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From Amandeep Khurana <ama...@gmail.com>
Subject Re: HDFS architecture based on GFS?
Date Mon, 16 Feb 2009 04:43:54 GMT
I dont know much about Hadoop streaming and have a quick question here.

The snippets of code/programs that you attach into the map reduce job might
want to access outside resources (like you mentioned). Now these might not
need to go to the namenode right? For example a python script. How would it
access the data? Would it ask the parent java process (in the tasktracker)
to get the data or would it go and do stuff on its own?


Amandeep Khurana
Computer Science Graduate Student
University of California, Santa Cruz


On Sun, Feb 15, 2009 at 8:23 PM, Matei Zaharia <matei@cloudera.com> wrote:

> Nope, typically the JobTracker just starts the process, and the tasktracker
> talks directly to the namenode to get a pointer to the datanode, and then
> directly to the datanode.
>
> On Sun, Feb 15, 2009 at 8:07 PM, Amandeep Khurana <amansk@gmail.com>
> wrote:
>
> > Alright.. Got it.
> >
> > Now, do the task trackers talk to the namenode and the data node directly
> > or
> > do they go through the job tracker for it? So, if my code is such that I
> > need to access more files from the hdfs, would the job tracker get
> involved
> > or not?
> >
> >
> >
> >
> > Amandeep Khurana
> > Computer Science Graduate Student
> > University of California, Santa Cruz
> >
> >
> > On Sun, Feb 15, 2009 at 7:20 PM, Matei Zaharia <matei@cloudera.com>
> wrote:
> >
> > > Normally, HDFS files are accessed through the namenode. If there was a
> > > malicious process though, then I imagine it could talk to a datanode
> > > directly and request a specific block.
> > >
> > > On Sun, Feb 15, 2009 at 7:15 PM, Amandeep Khurana <amansk@gmail.com>
> > > wrote:
> > >
> > > > Ok. Got it.
> > > >
> > > > Now, when my job needs to access another file, does it go to the
> > Namenode
> > > > to
> > > > get the block ids? How does the java process know where the files are
> > and
> > > > how to access them?
> > > >
> > > >
> > > > Amandeep Khurana
> > > > Computer Science Graduate Student
> > > > University of California, Santa Cruz
> > > >
> > > >
> > > > On Sun, Feb 15, 2009 at 7:05 PM, Matei Zaharia <matei@cloudera.com>
> > > wrote:
> > > >
> > > > > I mentioned this case because even jobs written in Java can use the
> > > HDFS
> > > > > API
> > > > > to talk to the NameNode and access the filesystem. People often do
> > this
> > > > > because their job needs to read a config file, some small data
> table,
> > > etc
> > > > > and use this information in its map or reduce functions. In this
> > case,
> > > > you
> > > > > open the second file separately in your mapper's init function and
> > read
> > > > > whatever you need from it. In general I wanted to point out that
> you
> > > > can't
> > > > > know which files a job will access unless you look at its source
> code
> > > or
> > > > > monitor the calls it makes; the input file(s) you provide in the
> job
> > > > > description are a hint to the MapReduce framework to place your job
> > on
> > > > > certain nodes, but it's reasonable for the job to access other
> files
> > as
> > > > > well.
> > > > >
> > > > > On Sun, Feb 15, 2009 at 6:14 PM, Amandeep Khurana <
> amansk@gmail.com>
> > > > > wrote:
> > > > >
> > > > > > Another question that I have here - When the jobs run arbitrary
> > code
> > > > and
> > > > > > access data from the HDFS, do they go to the namenode to get
the
> > > block
> > > > > > information?
> > > > > >
> > > > > >
> > > > > > Amandeep Khurana
> > > > > > Computer Science Graduate Student
> > > > > > University of California, Santa Cruz
> > > > > >
> > > > > >
> > > > > > On Sun, Feb 15, 2009 at 6:00 PM, Amandeep Khurana <
> > amansk@gmail.com>
> > > > > > wrote:
> > > > > >
> > > > > > > Assuming that the job is purely in Java and not involving
> > streaming
> > > > or
> > > > > > > pipes, wouldnt the resources (files) required by the job
as
> > inputs
> > > be
> > > > > > known
> > > > > > > beforehand? So, if the map task is accessing a second file,
how
> > > does
> > > > it
> > > > > > make
> > > > > > > it different except that there are multiple files. The
> JobTracker
> > > > would
> > > > > > know
> > > > > > > beforehand that multiple files would be accessed. Right?
