e) {
// Presume legit interrupt.
Thread.currentThread().interrupt();
} catch (Throwable t) {
LOG.warn("UNEXPECTED, continuing", t);
}
}
}
}
/**
* Schedule a log roll if needed.
*/
private void checkLogRoll() {
// If we have already requested a roll, do nothing
if (isLogRollRequested()) {
return;
}
// Will return immediately if we are in the middle of a WAL log roll currently.
if (!rollWriterLock.tryLock()) {
return;
}
try {
if (doCheckLogLowReplication()) {
LOG.warn("Requesting log roll because of low replication, current pipeline: " +
Arrays.toString(getPipeline()));
requestLogRoll(LOW_REPLICATION);
} else if (writer != null && writer.getLength() > logrollsize) {
if (LOG.isDebugEnabled()) {
LOG.debug("Requesting log roll because of file size threshold; length=" +
writer.getLength() + ", logrollsize=" + logrollsize);
}
requestLogRoll(SIZE);
} else if (doCheckSlowSync()) {
// We log this already in checkSlowSync
requestLogRoll(SLOW_SYNC);
}
} finally {
rollWriterLock.unlock();
}
}
/**
* @return true if number of replicas for the WAL is lower than threshold
*/
@Override
protected boolean doCheckLogLowReplication() {
boolean logRollNeeded = false;
// if the number of replicas in HDFS has fallen below the configured
// value, then roll logs.
try {
int numCurrentReplicas = getLogReplication();
if (numCurrentReplicas != 0 && numCurrentReplicas < this.minTolerableReplication) {
if (this.lowReplicationRollEnabled) {
if (this.consecutiveLogRolls.get() < this.lowReplicationRollLimit) {
LOG.warn("HDFS pipeline error detected. " + "Found " + numCurrentReplicas
+ " replicas but expecting no less than " + this.minTolerableReplication
+ " replicas. " + " Requesting close of WAL. current pipeline: "
+ Arrays.toString(getPipeline()));
logRollNeeded = true;
// If rollWriter is requested, increase consecutiveLogRolls. Once it
// is larger than lowReplicationRollLimit, disable the
// LowReplication-Roller
this.consecutiveLogRolls.getAndIncrement();
} else {
LOG.warn("Too many consecutive RollWriter requests, it's a sign of "
+ "the total number of live datanodes is lower than the tolerable replicas.");
this.consecutiveLogRolls.set(0);
this.lowReplicationRollEnabled = false;
}
}
} else if (numCurrentReplicas >= this.minTolerableReplication) {
if (!this.lowReplicationRollEnabled) {
// The new writer's log replicas is always the default value.
// So we should not enable LowReplication-Roller. If numEntries
// is lower than or equals 1, we consider it as a new writer.
if (this.numEntries.get() <= 1) {
return logRollNeeded;
}
// Once the live datanode number and the replicas return to normal,
// enable the LowReplication-Roller.
this.lowReplicationRollEnabled = true;
LOG.info("LowReplication-Roller was enabled.");
}
}
} catch (Exception e) {
LOG.warn("DFSOutputStream.getNumCurrentReplicas failed because of " + e + ", continuing...");
}
return logRollNeeded;
}
protected long getSequenceOnRingBuffer() {
return this.disruptor.getRingBuffer().next();
}
private SyncFuture publishSyncOnRingBuffer(boolean forceSync) {
long sequence = getSequenceOnRingBuffer();
return publishSyncOnRingBuffer(sequence, forceSync);
}
protected SyncFuture publishSyncOnRingBuffer(long sequence, boolean forceSync) {
// here we use ring buffer sequence as transaction id
SyncFuture syncFuture = getSyncFuture(sequence, forceSync);
try {
RingBufferTruck truck = this.disruptor.getRingBuffer().get(sequence);
truck.load(syncFuture);
} finally {
this.disruptor.getRingBuffer().publish(sequence);
}
return syncFuture;
}
// Sync all known transactions
private void publishSyncThenBlockOnCompletion(TraceScope scope, boolean forceSync) throws IOException {
SyncFuture syncFuture = publishSyncOnRingBuffer(forceSync);
blockOnSync(syncFuture);
}
/**
* {@inheritDoc}
*
* If the pipeline isn't started yet or is empty, you will get the default replication factor.
