[
https://issues.apache.org/jira/browse/HDFS-16659?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17584513#comment-17584513
]
ASF GitHub Bot commented on HDFS-16659:
---------------------------------------
xkrogen commented on PR #4560:
URL: https://github.com/apache/hadoop/pull/4560#issuecomment-1226631416
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
> 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}
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
---------------------------------------------------------------------
To unsubscribe, e-mail: hdfs-issues-unsubscr...@hadoop.apache.org
For additional commands, e-mail: hdfs-issues-h...@hadoop.apache.org
