HDFS-16659. JournalNode should throw CacheMissException when SinceTxId is bigger than HighestWrittenTxId

[ZanderXu]

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Description of PR

JournalNode should throw CacheMissException if sinceTxId is bigger than highestWrittenTxId during handling getJournaledEdits rpc from NNs. Current logic may cause in-progress EditlogTailer cannot replay any Edits from JournalNodes in some corner cases, resulting in ObserverNameNode cannot handle requests from clients.

Suppose there are 3 journalNodes, JN0 ~ JN1.

• JN0 has some abnormal cases when Active Namenode is syncing 10 Edits with first txid 11
• NameNode just ignore the abnormal JN0 and continue to sync Edits to Journal 1 and 2
• JN0 backed to health
• NameNode continue sync 10 Edits with first txid 21.
• At this point, there are no Edits 11 ~ 30 in the cache of JN0
• Observer NameNode try to select EditLogInputStream through getJournaledEdits with since txId 21
• Journal 2 has some abnormal cases and caused a slow response

The expected result is: Response should contain 20 Edits from txId 21 to txId 30 from JN1 and JN2. Because Active NameNode successfully write these Edits to JN1 and JN2 and failed write these edits to JN0.

But in the current implementation, the response is [Response(0) from JN0, Response(10) from JN1], because there are some abnormal cases in JN2, such as GC, bad network, cause a slow response. So the maxAllowedTxns will be 0, NameNode will not replay any Edits.

As above, the root case is that JournalNode should throw Miss Cache Exception when sinceTxid is more than highestWrittenTxId.

And the bug code as blew:

if (sinceTxId > getHighestWrittenTxId()) {
    // Requested edits that don't exist yet; short-circuit the cache here
    metrics.rpcEmptyResponses.incr();
    return GetJournaledEditsResponseProto.newBuilder().setTxnCount(0).build(); 
}

[hadoop-yetus]

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[ZanderXu]

@tomscut Are you interesting to review this bug about selecting EditLogInputStreams?

[ZanderXu]

@jojochuang @goiri Can you help me review this patch? Thanks

[ZanderXu]

@jojochuang @Hexiaoqiao Can you help me review this bug?

It will cause Observer Namenode cannot handle requests from Client in corner case. We encountered this bug in our prod environment, because our JournalNode cluster is deployed cross-dc to support cross-dc disaster recovery.

[ZanderXu]

@xkrogen Master, can you help me review this patch?

[xkrogen]

Thanks for trying to tackle this issue! Actually @shvachko and I discussed this potential issue long ago but had not observed problems in practice; I guess it is made much worse by using cross-DC JNs.

I don't feel that a CacheMissException is correct. The situation where the NN requests edits newer than what the JNs have is expected to be common, especially if the transaction rate is low, since in this situation the NN will constantly poll the JNs for new edits by sending sinceTxID = highestWrittenTxId + 1. I see you're trying to handle this by special-casing when the sinceTxId is getHighestWrittenTxId() + 1, but it seems pretty hacky/brittle.

My initial thought is that we should make a special-case return value when sinceTxId > highestWrittenTxId (maybe -1) and on the NN side, if you find some responses with txnCount > 0 and some responses with txnCount < 0, then you only use the responses with txnCount > 0. The main issue I see with this is that AsyncLoggerSet#waitForWriteQuorum() isn't set up to handle this kind of situation; it will just return as soon as there are a quorum of non-error responses.

As an alternative, we could create a new exception different from CacheMissException, like NewerTxnIdException, which the JN throws in the situation of startTxId > highestWrittenTxId. Since it's an exception, waitForWriteQuorum() will try to throw away JNs that threw it. If only some JNs throw the exception, then we still get a valid result from waitForWriteQuorum(). If too many JNs throw the exception, then we can catch it on the NN side and swallow the exception to treat it as a normal/expected situation. I think this would avoid us having to special-case startTxId + 1 on the JN side.

WDYT?

