Grokbase Groups Pig user March 2011
FAQ
Seems that the attachment got lost.
Anyway I tried something much simple and it seems that the problem lies in the registering of my jar file?!
I'm running

raw = load 'data/example.xml' using PigStorage();
dump raw

which succeeds, but when running

register ./laiw.jar;
raw = load 'data/example.xml' using PigStorage();
dump raw

it fails with

java.lang.ClassCastException: org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigOutputFormat cannot be cast to org.apache.hadoop.mapreduce.OutputFormat
at org.apache.hadoop.mapred.Task.initialize(Task.java:413)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:288)
at org.apache.hadoop.mapred.Child.main(Child.java:170)

in the jobtracker.

-----Original Message-----
From: Lai Will
Sent: Thursday, March 03, 2011 8:50 PM
To: user@pig.apache.org
Subject: RE: PigOutputFormat cannot be cast to OutputFormat

Please find it attached.

-----Original Message-----
From: Dmitriy Ryaboy
Sent: Thursday, March 03, 2011 7:58 PM
To: user@pig.apache.org
Subject: Re: PigOutputFormat cannot be cast to OutputFormat

Can you share your loader code?
On Thu, Mar 3, 2011 at 6:08 AM, Lai Will wrote:

Hello,

First of all thank you all, I really appreciate the help I get here :)
I've now written some UDFs and did some successful local runs.

When copying the date to HDFS and trying to run my pig file I get the
job fails.

Looking at the jobtracker I have the following error several times:

Job Setup: Failed<
http://shrek01.ethz.ch:50030/jobtasks.jsp?jobid=job_201103031425_0011&
type=setup&pagenum=1&state=killed
java.lang.ClassCastException:
org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigOutput
Format cannot be cast to org.apache.hadoop.mapreduce.OutputFormat
at org.apache.hadoop.mapred.Task.initialize(Task.java:413)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:288)
at org.apache.hadoop.mapred.Child.main(Child.java:170)

I tried to use DUMP instead of STORE (using PigStorage()) but the same
error remains.

Any idea what I did wrong?

Best,
Will


From: Dmitriy Ryaboy
Sent: Wednesday, March 02, 2011 6:30 PM
To: user@pig.apache.org
Cc: Lai Will
Subject: Re: Shared resources

In practice, I have never seen an EvalFunc get instantiated twice on
the same task, except when you are actually using it in two different
places (foreach x generate Foo(field1), Foo(field2)), or when things
like combiners are involved. In any case, number of instantiations is
number of invocations, to the degree that it might as well be one
even when it's not actually 1.

Put whatever initialization logic you need into the constructor. You
can pass (string) arguments to UDF constructors in Pig using the DEFINE keyword:

define myFoo com.mycompany.Foo('some', 'arguments');

foreach x generate myFoo(field1);

D

On Wed, Mar 2, 2011 at 9:11 AM, Lai Will <laiw@student.ethz.ch<mailto:
laiw@student.ethz.ch>> wrote:
I understand that the is not inter-task communication at all.
However my question arises within one task. The documentation says
that we should not make any assumptions on how may EvalFunc instances
(of the same
class) are instantiated.
Therefore I assume that within the same task, there might be several
instances of my EvalFunc and if every one of them is doing the parsing
of resource files into data structures a lot of memory and computing
power would be wasted.. so it's not about inter-task communication but
about inter-instance communication.

Thank you for your help.

Best,
Will

-----Original Message-----
From: Alan Gates
[mailto:gates@yahoo-inc.com Sent: Wednesday, March 02, 2011 5:17 PM
To: user@pig.apache.org Subject: Re: Shared resources

There is no shared inter-task processing in Hadoop. Each task runs in
a separate JVM and is locked off from all other tasks. This is partly
because you do not know a priori which tasks will run together in
which nodes, and partly for security. Data can be shared by all tasks
on a node via the distributed cache. If all your work could be done
once on the front end and then serialized to be later read by all
tasks you could use this mechanism to share it. With the code in
trunk UDFs can store data to the distributed cache, though this feature is not in a release yet.

Alan.
On Mar 2, 2011, at 7:54 AM, Lai Will wrote:

So I still get the redundant work whenever the same clusternode/vm
creates multiple instances of my EvalFunc?
And is it usual to have several instance of the EvalFunc on the same
clusternode/vm?

Will

-----Original Message-----
From: Alan Gates
[mailto:gates@yahoo-inc.com >]
Sent: Wednesday, March 02, 2011 4:49 PM
To: user@pig.apache.org > Subject: Re: Shared resources

There is no method in the eval func that gets called on the backend
before any exec calls. You can keep a flag that tracks whether you
have done the initialization so that you only do it the first time.

Alan.
On Mar 2, 2011, at 5:29 AM, Lai Will wrote:

Hello,

I wrote a EvalFunc implementation that


1) Parses a SQL Query

2) Scans a folder for resource files and creates an index on
these files

3) According to certain properties of the SQL Query accesses
the corresponding file and creates a Java objects holding relevant
the information of the file (for reuse).

4) Does some computation with the SQL Query and the information
found in the file

5) Outputs a transformed SQL Query

Currently I'm doing local tests without Hadoop and the code works
fine.

The problem I see, is that right now I initialize my parser in the
EvalFunc, so that every time It gets instantiated a new instance of
the parser is generated. Ideally only on instance per machine would
be created.
Even worse right now I create the index and parse the corresponding
resource file once per call exec in EvalFunc and therefore do a
lot of redundant computation.

Just because I don't know where and how to put this shared
computation.
Does anybody have a solution on that?

Best,
Will

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