As long as your reduce task can kick off the MR job asynchronously then it shouldn't be too much of an issue but it could very quickly result in a deadlock otherwise. If you set this up as two stages 1) to kick off the recursive MR and 2) analyze the final result set then it should work but off the top of my head I'm not sure how to sync when the second stage job runs unless you can just watch for a set of files to appear.
Matt From: Yaron Gonen [mailto:yaron.go...@gmail.com] Sent: Tuesday, October 04, 2011 10:46 AM To: mapreduce-user@hadoop.apache.org Subject: Submitting a Hadoop task from withing a reducer Hi, Hadoop tasks are always stacked to form a linear user-managed workflow (a reduce step cannot start before all previous mappers have stopped etc). This may be problematic in recursive tasks: for example in a BFS we will not get any output until the longest branch has been reached. In order to solve than, an idea came up of submitting a whole Hadoop task from within a reducer. Have anyone tried it? Thanks. This e-mail message may contain privileged and/or confidential information, and is intended to be received only by persons entitled to receive such information. If you have received this e-mail in error, please notify the sender immediately. Please delete it and all attachments from any servers, hard drives or any other media. Other use of this e-mail by you is strictly prohibited. All e-mails and attachments sent and received are subject to monitoring, reading and archival by Monsanto, including its subsidiaries. The recipient of this e-mail is solely responsible for checking for the presence of "Viruses" or other "Malware". Monsanto, along with its subsidiaries, accepts no liability for any damage caused by any such code transmitted by or accompanying this e-mail or any attachment. The information contained in this email may be subject to the export control laws and regulations of the United States, potentially including but not limited to the Export Administration Regulations (EAR) and sanctions regulations issued by the U.S. Department of Treasury, Office of Foreign Asset Controls (OFAC). As a recipient of this information you are obligated to comply with all applicable U.S. export laws and regulations.