* Therefore, if this function returns 0, it means you are not properly running with the HDFS-826
* patch.
*/
@Override
int getLogReplication() {
try {
// in standalone mode, it will return 0
if (this.hdfs_out instanceof HdfsDataOutputStream) {
return ((HdfsDataOutputStream) this.hdfs_out).getCurrentBlockReplication();
}
} catch (IOException e) {
LOG.info("", e);
}
return 0;
}
@Override
public void sync() throws IOException {
sync(useHsync);
}
@Override
public void sync(boolean forceSync) throws IOException {
try (TraceScope scope = TraceUtil.createTrace("FSHLog.sync")) {
publishSyncThenBlockOnCompletion(scope, forceSync);
}
}
@Override
public void sync(long txid) throws IOException {
sync(txid, useHsync);
}
@Override
public void sync(long txid, boolean forceSync) throws IOException {
if (this.highestSyncedTxid.get() >= txid) {
// Already sync'd.
return;
}
try (TraceScope scope = TraceUtil.createTrace("FSHLog.sync")) {
publishSyncThenBlockOnCompletion(scope, forceSync);
}
}
boolean isLowReplicationRollEnabled() {
return lowReplicationRollEnabled;
}
public static final long FIXED_OVERHEAD = ClassSize
.align(ClassSize.OBJECT + (5 * ClassSize.REFERENCE) + (2 * ClassSize.ATOMIC_INTEGER)
+ (3 * Bytes.SIZEOF_INT) + (4 * Bytes.SIZEOF_LONG));
/**
* This class is used coordinating two threads holding one thread at a 'safe point' while the
* orchestrating thread does some work that requires the first thread paused: e.g. holding the WAL
* writer while its WAL is swapped out from under it by another thread.
*
* Thread A signals Thread B to hold when it gets to a 'safe point'. Thread A wait until Thread B
* gets there. When the 'safe point' has been attained, Thread B signals Thread A. Thread B then
* holds at the 'safe point'. Thread A on notification that Thread B is paused, goes ahead and
* does the work it needs to do while Thread B is holding. When Thread A is done, it flags B and
* then Thread A and Thread B continue along on their merry way. Pause and signalling 'zigzags'
* between the two participating threads. We use two latches -- one the inverse of the other --
* pausing and signaling when states are achieved.
*
* To start up the drama, Thread A creates an instance of this class each time it would do this
* zigzag dance and passes it to Thread B (these classes use Latches so it is one shot only).
* Thread B notices the new instance (via reading a volatile reference or how ever) and it starts
* to work toward the 'safe point'. Thread A calls {@link #waitSafePoint(SyncFuture)} when it cannot proceed
* until the Thread B 'safe point' is attained. Thread A will be held inside in
* {@link #waitSafePoint(SyncFuture)} until Thread B reaches the 'safe point'. Once there, Thread B frees
* Thread A by calling {@link #safePointAttained()}. Thread A now knows Thread B is at the 'safe
* point' and that it is holding there (When Thread B calls {@link #safePointAttained()} it blocks
* here until Thread A calls {@link #releaseSafePoint()}). Thread A proceeds to do what it needs
* to do while Thread B is paused. When finished, it lets Thread B lose by calling
* {@link #releaseSafePoint()} and away go both Threads again.
*/
static class SafePointZigZagLatch {
/**
* Count down this latch when safe point attained.
*/
private volatile CountDownLatch safePointAttainedLatch = new CountDownLatch(1);
/**
* Latch to wait on. Will be released when we can proceed.
*/
private volatile CountDownLatch safePointReleasedLatch = new CountDownLatch(1);
private void checkIfSyncFailed(SyncFuture syncFuture) throws FailedSyncBeforeLogCloseException {
Throwable t = syncFuture.getThrowable();
if (t != null) {
throw new FailedSyncBeforeLogCloseException(t);
}
}
/**
* For Thread A to call when it is ready to wait on the 'safe point' to be attained. Thread A
* will be held in here until Thread B calls {@link #safePointAttained()}
* @param syncFuture We need this as barometer on outstanding syncs. If it comes home with an
* exception, then something is up w/ our syncing.