> > > > > > >
> > > > > > > I am slightly confused why you have mentioned this case
> > > separately...
> > > > > Can
> > > > > > > you elaborate on it a little bit?
> > > > > > >
> > > > > > > Amandeep
> > > > > > >
> > > > > > >
> > > > > > > Amandeep Khurana
> > > > > > > Computer Science Graduate Student
> > > > > > > University of California, Santa Cruz
> > > > > > >
> > > > > > >
> > > > > > > On Sun, Feb 15, 2009 at 4:47 PM, Matei Zaharia <
> > matei@cloudera.com
> > > >
> > > > > > wrote:
> > > > > > >
> > > > > > >> Typically the data flow is like this:1) Client submits
a job
> > > > > description
> > > > > > >> to
> > > > > > >> the JobTracker.
> > > > > > >> 2) JobTracker figures out block locations for the input
> file(s)
> > by
> > > > > > talking
> > > > > > >> to HDFS NameNode.
> > > > > > >> 3) JobTracker creates a job description file in HDFS
which
> will
> > be
> > > > > read
> > > > > > by
> > > > > > >> the nodes to copy over the job's code etc.
> > > > > > >> 4) JobTracker starts map tasks on the slaves (TaskTrackers)
> with
> > > the
> > > > > > >> appropriate data blocks.
> > > > > > >> 5) After running, maps create intermediate output files
on
> those
> > > > > slaves.
> > > > > > >> These are not in HDFS, they're in some temporary storage
used
> by
> > > > > > >> MapReduce.
> > > > > > >> 6) JobTracker starts reduces on a series of slaves,
which copy
> > > over
> > > > > the
> > > > > > >> appropriate map outputs, apply the reduce function,
and write
> > the
> > > > > > outputs
> > > > > > >> to
> > > > > > >> HDFS (one output file per reducer).
> > > > > > >> 7) Some logs for the job may also be put into HDFS
by the
> > > > JobTracker.
> > > > > > >>
> > > > > > >> However, there is a big caveat, which is that the map
and
> reduce
> > > > tasks
> > > > > > run
> > > > > > >> arbitrary code. It is not unusual to have a map that
opens a
> > > second
> > > > > HDFS
> > > > > > >> file to read some information (e.g. for doing a join
of a
> small
> > > > table
> > > > > > >> against a big file). If you use Hadoop Streaming or
Pipes to
> > write
> > > a
> > > > > job
> > > > > > >> in
> > > > > > >> Python, Ruby, C, etc, then you are launching arbitrary
> processes
> > > > which
> > > > > > may
> > > > > > >> also access external resources in this manner. Some
people
> also
> > > > > > read/write
> > > > > > >> to DBs (e.g. MySQL) from their tasks. A comprehensive
security
> > > > > solution
> > > > > > >> would ideally deal with these cases too.
> > > > > > >>
> > > > > > >> On Sun, Feb 15, 2009 at 3:22 PM, Amandeep Khurana <
> > > amansk@gmail.com
> > > > >
> > > > > > >> wrote:
> > > > > > >>
> > > > > > >> > A quick question here. How does a typical hadoop
job work at
> > the
> > > > > > system
> > > > > > >> > level? What are the various interactions and how
does the
> data
> > > > flow?
> > > > > > >> >
> > > > > > >> > Amandeep
> > > > > > >> >
> > > > > > >> >
> > > > > > >> > Amandeep Khurana
> > > > > > >> > Computer Science Graduate Student
> > > > > > >> > University of California, Santa Cruz
> > > > > > >> >
> > > > > > >> >
> > > > > > >> > On Sun, Feb 15, 2009 at 3:20 PM, Amandeep Khurana
<
> > > > amansk@gmail.com
> > > > > >
> > > > > > >> > wrote:
> > > > > > >> >
> > > > > > >> > > Thanks Matei. If the basic architecture is
similar to the
> > > Google
> > > > > > >> stuff, I
> > > > > > >> > > can safely just work on the project using
the information
> > from
> > > > the
> > > > > > >> > papers.
> > > > > > >> > >
> > > > > > >> > > I am aware of the 4487 jira and the current
status of the
> > > > > > permissions
> > > > > > >> > > mechanism. I had a look at them earlier.
