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From "Francesco Salbaroli (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4586) Fault tolerant Hadoop Job Tracker
Date Fri, 09 Jan 2009 10:30:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12662309#action_12662309

Francesco Salbaroli commented on HADOOP-4586:

>Sorry for the late comments:
>For a master/slave HA solution, two main problems are:
>1. Mechanism that determines a master in a cluster during startup and failover.

The JGroups library (whose manual can be found here: [http://www.jgroups.org/javagroupsnew/docs/manual/pdf/manual.pdf]
) handles automatically the election of a group coordinator. The node elected group coordinator
is also the master of the cluster. In case of a failure a new group coordinator (and, consequentially,
a new cluster master) will be elected.

>Handling loss of quorum, 

The shared state resides entirely on HDFS (see issues HADOOP-1876 and HADOOP-3245) so, until
now, there is no shared soft-state between nodes. However the facilities for managing a shared
state are present and can be used in a future update.

>split-brain and fencing in case of split-brain.

The JGroups library tries to automatically handle network partitions and merging, but given

*There is no shared soft-state
*There is only one access point in the whole Hadoop cluster to the HDFS (the NameNode)

the network partition problem should not be an issue (only one partition at a time can access
the HDFS). In future versions a more elegant way of dealing with network partitions should
be added.

> It also >requires comprehensive management tools for configuring, managing and monitoring

I am now adding JMX support. After the initial testing phase I will post result and an updated

>2. Sharing state information between master and slave, so that a slave node can take over
as master.
>Currenly the proposed solution addresses mainly the second problem. I have not seen much
information on how the first problem is addressed. While the sharing >of information between
master and slave can be done in many ways, managing the master/slave cluster is a more complicated
problem. Could you please add >more information on how the design handles these issues
and some notes on how administrator uses this functionality to manage the cluster.

I hope I have given an answer to your question. If you need more, feel free to contact me.

>Also analysis of the impact of job tracker performance due to the introduction of this
feature needs to be done.

I am about to begin the testing phase, results will follow


> Fault tolerant Hadoop Job Tracker
> ---------------------------------
>                 Key: HADOOP-4586
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4586
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: mapred
>    Affects Versions: 0.18.0, 0.18.1, 0.18.2
>         Environment: High availability enterprise system
>            Reporter: Francesco Salbaroli
>            Assignee: Francesco Salbaroli
>             Fix For: 0.21.0
>         Attachments: Enhancing the Hadoop MapReduce framework by adding fault.ppt, FaultTolerantHadoop.pdf,
HADOOP-4586-0.1.patch, jgroups-all.jar
> The Hadoop framework has been designed, in an eort to enhance perfor-
> mances, with a single JobTracker (master node). It's responsibilities varies
> from managing job submission process, compute the input splits, schedule
> the tasks to the slave nodes (TaskTrackers) and monitor their health.
> In some environments, like the IBM and Google's Internet-scale com-
> puting initiative, there is the need for high-availability, and performances
> becomes a secondary issue. In this environments, having a system with
> a Single Point of Failure (such as Hadoop's single JobTracker) is a major
> concern.
> My proposal is to provide a redundant version of Hadoop by adding
> support for multiple replicated JobTrackers. This design can be approached
> in many dierent ways. 
> In the document at: http://sites.google.com/site/hadoopthesis/Home/FaultTolerantHadoop.pdf?attredirects=0
> I wrote an overview of the problem and some approaches to solve it.
> I post this to the community to gather feedback on the best way to proceed in my work.
> Thank you!

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