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https://issues.apache.org/jira/browse/HDFS-17166?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17759192#comment-17759192
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ASF GitHub Bot commented on HDFS-17166:
---------------------------------------
KeeProMise commented on code in PR #5990:
URL: https://github.com/apache/hadoop/pull/5990#discussion_r1306285491
##########
hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/resolver/MembershipNamenodeResolver.java:
##########
@@ -478,4 +476,27 @@ private List<MembershipState>
getRecentRegistrationForQuery(
public void setRouterId(String router) {
this.routerId = router;
}
+
+ /**
+ * Shuffle cache, to ensure that the current nn will not be accessed first
next time.
+ *
+ *
+ * @param nsId name service id
+ * @param namenode namenode contexts
+ */
+ @Override
+ public synchronized void shuffleCache(String nsId, FederationNamenodeContext
namenode) {
+ cacheNS.compute(Pair.of(nsId, false), (ns, namenodeContexts) -> {
+ if (namenodeContexts != null
+ && namenodeContexts.size() > 0
Review Comment:
The reason for not judging outside here is that we should ensure that get
cache and modify cache are atomic, considering the following situation:
1. After **Thread1** judges that the cache is not empty, it gets the ns no
active in the cache
2. **Thread2** (loadCache thread )clear the cache
3. **Thread3** processes the client request and finds that the cache is
empty, and updates the cache. At this time, the ns - in the cache has active nn
4. **Thread1** rotates the previously acquired data of non-active nn and
writes it into the cache, causing ns in the cache to have no active nn
> RBF: Throwing NoNamenodesAvailableException for a long time, when failover
> --------------------------------------------------------------------------
>
> Key: HDFS-17166
> URL: https://issues.apache.org/jira/browse/HDFS-17166
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Jian Zhang
> Priority: Major
> Labels: pull-request-available
> Attachments:
> fix_NoNamenodesAvailableException_long_time_when_ns_failover.patch,
> image-2023-08-26-00-24-02-016.png, image-2023-08-26-00-25-42-086.png
>
>
> When ns failover， the router may record that the ns have no active namenode,
> the router cannot find the active nn in the ns for about 1 minute. The client
> will report an error after consuming the number of retries, and the router
> will be unable to provide services for the ns for a long time.
> 11:52:44 Start reporting
> !image-2023-08-26-00-24-02-016.png|width=800,height=100!
> 11:53:46 end reporting
> !image-2023-08-26-00-25-42-086.png|width=800,height=50!
>
> At this point, the failover has been successfully completed in the ns, and
> the client can directly connect to the active namenode to access it
> successfully, but the client cannot access the ns through router for up to a
> minute
>
> *There is a bug in this logic：*
> * A certain ns starts to fail over,
> * There is a state where there is no active nn in ns, Router reports the
> status (no active nn) to the state store
> * After a period of time, the router pulls the state store data to update the
> cache, and the cache records that the ns has no active nn
> * Failover successfully completed, at which point the ns actually has an
> active nn
> * Assuming it's not time for router to update the cache yet
> * The client sent a request to the router for the ns, and the router
> accessed the first nn of the ns in the router’s cache (no active nn)
> * Unfortunately, the nn is really standby, so the request went wrong and
> entered the exception handling logic. The router found that there is no
> active nn for the ns in the cache and throw NoNamenodesAvailableException
> * The NoNamenodesAvailableException exception is wrapped as a
> RetrieveException, which causes the client to retry. Since each router
> retrieves the true standby nn in the cache (because it is always the first
> one in the cache and has a high priority), a NoNamenodesAvailableException is
> thrown every time until the router updates the cache from the state store
>
> *Fix the bug*
> When an ns in the router's cache does not have an active nn, but in reality,
> the ns has an active nn, and the client requests to throw a
> NoNamenodesAvailableException, it is proven that the requested nn is a real
> standby nn. The priority of this nn should be lowered so that the next
> request will find the real active nn, avoiding constantly requesting the real
> standby nn, which will cause the cache to be updated before the next time,
> The router is unable to provide services for the ns to the client.
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