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From "Rushabh S Shah (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-15129) Datanode caches namenode DNS lookup failure and cannot startup
Date Tue, 19 Dec 2017 14:40:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-15129?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16296888#comment-16296888
] 

Rushabh S Shah commented on HADOOP-15129:
-----------------------------------------

Isn't this dupe of HDFS-8068 ?

> Datanode caches namenode DNS lookup failure and cannot startup
> --------------------------------------------------------------
>
>                 Key: HADOOP-15129
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15129
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: ipc
>    Affects Versions: 2.8.2
>         Environment: Google Compute Engine.
> I'm using Java 8, Debian 8, Hadoop 2.8.2.
>            Reporter: Karthik Palaniappan
>            Priority: Minor
>         Attachments: HADOOP-15129.001.patch
>
>
> On startup, the Datanode creates an InetSocketAddress to register with each namenode.
Though there are retries on connection failure throughout the stack, the same InetSocketAddress
is reused.
> InetSocketAddress is an interesting class, because it resolves DNS names to IP addresses
on construction, and it is never refreshed. Hadoop re-creates an InetSocketAddress in some
cases just in case the remote IP has changed for a particular DNS name: https://issues.apache.org/jira/browse/HADOOP-7472.
> Anyway, on startup, you cna see the Datanode log: "Namenode...remains unresolved" --
referring to the fact that DNS lookup failed.
> {code:java}
> 2017-11-02 16:01:55,115 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Refresh
request received for nameservices: null
> 2017-11-02 16:01:55,153 WARN org.apache.hadoop.hdfs.DFSUtilClient: Namenode for null
remains unresolved for ID null. Check your hdfs-site.xml file to ensure namenodes are configured
properly.
> 2017-11-02 16:01:55,156 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Starting
BPOfferServices for nameservices: <default>
> 2017-11-02 16:01:55,169 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Block pool
<registering> (Datanode Uuid unassigned) service to cluster-32f5-m:8020 starting to
offer service
> {code}
> The Datanode then proceeds to use this unresolved address, as it may work if the DN is
configured to use a proxy. Since I'm not using a proxy, it forever prints out this message:
> {code:java}
> 2017-12-15 00:13:40,712 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Problem
connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:13:45,712 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Problem
connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:13:50,712 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Problem
connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:13:55,713 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Problem
connecting to server: cluster-32f5-m:8020
> 2017-12-15 00:14:00,713 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Problem
connecting to server: cluster-32f5-m:8020
> {code}
> Unfortunately, the log doesn't contain the exception that triggered it, but the culprit
is actually in IPC Client: https://github.com/apache/hadoop/blob/trunk/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Client.java#L444.
> This line was introduced in https://issues.apache.org/jira/browse/HADOOP-487 to give
a clear error message when somebody mispells an address.
> However, the fix in HADOOP-7472 doesn't apply here, because that code happens in Client#getConnection
after the Connection is constructed.
> My proposed fix (will attach a patch) is to move this exception out of the constructor
and into a place that will trigger HADOOP-7472's logic to re-resolve addresses. If the DNS
failure was temporary, this will allow the connection to succeed. If not, the connection will
fail after ipc client retries (default 10 seconds worth of retries).
> I want to fix this in ipc client rather than just in Datanode startup, as this fixes
temporary DNS issues for all of Hadoop.



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