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Re: Datanode registration, port number
Hi Harsh,

Thanks a lot for the response. As it turns out, I figured out the
registration mechanism this evening and how the sourceId is relayed to the

As for your question about the cache layer it is a similar basic concept as
the plan mentioned, but the technical details differ significantly. First
of all, instead of having the user tell the namenode to perform caching (as
it seems from the proposal on JIRA), there is a distributed caching
algorithm that decides what files will be cached. Secondly, I am
implementing a hook-in with the job scheduler that arranges jobs according
to what files are cached at a given point in time (and also allows files to
be cached based on what jobs are to be run).

Also, the cache layer does a bit of metadata caching; the numbers on it are
not all in, but thus far, some of the *metadata* caching surprisingly gives
a pretty nice reduction in response time.

Any thoughts on the cache layer would be greatly appreciated.


On Mon, Dec 23, 2013 at 11:46 PM, Harsh J <[EMAIL PROTECTED]> wrote:

> Hi,
> On Mon, Dec 23, 2013 at 9:41 AM, Dhaivat Pandya <[EMAIL PROTECTED]>
> wrote:
> > Hi,
> >
> > I'm currently trying to build a cache layer that should sit "on top" of
> the
> > datanode. Essentially, the namenode should know the port number of the
> > cache layer instead of that of the datanode (since the namenode then
> relays
> > this information to the default HDFS client). All of the communication
> > between the datanode and the namenode currently flows through my cache
> > layer (including heartbeats, etc.)
> Curious Q: What does your cache layer aim to do btw? If its a data
> cache, have you checked out the design being implemented currently by
> https://issues.apache.org/jira/browse/HDFS-4949?
> > *First question*: is there a way to tell the namenode where a datanode
> > should be? Any way to trick it into thinking that the datanode is on a
> port
> > number where it actually isn't? As far as I can tell, the port number is
> > obtained from the DatanodeId object; can this be set in the configuration
> > so that the port number derived is that of the cache layer?
> The NN receives a DN host and port from the DN directly. The DN sends
> it whatever its running on. See
> https://github.com/apache/hadoop-common/blob/release-2.2.0/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java#L690
> > I spent quite a bit of time on the above question and I could not find
> any
> > sort of configuration option that would let me do that. So, I delved into
> > the HDFS source code and tracked down the DatanodeRegistration class.
> > However, I can't seem to find out *how* the NameNode figures out the
> > Datanode's port number or if I could somehow change the packets to
> reflect
> > the port number of cache layer?
> See
> https://github.com/apache/hadoop-common/blob/release-2.2.0/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java#L690
> (as above) for how the DN emits it. And no, IMO, that ("packet
> changes") is not the right way to go about it if you're planning an
> overhaul. Its easier and more supportable to make proper code changes
> instead.
> > *Second question: *how does the namenode
> > figure out a newly-registered Datanode's port number?
> Same as before. Registration sends the service addresses (so NN may
> use them for sending to clients), beyond which the DN's heartbeats are
> mere client-like connections to the NN, carried out on regular
> ephemeral ports.
> --
> Harsh J