Recovering running/scheduled jobs after JobTracker failure
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Key: HADOOP-1121
URL: https://issues.apache.org/jira/browse/HADOOP-1121
Project: Hadoop
Issue Type: New Feature
Components: mapred
Environment: all
Reporter: Alejandro Abdelnur
Currently all running/scheduled jobs are kept in memory in the JobTracker. If
the JobTracker goes down all the running/scheduled jobs have to be resubmitted.
Proposal:
(1) On job submission the JobTracker would save the job configuration (job.xml)
in a jobs DFS directory using the jobID as name.
(2) On job completion (success, failure, klll) it would delete the job
configuration from the jobs DFS directory.
(3) On JobTracker failure the jobs DFS directory will have all
running/scheduled jobs at failure time.
(4) On startup the JobTracker would check the jobs DFS directory for job config
files. if there is none it means no failure happened on last stop, there is
nothing to be done. If there are job config files in the jobs DFS directory
continue with the following recovery steps.
(A) rename all job config files to $JOB_CONFIG_FILE.recover.
(B) for each $JOB_CONFIG_FILE.recover: delete the output directory if it
exists, schedule the job using the original job ID, delete the
$JOB_CONFIG_FILE.recover (as a new $JOB_CONFIG_FILE will be there per
scheduling (per step #1).
(C) when B is completed start accepting new job submissions.
Other details:
A configuration flag would enable/disable the above behavior, if switched off
(default behavior) nothing of the above happens.
A startup flag could switch off job recovery for systems with the recover set
to ON.
Changes to the job ID generation should be put in place to avoid Job ID
collision with jobs IDs from previous failed runs, for example appending a JT
startup timestamp to the job IDs would do.
Further improvements on top of this one:
This mechanism would allow having a JobTracker node in standby to be started in
case of main JobTracker failure. The standby JobTracker would be started on
main JobTracker failure. Making things a little more comprehensive they backup
JobTrackers could be running in warm mode and hearbeats and ping calls among
them would activate a warm stand by JobTracker as new main JobTracker. Together
with an enhancement in the JobClient (keeping a list of backup JobTracker URLs)
would enable client fallback to backup JobTrackers.
State about partially run jobs could be kept, tasks
completed/in-progress/pending. This would enable to recover jobs half way
instead restarting them.
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