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https://issues.apache.org/jira/browse/HDFS-16659?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17582115#comment-17582115
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ASF GitHub Bot commented on HDFS-16659:
---------------------------------------
ZanderXu commented on PR #4560:
URL: https://github.com/apache/hadoop/pull/4560#issuecomment-1221229781
> 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.
> JournalNode should throw CacheMissException if SinceTxId is bigger than
> HighestWrittenTxId
> ------------------------------------------------------------------------------------------
>
> Key: HDFS-16659
> URL: https://issues.apache.org/jira/browse/HDFS-16659
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: ZanderXu
> Assignee: ZanderXu
> Priority: Critical
> Labels: pull-request-available
> Time Spent: 1h
> Remaining Estimate: 0h
>
> 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:
> {code:java}
> if (sinceTxId > getHighestWrittenTxId()) {
> // Requested edits that don't exist yet; short-circuit the cache here
> metrics.rpcEmptyResponses.incr();
> return
> GetJournaledEditsResponseProto.newBuilder().setTxnCount(0).build();
> }
> {code}
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