> > > > > > >> > >
> > > > > > >> > > Cheers
> > > > > > >> > > Amandeep
> > > > > > >> > >
> > > > > > >> > >
> > > > > > >> > > Amandeep Khurana
> > > > > > >> > > Computer Science Graduate Student
> > > > > > >> > > University of California, Santa Cruz
> > > > > > >> > >
> > > > > > >> > >
> > > > > > >> > > On Sun, Feb 15, 2009 at 2:40 PM, Matei Zaharia
<
> > > > > matei@cloudera.com>
> > > > > > >> > wrote:
> > > > > > >> > >
> > > > > > >> > >> Forgot to add, this JIRA details the
latest security
> > features
> > > > > that
> > > > > > >> are
> > > > > > >> > >> being
> > > > > > >> > >> worked on in Hadoop trunk:
> > > > > > >> > >> https://issues.apache.org/jira/browse/HADOOP-4487.
> > > > > > >> > >> This document describes the current status
and
> limitations
> > of
> > > > the
> > > > > > >> > >> permissions mechanism:
> > > > > > >> > >>
> > > > > > >>
> > > > >
> > http://hadoop.apache.org/core/docs/current/hdfs_permissions_guide.html
> > > .
> > > > > > >> > >>
> > > > > > >> > >> On Sun, Feb 15, 2009 at 2:35 PM, Matei
Zaharia <
> > > > > matei@cloudera.com
> > > > > > >
> > > > > > >> > >> wrote:
> > > > > > >> > >>
> > > > > > >> > >> > I think it's safe to assume that
Hadoop works like
> > > > > MapReduce/GFS
> > > > > > at
> > > > > > >> > the
> > > > > > >> > >> > level described in those papers.
In particular, in
> HDFS,
> > > > there
> > > > > is
> > > > > > a
> > > > > > >> > >> master
> > > > > > >> > >> > node containing metadata and a number
of slave nodes
> > > > > (datanodes)
> > > > > > >> > >> containing
> > > > > > >> > >> > blocks, as in GFS. Clients start
by talking to the
> master
> > > to
> > > > > list
> > > > > > >> > >> > directories, etc. When they want
to read a region of
> some
> > > > file,
> > > > > > >> they
> > > > > > >> > >> tell
> > > > > > >> > >> > the master the filename and offset,
and they receive a
> > list
> > > > of
> > > > > > >> block
> > > > > > >> > >> > locations (datanodes). They then
contact the individual
> > > > > datanodes
> > > > > > >> to
> > > > > > >> > >> read
> > > > > > >> > >> > the blocks. When clients write a
file, they first
> obtain
> > a
> > > > new
> > > > > > >> block
> > > > > > >> > ID
> > > > > > >> > >> and
> > > > > > >> > >> > list of nodes to write it to from
the master, then
> > contact
> > > > the
> > > > > > >> > datanodes
> > > > > > >> > >> to
> > > > > > >> > >> > write it (actually, the datanodes
pipeline the write as
> > in
> > > > GFS)
> > > > > > and
> > > > > > >> > >> report
> > > > > > >> > >> > when the write is complete. HDFS
actually has some
> > security
> > > > > > >> mechanisms
> > > > > > >> > >> built
> > > > > > >> > >> > in, authenticating users based on
their Unix ID and
> > > providing
> > > > > > >> > Unix-like
> > > > > > >> > >> file
> > > > > > >> > >> > permissions. I don't know much about
how these are
> > > > implemented,
> > > > > > but
> > > > > > >> > they
> > > > > > >> > >> > would be a good place to start looking.
> > > > > > >> > >> >
> > > > > > >> > >> > On Sun, Feb 15, 2009 at 1:36 PM,
Amandeep Khurana <
> > > > > > >> amansk@gmail.com
> > > > > > >> > >> >wrote:
> > > > > > >> > >> >
> > > > > > >> > >> >> Thanks Matie
> > > > > > >> > >> >>
> > > > > > >> > >> >> I had gone through the architecture
document online. I
> > am
> > > > > > >> currently
> > > > > > >> > >> >> working
> > > > > > >> > >> >> on a project towards Security
in Hadoop. I do know how
> > the
> > > > > data
> > > > > > >> moves
> > > > > > >> > >> >> around
> > > > > > >> > >> >> in the GFS but wasnt sure how
much of that does HDFS
> > > follow
> > > > > and
> > > > > > >> how
> > > > > > >> > >> >> different it is from GFS. Can
you throw some light on
> > > that?
> > > > > > >> > >> >>
> > > > > > >> > >> >> Security would also involve
the Map Reduce jobs
> > following
> > > > the
> > > > > > same
> > > > > > >> > >> >> protocols. Thats why the question
about how does the
> > > Hadoop
> > > > > > >> framework
> > > > > > >> > >> >> integrate with the HDFS, and
how different is it from
> > Map
> > > > > Reduce
> > > > > > >> and
> > > > > > >> > >> GFS.