[ZanderXu]

we could create a new exception different from CacheMissException, like NewerTxnIdException, which the JN throws in the situation of startTxId > highestWrittenTxId

@xkrogen Master, thanks for your nice suggestion. About startTxId = highestWrittenTxId + 1, this case is common, so it should returns one JournaledEditsResponse with txtCount=0. if sinceTxId > highestTxId + 1, just throw NewerTxnIdException and let namenode ignore this abnormal journalnode.

I have updated this patch, please help me review this patch. Thanks, Master!

[hadoop-yetus]

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[ZanderXu]

What's the concern with throwing the exception in in the highestTxId + 1

@xkrogen Master, Maybe our understanding of sinceTxId == highestTxId + 1 is a bit ambiguous. Please correct me, if I'm wrong.

• sinceTxId == highestTxId + 1 is normal case, especially if the transaction rate is low. So JournalNode should return one empty GetJournaledEditsResponseProto to NameNode, not throw NewerTxnIdException.
• Conversely, if Journal throws a NewerTxnIdException to namenode, namenode will fail back to selectStreamingInputStreams with getEditLogManifest with this sinceTxId. Because there is no new edits in JournalNodes, so selectStreamingInputStreams will get an empty response too.

I try to guess that you mean to use nextTxId? If we use nextTxId, maybe we can change this code as bellow:

if (sinceTxId > nextTxId) {
  throw new JournaledEditsCache.NewerTxnIdException(...);
}

Because sinceTxId == nextTxId is a normal case, JournalNode should return an empty GetJournaledEditsResponseProto.

[xkrogen]

I am suggesting that we would also modify QuorumJournalManager#selectInputStreams() like:

      try {
        Collection<EditLogInputStream> rpcStreams = new ArrayList<>();
        selectRpcInputStreams(rpcStreams, fromTxnId, onlyDurableTxns);
        streams.addAll(rpcStreams);
        return;
      } catch (NewerTxnIdException ntie) {
        // normal situation, we requested newer IDs than any journal has. no new streams
        return;
      } catch (IOException ioe) {
        LOG.warn("Encountered exception while tailing edits >= " + fromTxnId +
            " via RPC; falling back to streaming.", ioe);
      }

I say this mainly because we want to use NewerTxnIdException to detect when a JN is lagging, right? But if we special-case sinceTxId == highestTxId + 1, then we might not detect the case where a JN is lagging by one txn.

So let's say we have: JN0 with ID 1, JN1 with ID 2, JN2 with ID 2 (so JN0 lags by one txn). Now we send out getJournaledEdits() RPCs. JN2 happens to respond slow, so we get response from JN0 and JN1. Now it looks like only txn 1 is durably committed and we never load txn 2 -- the same issue you described in your original bug description.

But by throwing NewerTxnIdException, AsyncLoggerSet will instead ignore the response from JN0, so we wait for response from JN1 and JN2, and we correctly see that up to txn 2 is committed durably.

Does this clarify? I agree the situation I describe should be rare, but I feel that we can cleanly solve it by using NewerTxnIdException.

[ZanderXu]

@xkrogen Master, thanks for your detailed explanation and nice suggestion. I will modify this path whit this nice idea.

[hadoop-yetus]

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[hadoop-yetus]

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[ZanderXu]

@xkrogen Sir, sorry to ping you again. Please help me review this patch again.

[hadoop-yetus]

:confetti_ball: +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 :ok:	reexec	0m 48s		Docker mode activated.
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			_ Other Tests _
+1 :green_heart:	unit	253m 4s		hadoop-hdfs in the patch passed.
+1 :green_heart:	asflicense	1m 12s		The patch does not generate ASF License warnings.
		364m 30s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4560/5/artifact/out/Dockerfile
GITHUB PR	https://github.com/apache/hadoop/pull/4560
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uname	Linux 12111e311603 4.15.0-191-generic #202-Ubuntu SMP Thu Aug 4 01:49:29 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
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Max. process+thread count	2920 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4560/5/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[hadoop-yetus]

