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Date: Mon, 30 Apr 2012 18:51:49 +0000 (UTC)
From: "Tsz Wo (Nicholas), SZE (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (HADOOP-8325) Add a ShutdownHookManager to be
 used by different components instead of the JVM shutdownhook
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    [ https://issues.apache.org/jira/browse/HADOOP-8325?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13265107#comment-13265107 ] 

Tsz Wo (Nicholas), SZE commented on HADOOP-8325:
------------------------------------------------

Hi Alejandro,

hooks is a synchronized set but we still have to manually synchronize it when iterating it.  So, we need to synchronize the first line of getShutdownHooksInOrder() which copies the hooks to a new list.
{code}
+  List<Runnable> getShutdownHooksInOrder() {
+    List<HookEntry> list = new ArrayList<HookEntry>(MGR.hooks);
{code}

BTW, what should be expected for adding/removing hooks during shutdown?  I think we probably should throw an IllegalStateException.
                
> Add a ShutdownHookManager to be used by different components instead of the JVM shutdownhook
> --------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-8325
>                 URL: https://issues.apache.org/jira/browse/HADOOP-8325
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: fs
>    Affects Versions: 2.0.0
>            Reporter: Alejandro Abdelnur
>            Assignee: Alejandro Abdelnur
>            Priority: Critical
>             Fix For: 2.0.0
>
>         Attachments: HADOOP-8325.patch, HADOOP-8325.patch, HADOOP-8325.patch, HADOOP-8325.patch, HADOOP-8325.patch, HADOOP-8325.patch, HADOOP-8325.patch
>
>
> FileSystem adds a JVM shutdown hook when a filesystem instance is cached.
> MRAppMaster also uses a JVM shutdown hook, among other things, the MRAppMaster JVM shutdown hook is used to ensure state are written to HDFS.
> This creates a race condition because each JVM shutdown hook is a separate thread and if there are multiple JVM shutdown hooks there is not assurance of order of execution, they could even run in parallel.

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