> > > > > > >> > >> >> The GFS and Map Reduce papers
give a good information
> on
> > > how
> > > > > > those
> > > > > > >> > >> systems
> > > > > > >> > >> >> are designed but there is nothing
that concrete for
> > Hadoop
> > > > > that
> > > > > > I
> > > > > > >> > have
> > > > > > >> > >> >> been
> > > > > > >> > >> >> able to find.
> > > > > > >> > >> >>
> > > > > > >> > >> >> Amandeep
> > > > > > >> > >> >>
> > > > > > >> > >> >>
> > > > > > >> > >> >> Amandeep Khurana
> > > > > > >> > >> >> Computer Science Graduate Student
> > > > > > >> > >> >> University of California, Santa
Cruz
> > > > > > >> > >> >>
> > > > > > >> > >> >>
> > > > > > >> > >> >> On Sun, Feb 15, 2009 at 12:07
PM, Matei Zaharia <
> > > > > > >> matei@cloudera.com>
> > > > > > >> > >> >> wrote:
> > > > > > >> > >> >>
> > > > > > >> > >> >> > Hi Amandeep,
> > > > > > >> > >> >> > Hadoop is definitely inspired
by MapReduce/GFS and
> > aims
> > > to
> > > > > > >> provide
> > > > > > >> > >> those
> > > > > > >> > >> >> > capabilities as an open-source
project. HDFS is
> > similar
> > > to
> > > > > GFS
> > > > > > >> > (large
> > > > > > >> > >> >> > blocks, replication, etc);
some notable things
> missing
> > > are
> > > > > > >> > read-write
> > > > > > >> > >> >> > support in the middle of
a file (unlikely to be
> > provided
> > > > > > because
> > > > > > >> > few
> > > > > > >> > >> >> Hadoop
> > > > > > >> > >> >> > applications require it)
and multiple appenders (the
> > > > record
> > > > > > >> append
> > > > > > >> > >> >> > operation). You can read
about HDFS architecture at
> > > > > > >> > >> >> >
> > > > http://hadoop.apache.org/core/docs/current/hdfs_design.html
> > > > > .
> > > > > > >> The
> > > > > > >> > >> >> MapReduce
> > > > > > >> > >> >> > part of Hadoop interacts
with HDFS in the same way
> > that
> > > > > > Google's
> > > > > > >> > >> >> MapReduce
> > > > > > >> > >> >> > interacts with GFS (shipping
computation to the
> data),
> > > > > > although
> > > > > > >> > >> Hadoop
> > > > > > >> > >> >> > MapReduce also supports
running over other
> distributed
> > > > > > >> filesystems.
> > > > > > >> > >> >> >
> > > > > > >> > >> >> > Matei
> > > > > > >> > >> >> >
> > > > > > >> > >> >> > On Sun, Feb 15, 2009 at
11:57 AM, Amandeep Khurana <
> > > > > > >> > amansk@gmail.com
> > > > > > >> > >> >
> > > > > > >> > >> >> > wrote:
> > > > > > >> > >> >> >
> > > > > > >> > >> >> > > Hi
> > > > > > >> > >> >> > >
> > > > > > >> > >> >> > > Is the HDFS architecture
completely based on the
> > > Google
> > > > > > >> > Filesystem?
> > > > > > >> > >> If
> > > > > > >> > >> >> it
> > > > > > >> > >> >> > > isnt, what are the
differences between the two?
> > > > > > >> > >> >> > >
> > > > > > >> > >> >> > > Secondly, is the coupling
between Hadoop and HDFS
> > same
> > > > as
> > > > > > how
> > > > > > >> it
> > > > > > >> > is
> > > > > > >> > >> >> > between
> > > > > > >> > >> >> > > the Google's version
of Map Reduce and GFS?
> > > > > > >> > >> >> > >
> > > > > > >> > >> >> > > Amandeep
> > > > > > >> > >> >> > >
> > > > > > >> > >> >> > >
> > > > > > >> > >> >> > > Amandeep Khurana
> > > > > > >> > >> >> > > Computer Science Graduate
Student
> > > > > > >> > >> >> > > University of California,
Santa Cruz
> > > > > > >> > >> >> > >
> > > > > > >> > >> >> >
> > > > > > >> > >> >>
> > > > > > >> > >> >
> > > > > > >> > >> >
> > > > > > >> > >>
> > > > > > >> > >
> > > > > > >> > >
> > > > > > >> >
> > > > > > >>
> > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
>

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