:confetti_ball: +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 :ok:	reexec	0m 42s		Docker mode activated.
			_ Prechecks _
+1 :green_heart:	dupname	0m 0s		No case conflicting files found.
+0 :ok:	codespell	0m 1s		codespell was not available.
+0 :ok:	detsecrets	0m 1s		detect-secrets was not available.
+1 :green_heart:	@author	0m 0s		The patch does not contain any @author tags.
+1 :green_heart:	test4tests	0m 0s		The patch appears to include 1 new or modified test files.
			_ trunk Compile Tests _
+1 :green_heart:	mvninstall	38m 45s		trunk passed
+1 :green_heart:	compile	1m 34s		trunk passed with JDK Ubuntu-11.0.16+8-post-Ubuntu-0ubuntu120.04
+1 :green_heart:	compile	1m 32s		trunk passed with JDK Private Build-1.8.0_342-8u342-b07-0ubuntu1~20.04-b07
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			_ Patch Compile Tests _
+1 :green_heart:	mvninstall	1m 19s		the patch passed
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+1 :green_heart:	javac	1m 24s		the patch passed
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+1 :green_heart:	javac	1m 16s		the patch passed
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+1 :green_heart:	javadoc	1m 29s		the patch passed with JDK Private Build-1.8.0_342-8u342-b07-0ubuntu1~20.04-b07
+1 :green_heart:	spotbugs	3m 16s		the patch passed
+1 :green_heart:	shadedclient	22m 28s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 :green_heart:	unit	237m 30s		hadoop-hdfs in the patch passed.
+1 :green_heart:	asflicense	1m 0s		The patch does not generate ASF License warnings.
		346m 21s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4560/6/artifact/out/Dockerfile
GITHUB PR	https://github.com/apache/hadoop/pull/4560
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uname	Linux f9bd71d68f1c 4.15.0-191-generic #202-Ubuntu SMP Thu Aug 4 01:49:29 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
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Default Java	Private Build-1.8.0_342-8u342-b07-0ubuntu1~20.04-b07
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.16+8-post-Ubuntu-0ubuntu120.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_342-8u342-b07-0ubuntu1~20.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4560/6/testReport/
Max. process+thread count	3519 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4560/6/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[xkrogen]

Merged to trunk. Thanks for the contribution @ZanderXu !

[ZanderXu]

@xkrogen Sir, thank you for your patient answers and reviews, as well as nice suggestions.

[ZanderXu]

@xkrogen After deep thinking and do some verifications, I found there are two places should be fixed for the case that sinceTxId = highestTxId + 1.

Currently Journal throws one NewerTxnIdException to namenode, we expect namenode can catch NewerTxnIdException during selectRpcInputStreams and ignore it.

But the namenode throws a QuorumException during selectRpcInputStreams because there are a majority of NewerTxnIdException. Then the namenode fallbacks to selectStreamingInputStreams.

Beside this problem, JournalNodeRpcServer shouldn't print any logs about NewerTxnIdException when sinceTxId = highestTxId + 1, but it should print some logs about NewerTxnIdException when sinceTxId > highestTxId + 1.

So as above cases, how about handling them differently? such as

    long highestTxId = getHighestWrittenTxId();
    if (sinceTxId == highestTxId + 1) {
      // This is normal case and will return one response with 0 txnCount.
      metrics.rpcEmptyResponses.incr();
      return GetJournaledEditsResponseProto.newBuilder().setTxnCount(0).build();
    } else if (sinceTxId > highestTxId) {
      // Requested edits that don't exist yet and is newer than highestTxId.
      metrics.rpcEmptyResponses.incr();
      throw new NewerTxnIdException(
          "Highest txn ID available in the journal is %d, but requested txns starting at %d.",
          highestTxId, sinceTxId);
    }

[ZanderXu]

@xkrogen Sir, I create a new ticket HDFS-16771 to resolve this problems. If you have any good ideas, please share with me. Thanks.