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conf
ivy
src
README
build.xml
ivy.xml

README

# Copyright 2008 The Apache Software Foundation Licensed under the
# Apache License, Version 2.0 (the "License"); you may not use this
# file except in compliance with the License.  You may obtain a copy
# of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless
# required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.  See the License for the specific language governing
# permissions and limitations under the License.

/**
 * The AvatarNode is your answer for High Availability of the 
 * NameNode.  The AvatarNode has two avatars.. the Standby avatar 
 * and the Active avatar.
 * 
 * In the Standby avatar, the AvatarNode is consuming transaction logs
 * generated by the primary (via a transaction log stored in a shared device).
 * Typically, the primary Namenode is writing transactions to a NFS filesystem
 * and the Standby is reading the log from the same NFS filesystem. The 
 * Standby is also making periodic checkpoints to the primary namenode.
 * 
 * A manual command can switch the AvatarNode from the Standby avatar
 * to the Active avatar. In the Active avatar, the AvatarNode performs precisely
 * the same functionality as a real usual Namenode. The switching from 
 * Standby avatar to the Active avatar is fast and can typically occur 
 * within seconds.
 *
 * Typically, an adminstrator will run require two shared mount points for
 * transaction logs. It has to be set in fs.name.dir.shared0 and
 * fs.name.dir.shared1 (similarly for edits). Then the adminstrator starts
 * the AvatarNode on two different machines as follows:
 *
 * bin/hadoop org.apache.hadoop.hdfs.server.namenode.AvatarNode -zero -active
 * bin/hadoop org.apache.hadoop.hdfs.server.namenode.AvatarNode -one -standby
 * The first  AvatarNode uses  fs.name.dir.shared0 while the second
 * AvatarNode uses fs.name.dir.shared1 to write its transaction logs.
 * Also, at startup, the first instance is the primary Namenode and the
 * second instance is the Standby
 *
 * After a while, the adminstrator decides to change the avatar of the
 * second instance to Active. In this case, he/she has to first ensure that the
 * first instance is really really dead. This code does not handle the
 * split-brain scenario where there are two active namenodes in one cluster.
 *

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BUILDING:

In HADOOP_HOME, run ant package to build Hadoop and its contrib packages.

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INSTALLING and CONFIGURING:

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OPTIONAL CONFIGIURATION:

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ADMINISTRATION:

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IMPLEMENTATION: