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https://issues.apache.org/jira/browse/HDFS-5846?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13924512#comment-13924512
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Chris Nauroth commented on HDFS-5846:
-------------------------------------
Hi, [~nikola.vujic]. This patch mostly looks good. Just a few minor comments:
# There are a few small code formatting issues. The Hadoop codebase wraps
lines at 80 characters, and I saw some lines longer than that. The standard
for indentation is 2 spaces. I think I saw tab characters, which we don't use,
so please convert those to 2 spaces.
# I think the description in hdfs-default.xml needs to be updated. In trunk,
the property is called {{net.topology.script.file.name}}. (Note the "net."
prefix.) I wasn't sure about the mention of
{{topology.dependency.script.file.name}}.
# Let's add a unit test that turns on the configuration property and validates
that an unresolved host can't register. You can look at the {{StaticMapping}}
class and its usage inside {{MiniDFSCluster}} for some inspiration on how to
write tests that cover DNS queries.
> Assigning DEFAULT_RACK in resolveNetworkLocation method can break data
> resiliency
> ---------------------------------------------------------------------------------
>
> Key: HDFS-5846
> URL: https://issues.apache.org/jira/browse/HDFS-5846
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Nikola Vujic
> Assignee: Nikola Vujic
> Attachments: hdfs-5846.patch
>
>
> Medhod CachedDNSToSwitchMapping::resolve() can return NULL which requires
> careful handling. Null can be returned in two cases:
> • An error occurred with topology script execution (script crashes).
> • Script returns wrong number of values (other than expected)
> Critical handling is in the DN registration code. DN registration code is
> responsible for assigning proper topology paths to all registered datanodes.
> Existing code handles this NULL pointer on the following way
> ({{resolveNetworkLocation}} method):
> {code}
> / /resolve its network location
> List<String> rName = dnsToSwitchMapping.resolve(names);
> String networkLocation;
> if (rName == null) {
> LOG.error("The resolve call returned null! Using " +
> NetworkTopology.DEFAULT_RACK + " for host " + names);
> networkLocation = NetworkTopology.DEFAULT_RACK;
> } else {
> networkLocation = rName.get(0);
> }
> return networkLocation;
> {code}
> The line of code that is assigning default rack:
> {code} networkLocation = NetworkTopology.DEFAULT_RACK; {code}
> can cause a serious problem. This means if somehow we got NULL, then the
> default rack will be assigned as a DN's network location and DN's
> registration will finish successfully. Under this circumstances, we will be
> able to load data into cluster which is working with a wrong topology. Wrong
> topology means that fault domains are not honored.
> For the end user, it means that two data replicas can end up in the same
> fault domain and a single failure can cause loss of two, or more, replicas.
> Cluster would be in the inconsistent state but it would not be aware of that
> and the whole thing would work as if everything was fine. We can notice that
> something wrong happened almost only by looking in the log for the error:
> {code}
> LOG.error("The resolve call returned null! Using " +
> NetworkTopology.DEFAULT_RACK + " for host " + names);
> {code}
>
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