Handling exceptions in Threads

Ruby
1

Hello guys,

I have made an multithreaded algorithm that triggers around a hundred
concurrent threads. I have in fact, two versions of the same algorithm,
one with threads and another with recursive calls.

Each thread execute a query on an Oracle database and according to the
data, it either calls a new thread and ends, or it stores the result in
a
global array. The recursive version follow the same process, but instead
or running in parallel, it calls the method over and over sequentially.

The recursive version runs perfectly and it ends without problem. The
multithreaded version fails with an exception that I am not able to
identify.

If I leave the Thread.abort_on_exception = false, the algorithm ends
after
triggering about 10 threads. If I set it to Thread.abort_on_exception =
true, the algorithm end abnormally, but without printing any error
message.

I have added the following code at the end of the method called by the
thread:

rescue Exception => error
puts “#{error.class}: #{error.message}”

But I cannot get any error message in the console. I am sure that the
problem is something related with the database, but I do not know how to
solve the problem if I do not see the error message.

Any ideas?

Guillermo A.
Senior Engineer, Koiaka GmbH

Koiaka GmbH
Riesserkopfstr. 17
82467 Garmisch-Partenkirchen
Tel: +49 (0)8821 9679555
Fax: +49 (0)8821 730 9185
Mailto:[email protected]
http://www.koiaka.com

Amtsgericht München: HR B 161 041
Geschäftsführer: Guillermo A.
Sitz: Garmisch-Partenkirchen

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2

[email protected] wrote:

But I cannot get any error message in the console. I am sure that the
problem is something related with the database, but I do not know how to
solve the problem if I do not see the error message.

Why are you sure it is the database? If your other version (the
recursive one) works, then that suggests the db calls are ok, doesn’t
it?

3

On Jul 15, 2008, at 6:35 AM, [email protected] wrote:

rescue Exception => error
puts “#{error.class}: #{error.message}”

But I cannot get any error message in the console. I am sure that the
problem is something related with the database, but I do not know
how to
solve the problem if I do not see the error message.

Any ideas?

  1. STDERR.puts message

stdout bufferes and can get lost

  1. it’s highly unlikely the db handle is thread safe and, if it is,
    it’ll be native thread safe not green thread safe. aka - you will not
    get anything from using MT to execute your queries.

so, just use producer/consumer to both simplify and speed up your
application. for example

q = Queue.new

queries = list_of_queries

done = Object.new.freeze

producer = Thread.new do
Thread.current.abort_on_exception

queries.each do |query|
q.push db.execute(query)
end

q.push done
end

while( ( result = q.pop) != done )
handle_result result
end

you could handle the results in threads too using the same pattern.
the key is here is simply to eliminate the MT aspect of the db handle
usage - for sanity and correctness.

cheers.

a @ http://codeforpeople.com/

4

Hello,

I have solved the problem. It was something like this:

def mymethod
row = myselect.execute(select)
if row.next
Thread.new {mymethod(row.getcursorvalue)}
end
Rescue
puts error
end

As you can see the the variable “row” is local to the method, but when I
was including it inside of the thread block, it becomes local of the
thread. The database was answering with an error like this “No cursor
opened”, since the cursor is not valid in the thread context. The Rescue
clause was not responding, since the error was introduced before the
method was actually called.

you could handle the results in threads too using the same pattern.
the key is here is simply to eliminate the MT aspect of the db handle
usage - for sanity and correctness.

I am using JRuby 1.1.2 with the Oracle JDBC. The engine is an Oracle 11i
running on a separated server, but sometimes I use it in my own MacBook
PRO on a linux vmware virtual machine. Do you think it is not thread
safe?
also, why do you think that it is not good to run the queries in
parallel?

Cheers,

Guillermo A.
Senior Engineer, Koiaka GmbH

Koiaka GmbH
Riesserkopfstr. 17
82467 Garmisch-Partenkirchen
Tel: +49 (0)8821 9679555
Fax: +49 (0)8821 730 9185
Mailto:[email protected]
http://www.koiaka.com

Amtsgericht München: HR B 161 041
Geschäftsführer: Guillermo A.
Sitz: Garmisch-Partenkirchen

Diese Email kann vertrauliche und/oder rechtlich geschützte
Informationen
enthalten. Wenn Sie nicht der richtige Adressat sind oder diese Email
irrtümlich erhalten haben, dürfen Sie diese weder benutzen, kopieren,
weiterleiten oder irgend eine Maßnahme einleiten, die im Zusammenhang
mit
dem Inhalt dieser Email steht. Informieren Sie bitte sofort den Absender
und vernichten Sie die irrtümlich erhaltene Email vollständig.
Vielen Dank!

This e-mail message may contain confidential and/or privileged
information. If you are not an addressee or otherwise authorized to
receive this message, you should not use, copy, disclose or take any
action based on this e-mail or any information contained in the message.
If you have received this material in error, please advise the sender
immediately by reply e-mail and delete this message completely.
Thank you!

From:
“ara.t.howard” [email protected]
To:
[email protected] (ruby-talk ML)
Date:
15.07.2008 18:24
Subject:
Re: Handling exceptions in Threads

On Jul 15, 2008, at 6:35 AM, [email protected] wrote:

rescue Exception => error
puts “#{error.class}: #{error.message}”

But I cannot get any error message in the console. I am sure that the
problem is something related with the database, but I do not know
how to
solve the problem if I do not see the error message.

Any ideas?

  1. STDERR.puts message

stdout bufferes and can get lost

  1. it’s highly unlikely the db handle is thread safe and, if it is,
    it’ll be native thread safe not green thread safe. aka - you will not
    get anything from using MT to execute your queries.

so, just use producer/consumer to both simplify and speed up your
application. for example

q = Queue.new

queries = list_of_queries

done = Object.new.freeze

producer = Thread.new do
Thread.current.abort_on_exception

queries.each do |query|
q.push db.execute(query)
end

q.push done
end

while( ( result = q.pop) != done )
handle_result result
end

you could handle the results in threads too using the same pattern.
the key is here is simply to eliminate the MT aspect of the db handle
usage - for sanity and correctness.

cheers.

a @ http://codeforpeople.com/