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Date: Tue, 29 Aug 2006 14:54:23 -0700 (PDT)
From: "Doug Cutting (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Commented: (HADOOP-489) Seperating user logs from system
 logs in map reduce
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    [ http://issues.apache.org/jira/browse/HADOOP-489?page=comments#action_12431386 ] 
            
Doug Cutting commented on HADOOP-489:
-------------------------------------

Couldn't last-N lines be implemented with 'exec tail'?  We could add a servlet for this, that has parameters like the user, jobid, etc.  User log files could be written into a directory hierarchy so that there's a file per task, directory per user, subdir per job.

> Seperating user logs from system logs in map reduce
> ---------------------------------------------------
>
>                 Key: HADOOP-489
>                 URL: http://issues.apache.org/jira/browse/HADOOP-489
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: mapred
>            Reporter: Mahadev konar
>         Assigned To: Mahadev konar
>            Priority: Minor
>
> Currently the user logs are a part of system logs in mapreduce. Anything logged by the user is logged into the tasktracker log files. This create two issues-
> 1) The system log files get cluttered with user output. If the user outputs a large amount of logs, the system logs need to be cleaned up pretty often.
> 2) For the user, it is difficult to get to each of the machines and look for the logs his/her job might have generated.
> I am proposing three solutions to the problem. All of them have issues with it -
> Solution 1.
> Output the user logs on the user screen as part of the job submission process. 
> Merits- 
> This will prevent users from printing large amount of logs and the user can get runtime feedback on what is wrong with his/her job.
> Issues - 
> This proposal will use the framework bandwidth while running jobs for the user. The user logs will need to pass from the tasks to the tasktrackers, from the tasktrackers to the jobtrackers and then from the jobtrackers to the jobclient using a lot of framework bandwidth if the user is printing out too much data.
> Solution 2.
> Output the user logs onto a dfs directory and then concatenate these files. Each task can create a file for the output in the log direcotyr for a given user and jobid.
> Issues -
> This will create a huge amount of small files in DFS which later can be concatenated into a single file. Also there is this issue that who would concatenate these files into a single file? This could be done by the framework (jobtracker) as part of the cleanup for the jobs - might stress the jobtracker.
>  
> Solution 3.
> Put the user logs into a seperate user log file in the log directory on each tasktrackers. We can provide some tools to query these local log files. We could have commands like for jobid j and for taskid t get me the user log output. These tools could run as a seperate map reduce program with each map grepping the user log files and a single recude aggregating these logs in to a single dfs file.
> Issues-
> This does sound like more work for the user. Also, the output might not be complete since a tasktracker might have went down after it ran the job. 
> Any thoughts?

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