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https://issues.apache.org/jira/browse/HADOOP-7777?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13138645#comment-13138645
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Steve Loughran commented on HADOOP-7777:
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This adds a new base class {{AbstractDNSToSwitchMapping}} to offer a common
implementation class for the bundled DNS mappings, and a new predicate
{{isSingleSwitch()}} which should be true if
a class believes that its topology is only on a single switch.
I initially tried to have the base class extend {{Configurable}} but that
triggered calls to things like {{ScriptBasedMapping.setConf()}} from
Configurable's constructor, which had lots of consequences as the subclasses
weren't ready for that yet. I added the workarounds -checks for other fields
being initialised- but this left the code brittle and made it harder to make
any custom mappers subclasses of this. I reverted those changes and now the
{{AbstractDNSToSwitchMapping}} class just implements {{Configurable}}.
This base class (and the sub classes) do what I wanted, namely indicate whether
or not they are single switch or not, with the presumption being "multi-switch
unless stated".
# {{ScriptBasedMapping}} is single rack if it has no script.
# {{CachedDNSToSwitchMapping}} relays its query to the raw mapping.
# {{StaticMapping}} says it is single rack if its map is empty (that is, all
resolved nodes
will be reported as being in the default rack).
There's tests for all this, and in {{TestSwitchMapping}} verification that
implementations of {{DNSToSwitchMapping}} are always considered multi-switch.
The tests also see what happens when you pass down null configurations (the
existing stuff mostly broke)
Once committed I can fix HDFS BlockManager to use the switch predicate to
decide whether to assume multi-rack placement or not.
> Implement a base class for DNSToSwitchMapping implementations that can offer
> extra topology information
> -------------------------------------------------------------------------------------------------------
>
> Key: HADOOP-7777
> URL: https://issues.apache.org/jira/browse/HADOOP-7777
> Project: Hadoop Common
> Issue Type: Improvement
> Components: util
> Affects Versions: 0.23.0
> Reporter: Steve Loughran
> Assignee: Steve Loughran
> Attachments: HADOOP-7777-switch.patch, HADOOP-7777-switch.patch
>
>
> HDFS-2492 has identified a need for DNSToSwitchMapping implementations to
> provide a bit more topology information (e.g. whether or not there are
> multiple switches). This could be done by writing an extended interface,
> querying its methods if present and coming up with a default action if there
> is no extended interface.
> Alternatively, we have a base class that all the standard mappings implement,
> with a boolean isMultiRack() method; all the standard subclasses would extend
> this, as could any third party topology provider. The advantage of this
> approach is that it is easier to add new operations without going into a
> multi-interface mess.
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