* @return The passed syncFuture
*/
SyncFuture waitSafePoint(SyncFuture syncFuture) throws InterruptedException,
FailedSyncBeforeLogCloseException {
while (!this.safePointAttainedLatch.await(1, TimeUnit.MILLISECONDS)) {
checkIfSyncFailed(syncFuture);
}
checkIfSyncFailed(syncFuture);
return syncFuture;
}
/**
* @return if the safepoint has been attained.
*/
@InterfaceAudience.Private
boolean isSafePointAttained() {
return this.safePointAttainedLatch.getCount() == 0;
}
/**
* Called by Thread B when it attains the 'safe point'. In this method, Thread B signals Thread
* A it can proceed. Thread B will be held in here until {@link #releaseSafePoint()} is called
* by Thread A.
*/
void safePointAttained() throws InterruptedException {
this.safePointAttainedLatch.countDown();
this.safePointReleasedLatch.await();
}
/**
* Called by Thread A when it is done with the work it needs to do while Thread B is halted.
* This will release the Thread B held in a call to {@link #safePointAttained()}
*/
void releaseSafePoint() {
this.safePointReleasedLatch.countDown();
}
/**
* @return True is this is a 'cocked', fresh instance, and not one that has already fired.
*/
boolean isCocked() {
return this.safePointAttainedLatch.getCount() > 0
&& this.safePointReleasedLatch.getCount() > 0;
}
}
/**
* Handler that is run by the disruptor ringbuffer consumer. Consumer is a SINGLE
* 'writer/appender' thread. Appends edits and starts up sync runs. Tries its best to batch up
* syncs. There is no discernible benefit batching appends so we just append as they come in
* because it simplifies the below implementation. See metrics for batching effectiveness (In
* measurement, at 100 concurrent handlers writing 1k, we are batching > 10 appends and 10 handler
* sync invocations for every actual dfsclient sync call; at 10 concurrent handlers, YMMV).
*
* Herein, we have an array into which we store the sync futures as they come in. When we have a
* 'batch', we'll then pass what we have collected to a SyncRunner thread to do the filesystem
* sync. When it completes, it will then call {@link SyncFuture#done(long, Throwable)} on each of
* SyncFutures in the batch to release blocked Handler threads.
*
* I've tried various effects to try and make latencies low while keeping throughput high. I've
* tried keeping a single Queue of SyncFutures in this class appending to its tail as the syncs
* coming and having sync runner threads poll off the head to 'finish' completed SyncFutures. I've
* tried linkedlist, and various from concurrent utils whether LinkedBlockingQueue or
* ArrayBlockingQueue, etc. The more points of synchronization, the more 'work' (according to
* 'perf stats') that has to be done; small increases in stall percentages seem to have a big
* impact on throughput/latencies. The below model where we have an array into which we stash the
* syncs and then hand them off to the sync thread seemed like a decent compromise. See HBASE-8755
* for more detail.
*/
class RingBufferEventHandler implements EventHandler, LifecycleAware {
private final SyncRunner[] syncRunners;
private final SyncFuture[] syncFutures;
// Had 'interesting' issues when this was non-volatile. On occasion, we'd not pass all
// syncFutures to the next sync'ing thread.
private AtomicInteger syncFuturesCount = new AtomicInteger();
private volatile SafePointZigZagLatch zigzagLatch;
/**
* Set if we get an exception appending or syncing so that all subsequence appends and syncs on
* this WAL fail until WAL is replaced.
*/
private Exception exception = null;
/**
* Object to block on while waiting on safe point.
*/
private final Object safePointWaiter = new Object();
private volatile boolean shutdown = false;
/**
* Which syncrunner to use next.
*/
private int syncRunnerIndex;
RingBufferEventHandler(final int syncRunnerCount, final int maxBatchCount) {
this.syncFutures = new SyncFuture[maxBatchCount];
this.syncRunners = new SyncRunner[syncRunnerCount];
for (int i = 0; i < syncRunnerCount; i++) {
this.syncRunners[i] = new SyncRunner("sync." + i, maxBatchCount);
}
}
private void cleanupOutstandingSyncsOnException(final long sequence, final Exception e) {
// There could be handler-count syncFutures outstanding.
for (int i = 0; i < this.syncFuturesCount.get(); i++) {
this.syncFutures[i].done(sequence, e);
}
offerDoneSyncsBackToCache();
}
/**
* Offers the finished syncs back to the cache for reuse.
