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From "Devaraj Das (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1985) Abstract node to switch mapping into a topology service class used by namenode and jobtracker
Date Tue, 30 Oct 2007 10:00:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12538717

Devaraj Das commented on HADOOP-1985:

Some thoughts - 
1) Make the DNS->Switch mapping an interface class. 
    1.1) interface DNStoSwitchMap {
               public String resolve(String dnsname);
    1.2) The switch string format is the same as it exists today (documented in https://issues.apache.org/jira/secure/attachment/12345251/Rack_aware_HDFS_proposal.pdf).
That will make things work in the non-typical setup with 3+ levels of nodes.
    1.3) The default implementation of the interface, packaged with hadoop, could simply look
up a table of dns->switch mapping created statically. 

2) The DataNode, today, takes the location as an argument. This is not needed anymore, and
hence the associated code would go away.
3) The DataNode sends the location information as part of the registration. The NetworkTopology
is derived at the NameNode. Using the interface mentioned in (1), the NameNode can create
the topology all by itself.

4) The JobTracker creates the NetworkTopology for the TaskTrackers exactly how the NameNode
does it.
5) The JobTracker assigns tasks first on node-locality basis, then on rack-locality basis.

In our environment,  "distance-basis" (o.a.h.n.NetworkTopology.getDistance), isn't that much
relevant. But we might make the framework ready for it as well (assuming it is not too much

Does the above make sense?

> Abstract node to switch mapping into a topology service class used by namenode and jobtracker
> ---------------------------------------------------------------------------------------------
>                 Key: HADOOP-1985
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1985
>             Project: Hadoop
>          Issue Type: New Feature
>            Reporter: eric baldeschwieler
>            Assignee: Devaraj Das
> In order to implement switch locality in MapReduce, we need to have switch location in
both the namenode and job tracker.  Currently the namenode asks the data nodes for this info
and they run a local script to answer this question.  In our environment and others that I
know of there is no reason to push this to each node.  It is easier to maintain a centralized
script that maps node DNS names to switch strings.
> I propose that we build a new class that caches known DNS name to switch mappings and
invokes a loadable class or a configurable system call to resolve unknown DNS to switch mappings.
 We can then add this to the namenode to support the current block to switch mapping needs
and simplify the data nodes.  We can also add this same callout to the job tracker and then
implement rack locality logic there without needing to chane the filesystem API or the split
planning API.
> Not only is this the least intrusive path to building racklocal MR I can ID, it is also
future compatible to future infrastructures that may derive topology on the fly, etc, etc...

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