[Hadoop Wiki] Trivial Update of "Hbase/MultipleMasters" by stack

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http://wiki.apache.org/hadoop/Hbase/MultipleMasters

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  == Single Master Setup ==
  
- The [http://hadoop.apache.org/hbase/docs/current/api/overview-summary.html#overview_description Getting Started] documentation gets you into that state. A failure of the Master server will not be damageable in the first few minutes but your regions will be unable to split. If it does happen, you can go on any other machine with the correct installation/configuration and do {{{$ ${HBASE_HOME}/bin/hbase-daemon.sh start master}}}. Currently the Hadoop Distributed Filesystem is '''not''' highly available so if the Namenode resides on the same machine is your Master, the cluster is still wedged and you will have to shut down HBase with a high probability of losing data.
+ The [http://hadoop.apache.org/hbase/docs/current/api/overview-summary.html#overview_description Getting Started] documentation describes set up with a single Master. A cluster can run without a Master for a number of few minutes but your regions will be unable to split. If it does happen, you can go to any other machine with the correct installation/configuration and do {{{$ ${HBASE_HOME}/bin/hbase-daemon.sh start master}}}.  This newly started Master will take over Master functions.
+ 
+ Currently the Hadoop Distributed Filesystem is '''not''' highly available so if the Namenode resides on the same machine is your Master, the cluster is still wedged and you will have to shut down HBase with a high probability of losing data.
  
  == Multiple Masters Setup ==
  
- Before setting up multiple Masters, you should already have built an HBase cluster with a single Master. If not, please refer to the Getting Started documentation.
+ Before setting up multiple Masters, you should already have built an HBase cluster with a single Master. If not, please refer to the [http://hadoop.apache.org/hbase/docs/current/api/overview-summary.html#overview_description Getting Started] documentation.
  
  === Basic knowledge ===
  
- The multi-master feature introduced in 0.20 does not add the cooperation of a score of Masters, there is still just one working Master while the other ''backups'' wait. For example, if you start 200 Masters only 1 will be active while the others wait for it to die. The switch usually takes zookeeper.session.timeout plus a couple of seconds to occur. See "How it works inside" for more information.
+ The multi-master feature introduced in 0.20 does not add cooperating Masters; there is still just one working Master while the other ''backups'' wait. For example, if you start 200 Masters only 1 will be active while the others wait for it to die. The switch usually takes zookeeper.session.timeout plus a couple of seconds to occur. See "How it works inside" below for more information.
  
  === Designing your highly available setup ===
  
- The rule of thumb here is to not put all your eggs in the same basket. You don't want a Namenode and a Master on the same machine because currently you can recover automatically from a Master failure but from a Namenode failure. Be sure that the Namenode has its own very reliable machine until Hadoop 0.21 comes in with ''Backup Namenodes''. Also you don't want to have a Region Server with a Master as that machine failure will imply first a Master failover and then the new Master will have to split the logs of the failed RS. 
+ The rule of thumb here is to not put all your eggs in the same basket. You don't want a Namenode and a Master on the same machine because currently you can recover automatically from a Master failure but not from a Namenode failure. Be sure that the Namenode has its own very reliable machine until Hadoop 0.21 comes in with ''Backup Namenodes''. Also you don't want to have a Region Server and a Master on the same node, as that machine failure will imply first a Master failover and then the new Master will have to split the logs of the failed RS. 
  
  Your ideal highly available cluster would have 5 or more dedicated Zookeeper servers, 2-3 dedicated Master servers (one per rack for example), 1 very reliable Namenode/Job Tracker server with redundant hardware and the rest is the usual Datanode/Task Tracker/Region Server stack. If you don't even have twice that amount of machines, you will have to evaluate some trade-offs. For example, you could try to keep a dedicated Master server and put the others along the Region Servers as the failure of a backup Master doesn't have any impact and you could do the same for the ZK servers.
  
@@ -26, +28 @@

  
  Currently handling the other Masters isn't really user friendly but it's getting worked on. When you start HBase, your first main Master will also be started. To start other Masters do {{{$ ${HBASE_HOME}/bin/hbase-daemon.sh start master}}} on all the nodes you want to, as long as the have the correct installation/configuration. You could also do {{{$ ${HBASE_HOME}/bin/hbase-daemons.sh start master}}} and that would start a Master on every machine listed in ­­{{{conf/regionserver}}}.
  
- To stop any Master without shutting down HBase, you currently have to {{{kill -9}}} it. If you kill the active Master, first make sure it's not splitting logs as you could lose data. To check that, tail the Master's log and watch for anything that says "Splitting logs # of #". 
+ To stop any Master '''without shutting down HBase''', you currently have to {{{kill -9}}} it (This is OK.  All state is maintained elsewhere off in ZooKeeper and out on RegionServers). If you kill the active Master, first make sure it's not splitting logs as you could lose data. To check that, tail the Master's log and watch for anything that says "Splitting logs # of #". 
  
  == How it works inside ==