*/
private void offerDoneSyncsBackToCache() {
for (int i = 0; i < this.syncFuturesCount.get(); i++) {
syncFutureCache.offer(syncFutures[i]);
}
this.syncFuturesCount.set(0);
}
/**
* @return True if outstanding sync futures still
*/
private boolean isOutstandingSyncs() {
// Look at SyncFutures in the EventHandler
for (int i = 0; i < this.syncFuturesCount.get(); i++) {
if (!this.syncFutures[i].isDone()) {
return true;
}
}
return false;
}
private boolean isOutstandingSyncsFromRunners() {
// Look at SyncFutures in the SyncRunners
for (SyncRunner syncRunner: syncRunners) {
if(syncRunner.isAlive() && !syncRunner.areSyncFuturesReleased()) {
return true;
}
}
return false;
}
@Override
// We can set endOfBatch in the below method if at end of our this.syncFutures array
public void onEvent(final RingBufferTruck truck, final long sequence, boolean endOfBatch)
throws Exception {
// Appends and syncs are coming in order off the ringbuffer. We depend on this fact. We'll
// add appends to dfsclient as they come in. Batching appends doesn't give any significant
// benefit on measurement. Handler sync calls we will batch up. If we get an exception
// appending an edit, we fail all subsequent appends and syncs with the same exception until
// the WAL is reset. It is important that we not short-circuit and exit early this method.
// It is important that we always go through the attainSafePoint on the end. Another thread,
// the log roller may be waiting on a signal from us here and will just hang without it.
try {
if (truck.type() == RingBufferTruck.Type.SYNC) {
this.syncFutures[this.syncFuturesCount.getAndIncrement()] = truck.unloadSync();
// Force flush of syncs if we are carrying a full complement of syncFutures.
if (this.syncFuturesCount.get() == this.syncFutures.length) {
endOfBatch = true;
}
} else if (truck.type() == RingBufferTruck.Type.APPEND) {
FSWALEntry entry = truck.unloadAppend();
//TODO handle htrace API change, see HBASE-18895
//TraceScope scope = Trace.continueSpan(entry.detachSpan());
try {
if (this.exception != null) {
// Return to keep processing events coming off the ringbuffer
return;
}
append(entry);
} catch (Exception e) {
// Failed append. Record the exception.
this.exception = e;
// invoking cleanupOutstandingSyncsOnException when append failed with exception,
// it will cleanup existing sync requests recorded in syncFutures but not offered to SyncRunner yet,
// so there won't be any sync future left over if no further truck published to disruptor.
cleanupOutstandingSyncsOnException(sequence,
this.exception instanceof DamagedWALException ? this.exception
: new DamagedWALException("On sync", this.exception));
// Return to keep processing events coming off the ringbuffer
return;
} finally {
entry.release();
}
} else {
// What is this if not an append or sync. Fail all up to this!!!
cleanupOutstandingSyncsOnException(sequence,
new IllegalStateException("Neither append nor sync"));
// Return to keep processing.
return;
}
// TODO: Check size and if big go ahead and call a sync if we have enough data.
// This is a sync. If existing exception, fall through. Else look to see if batch.
if (this.exception == null) {
// If not a batch, return to consume more events from the ring buffer before proceeding;
// we want to get up a batch of syncs and appends before we go do a filesystem sync.
if (!endOfBatch || this.syncFuturesCount.get() <= 0) {
return;
}
// syncRunnerIndex is bound to the range [0, Integer.MAX_INT - 1] as follows:
// * The maximum value possible for syncRunners.length is Integer.MAX_INT
// * syncRunnerIndex starts at 0 and is incremented only here
// * after the increment, the value is bounded by the '%' operator to
// [0, syncRunners.length), presuming the value was positive prior to
// the '%' operator.
// * after being bound to [0, Integer.MAX_INT - 1], the new value is stored in
// syncRunnerIndex ensuring that it can't grow without bound and overflow.
