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Date: Wed, 27 Jun 2012 21:32:44 +0000 (UTC)
From: "Todd Lipcon (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading
 and writing edit logs
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    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13402570#comment-13402570 ] 

Todd Lipcon commented on HDFS-3077:
-----------------------------------

bq. This suffers from a TOCTTOU (time of check to time of use) race, since another client can come along and delete the file in between the file.exists() and new FileReader(). So since you'll need to handle the FileNotFoundException, I think you can then drop the file.exists() check.

The problem is that Java throws FileNotFoundException for permissions errors or IO errors, too. And we don't want to return the default value in those cases. So I'd rather have the TOCTOU race, given that the assumption here is that the only process changing these files is the one calling this code. BTW, that code is from HDFS-3190 - probably best to comment on the JIRA associated with each piece of code.
                
> Quorum-based protocol for reading and writing edit logs
> -------------------------------------------------------
>
>                 Key: HDFS-3077
>                 URL: https://issues.apache.org/jira/browse/HDFS-3077
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: ha, name-node
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hdfs-3077-partial.txt, hdfs-3077.txt, qjournal-design.pdf, qjournal-design.pdf
>
>
> Currently, one of the weak points of the HA design is that it relies on shared storage such as an NFS filer for the shared edit log. One alternative that has been proposed is to depend on BookKeeper, a ZooKeeper subproject which provides a highly available replicated edit log on commodity hardware. This JIRA is to implement another alternative, based on a quorum commit protocol, integrated more tightly in HDFS and with the requirements driven only by HDFS's needs rather than more generic use cases. More details to follow.

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