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https://issues.apache.org/jira/browse/HADOOP-19218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17862617#comment-17862617
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ASF GitHub Bot commented on HADOOP-19218:
-----------------------------------------
shahrs87 commented on code in PR #6916:
URL: https://github.com/apache/hadoop/pull/6916#discussion_r1663310288
##########
hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Server.java:
##########
@@ -2516,13 +2509,15 @@ public int readAndProcess() throws IOException,
InterruptedException {
}
if (!RpcConstants.HEADER.equals(dataLengthBuffer)) {
+ final String hostName = addr == null ? this.hostAddress :
addr.getHostName();
Review Comment:
In what case addr would be null? We were not handling `addr == null` edge
case before. Wondering what has changed.
> Avoid DNS lookup while creating IPC Connection object
> -----------------------------------------------------
>
> Key: HADOOP-19218
> URL: https://issues.apache.org/jira/browse/HADOOP-19218
> Project: Hadoop Common
> Issue Type: Improvement
> Reporter: Viraj Jasani
> Assignee: Viraj Jasani
> Priority: Major
> Labels: pull-request-available
>
> Been running HADOOP-18628 in production for quite sometime, everything works
> fine as long as DNS servers in HA are available. Upgrading single NS server
> at a time is also a common case, not problematic. Every DNS lookup takes 1ms
> in general.
> However, recently we encountered a case where 2 out of 4 NS servers went down
> (temporarily but it's a rare case). With small duration DNS cache and 2s of
> NS fallback timeout configured in resolv.conf, now any client performing DNS
> lookup can encounter 4s+ delay. This caused namenode outage as listener
> thread is single threaded and it was not able to keep up with large num of
> unique clients (in direct proportion with num of DNS resolutions every few
> seconds) initiating connection on listener port.
> While having 2 out of 4 DNS servers offline is rare case and NS fallback
> settings could also be improved, it is important to note that we don't need
> to perform DNS resolution for every new connection if the intention is to
> improve the insights into VersionMistmatch errors thrown by the server.
> The proposal is the delay the DNS resolution until the server throws the
> error for incompatible header or version mismatch. This would also help with
> ~1ms extra time spent even for healthy DNS lookup.
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