// * note that the value after the increment must be positive, because the most it
// could have been prior was Integer.MAX_INT - 1 and we only increment by 1.
this.syncRunnerIndex = (this.syncRunnerIndex + 1) % this.syncRunners.length;
try {
// Below expects that the offer 'transfers' responsibility for the outstanding syncs to
// the syncRunner. We should never get an exception in here.
this.syncRunners[this.syncRunnerIndex].offer(sequence, this.syncFutures,
this.syncFuturesCount.get());
} catch (Exception e) {
// Should NEVER get here.
requestLogRoll(ERROR);
this.exception = new DamagedWALException("Failed offering sync", e);
}
}
// We may have picked up an exception above trying to offer sync
if (this.exception != null) {
cleanupOutstandingSyncsOnException(sequence, this.exception instanceof DamagedWALException
? this.exception : new DamagedWALException("On sync", this.exception));
}
attainSafePoint(sequence);
// It is critical that we offer the futures back to the cache for reuse here after the
// safe point is attained and all the clean up has been done. There have been
// issues with reusing sync futures early causing WAL lockups, see HBASE-25984.
offerDoneSyncsBackToCache();
} catch (Throwable t) {
LOG.error("UNEXPECTED!!! syncFutures.length=" + this.syncFutures.length, t);
}
}
SafePointZigZagLatch attainSafePoint() {
this.zigzagLatch = new SafePointZigZagLatch();
return this.zigzagLatch;
}
/**
* Check if we should attain safe point. If so, go there and then wait till signalled before we
* proceeding.
*/
private void attainSafePoint(final long currentSequence) {
if (this.zigzagLatch == null || !this.zigzagLatch.isCocked()) {
return;
}
// If here, another thread is waiting on us to get to safe point. Don't leave it hanging.
beforeWaitOnSafePoint();
try {
// Wait on outstanding syncers; wait for them to finish syncing (unless we've been
// shutdown or unless our latch has been thrown because we have been aborted or unless
// this WAL is broken and we can't get a sync/append to complete).
while ((!this.shutdown && this.zigzagLatch.isCocked()
&& highestSyncedTxid.get() < currentSequence &&
// We could be in here and all syncs are failing or failed. Check for this. Otherwise
// we'll just be stuck here for ever. In other words, ensure there syncs running.
isOutstandingSyncs())
// Wait for all SyncRunners to finish their work so that we can replace the writer
|| isOutstandingSyncsFromRunners()) {
synchronized (this.safePointWaiter) {
this.safePointWaiter.wait(0, 1);
}
}
// Tell waiting thread we've attained safe point. Can clear this.throwable if set here
// because we know that next event through the ringbuffer will be going to a new WAL
// after we do the zigzaglatch dance.
this.exception = null;
this.zigzagLatch.safePointAttained();
} catch (InterruptedException e) {
LOG.warn("Interrupted ", e);
Thread.currentThread().interrupt();
}
}
/**
* Append to the WAL. Does all CP and WAL listener calls.
*/
void append(final FSWALEntry entry) throws Exception {
try {
FSHLog.this.appendEntry(writer, entry);
} catch (Exception e) {
String msg = "Append sequenceId=" + entry.getKey().getSequenceId()
+ ", requesting roll of WAL";
LOG.warn(msg, e);
requestLogRoll(ERROR);
throw new DamagedWALException(msg, e);
}
}
@Override
public void onStart() {
for (SyncRunner syncRunner : this.syncRunners) {
syncRunner.start();
}
}
@Override
public void onShutdown() {
for (SyncRunner syncRunner : this.syncRunners) {
syncRunner.interrupt();
}
}
}
/**
* This method gets the pipeline for the current WAL.
*/
@Override
DatanodeInfo[] getPipeline() {
if (this.hdfs_out != null) {
if (this.hdfs_out.getWrappedStream() instanceof DFSOutputStream) {
return ((DFSOutputStream) this.hdfs_out.getWrappedStream()).getPipeline();
}
}
return new DatanodeInfo[0];
}
Writer getWriter() {
return this.writer;
}
void setWriter(Writer writer) {
this.writer = writer;
}
}