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From "Robert Nettleton" <rnettle...@hortonworks.com>
Subject Re: Review Request 37356: Fix synchronization on TopologyManager's outstanding requests
Date Wed, 12 Aug 2015 21:55:31 GMT


> On Aug. 11, 2015, 8:24 p.m., John Speidel wrote:
> > ambari-server/src/main/java/org/apache/ambari/server/topology/TopologyManager.java,
line 169
> > <https://reviews.apache.org/r/37356/diff/1/?file=1037718#file1037718line169>
> >
> >     Just got done discussing with Bob on a call.
> >     
> >     This synchronized block should be moved up to line 153 and end at the bottom
of the method.  The existing availbleHosts synchronization block on line 195 should be removed
due to the existence of the new availableHosts syncronization described above.  
> >     
> >     The important thing here is that we synchronize on a common mutual exclusion
monitor in both onHostRegistered and processRequest.  With this patch it will be the availableHosts
monitor.
> >     
> >     I agree with the other comments that this is dangerous any likely inefficient
and also agree that we should change as little as possible initially while still fixing the
issue.  If this patch causes a significant performance degradation then we can then look at
optimizing the locking in this class in another patch.

Version 2 of the patch addresses this issue.


- Robert


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https://reviews.apache.org/r/37356/#review94991
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On Aug. 12, 2015, 9:54 p.m., Robert Nettleton wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/37356/
> -----------------------------------------------------------
> 
> (Updated Aug. 12, 2015, 9:54 p.m.)
> 
> 
> Review request for Ambari, Jonathan Hurley, John Speidel, Mahadev Konar, Robert Levas,
and Sumit Mohanty.
> 
> 
> Bugs: AMBARI-12720
>     https://issues.apache.org/jira/browse/AMBARI-12720
> 
> 
> Repository: ambari
> 
> 
> Description
> -------
> 
> This patch provides a fix for AMBARI-12720. 
> 
> Blueprint Cluster deployments are failing under load, with clusters of 50+ nodes.  One
or more hosts in the cluster fail to be registered, even though the ambari-agent registration
occurs properly for these hosts.  This problem seems to occur when certain hosts are registered
at roughly the same time as the call to TopologyManager.processRequest().  
> 
> Upon further analysis, it appears that the TopologyManager.outstandingRequests object
is not fully sychronized.  
> 
> There are two paths involved in reading/writing to this data structure:
> 
> 1. TopologyManager.processRequest() (occurs on the main request thread handling the cluster
creation request).  When all known hosts have been processed, but some requests are pending,
the LogicalRequest instance is added to TopologyManager.outstandingRequests.  Note:  The call
to add this request is not synchronized on the outstandingRequests object, but is only synchronized
on TopologyManager.availableHosts. 
> 2. TopologyManager.onHostRegistered() (occurs on a separate thread that handles ambari-agent
registrations for new hosts joining the cluster).  This method does synchronize on TopologyManager.outstandingRequests,
and attempts to iterate over these requests to determine if the newly registered host can
satisfy the request.  
> 
> The incorrect synchronization on TopologyManager.outstandingRequests appears to be the
root of the intermittent cluster deployment problems described in AMBARI-12720.  Since this
datastructure is read/updated by both threads, and since this object is not protected by the
same locks, there appears to be a race condition in this code.  
> 
> To fix this problem, this patch implements the following:
> 
> 1. Updates the TopologyManager.onHostRegistered() method to lock on
>         --> availableHosts
>             --> outstandingRequests
>    prior to reading TopologyManager.outstandingRequests
>    
> 2. Updates the TopologyManager.processRequest() method to lock on
>         --> availableHosts
>             --> outstandingRequests
>    prior to adding a LogicalRequest instance to this collection
>    
> 
> The locking order (availableHosts, outstandingRequests) must be kept in place in order
to avoid any potential deadlocks from making a change like this.
> 
> 
> Diffs
> -----
> 
>   ambari-server/src/main/java/org/apache/ambari/server/topology/TopologyManager.java
4f51d47 
> 
> Diff: https://reviews.apache.org/r/37356/diff/
> 
> 
> Testing
> -------
> 
> 1. Ran the ambari-server unit test suite ("mvn clean test").  All tests are passing.
> 2. Deployed a 3-node HDFS HA cluster via a Blueprint, to make sure this change does not
break existing deployments.
> 3. Deployed a 45-node HDFS HA cluster via a Blueprint on the cloud, to make sure this
change does not break deployments of larger clusters.
> 
> 
> Thanks,
> 
> Robert Nettleton
> 
>


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