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https://issues.apache.org/jira/browse/HBASE-21503?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Duo Zhang updated HBASE-21503:
------------------------------
Resolution: Fixed
Hadoop Flags: Reviewed
Status: Resolved (was: Patch Available)
Pushed to branch-2.0+. Thanks [~zghaobac] for reviewing.
And thanks [~wchevreuil] for the great analyzing.
> Replication normal source can get stuck due potential race conditions between
> source wal reader and wal provider initialization threads.
> ----------------------------------------------------------------------------------------------------------------------------------------
>
> Key: HBASE-21503
> URL: https://issues.apache.org/jira/browse/HBASE-21503
> Project: HBase
> Issue Type: Bug
> Components: Replication
> Reporter: Wellington Chevreuil
> Assignee: Wellington Chevreuil
> Priority: Blocker
> Fix For: 3.0.0, 2.2.0, 2.0.3, 2.1.2
>
> Attachments: HBASE-21503-master.001.patch, HBASE-21503.patch
>
>
> Noticed replication sources could get stuck while doing some tests that
> involved RS restart. On these cases, upon RS restart, the newly created
> normal source was reaching wal end and not recognising it was open for write,
> what leads to remove it from source queue. Thus, no new OP get's replicated
> unless this log goes to a recovery queue.
> Checking this further, my understanding is that, during restart, RS will
> start replication services, which inits ReplicationSourceManager and
> ReplicationSources for each wal group id, in below sequence:
> {noformat}
> HRegionServer -> Replication.startReplicationService() ->
> ReplicationSourceManager.init() -> add ReplicationSource
> {noformat}
> At this point, ReplicationSources have no paths yet, so WAL reader thread is
> not running. ReplicationSourceManager is registered as a WAL listener, in
> order to get notified whenever new wal file is available. During
> ReplicationSourceManager and ReplicationSource instances creation, a
> WALFileLengthProvider instance is obtained from WALProvider and cached by
> both ReplicationSourceManager and ReplicationSource. The default
> implementation for this WALFileLengthProvider is below, on WALProvider
> interface:
> {noformat}
> default WALFileLengthProvider getWALFileLengthProvider() {
> return path -> getWALs().stream().map(w ->
> w.getLogFileSizeIfBeingWritten(path))
> .filter(o -> o.isPresent()).findAny().orElse(OptionalLong.empty());
> }
> {noformat}
> Notice that if WALProvider.getWALs returns an empty list, this
> WALFileLengthProvider instance is always going to return nothing. This is
> relevant because when ReplicationSource finally starts
> ReplicationSourceWALReader thread, it passes this WALFileLengthProvider,
> which is used by WALEntryStream (inside the wal reader) to determine if wal
> is being written (and should be kept in the queue) here:
> {noformat}
> private void tryAdvanceEntry() throws IOException {
> if (checkReader()) {
> boolean beingWritten = readNextEntryAndRecordReaderPosition();
> LOG.trace("reading wal file {}. Current open for write: {}",
> this.currentPath, beingWritten);
> if (currentEntry == null && !beingWritten) {
> // no more entries in this log file, and the file is already closed,
> i.e, rolled
> // Before dequeueing, we should always get one more attempt at
> reading.
> // This is in case more entries came in after we opened the reader,
> and the log is rolled
> // while we were reading. See HBASE-6758
> resetReader();
> readNextEntryAndRecordReaderPosition();
> if (currentEntry == null) {
> if (checkAllBytesParsed()) { // now we're certain we're done with
> this log file
> dequeueCurrentLog();
> if (openNextLog()) {
> readNextEntryAndRecordReaderPosition();
> }
> }
> }
> }
> ...
> {noformat}
> Here code snippet for WALEntryStream.readNextEntryAndRecordReaderPosition()
> method that relies on the WALFileLengthProvider:
> {noformat}
> ...
> #1 OptionalLong fileLength =
> walFileLengthProvider.getLogFileSizeIfBeingWritten(currentPath);
> if (fileLength.isPresent() && readerPos > fileLength.getAsLong()) {
> // see HBASE-14004, for AsyncFSWAL which uses fan-out, it is possible
> that we read uncommitted
> // data, so we need to make sure that we do not read beyond the
> committed file length.
> if (LOG.isDebugEnabled()) {
> LOG.debug("The provider tells us the valid length for " + currentPath
> + " is " +
> fileLength.getAsLong() + ", but we have advanced to " +
> readerPos);
> }
> resetReader();
> return true;
> }
> if (readEntry != null) {
> metrics.incrLogEditsRead();
> metrics.incrLogReadInBytes(readerPos - currentPositionOfEntry);
> }
> currentEntry = readEntry; // could be null
> this.currentPositionOfReader = readerPos;
> return fileLength.isPresent();
> ...
> {noformat}
> The problem can occur because when wal file is indeed created in
> AbstractFSWALProvider.getWAL() method (snippet shown below), line marked as
> #2 in below snippet triggers notification of registered WALListeners,
> including ReplicationSourceManager, which will start
> ReplicationSourceWALReader thread. If ReplicationSourceWALReader thread
> reaches the point #1 from snippet above before the thread running
> AbstractFSWALProvider.getWAL() method gets to point #3 from below snippet,
> then WALFileLengthProvider will return empty and the wal will not be
> considered as open, causing it to be dequeued:
> {noformat}
> @Override
> public T getWAL(RegionInfo region) throws IOException {
> T walCopy = wal;
> if (walCopy == null) {
> // only lock when need to create wal, and need to lock since
> // creating hlog on fs is time consuming
> synchronized (walCreateLock) {
> walCopy = wal;
> if (walCopy == null) {
> walCopy = createWAL();
> boolean succ = false;
> try {
> #2 walCopy.init();
> succ = true;
> } finally {
> if (!succ) {
> walCopy.close();
> }
> }
> #3 wal = walCopy;
> }
> }
> }
> return walCopy;
> }
> {noformat}
> This can be sorted by making AbstractFSWALProvider.getWALs reuse
> AbstractFSWALProvider.getWAL method to obtain the WAL instance. Do we really
> have scenarios where we want to return no WAL instance? Another possibility
> could be to synchronize getWALs on same lock currently used by getWAL.
> Am proposing an initial patch with the 1st solution, after some tests, it
> does seem to be enough to sort the problem.
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