HBASE-11961 Document region state transitions (Jimmy Xiang and Misty Stanley-Jones)
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Jimmy Xiang
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@ -3042,6 +3042,205 @@ ctime = Sat Jun 23 11:13:40 PDT 2012
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</para>
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</section>
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<section xml:id="regions.arch.states">
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<title>Region State Transition</title>
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<para> HBase maintains a state for each region and persists the state in META. The state
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of the META region itself is persisted in ZooKeeper. You can see the states of regions
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in transition in the Master web UI. Following is the list of possible region
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states.</para>
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<itemizedlist>
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<title>Possible Region States</title>
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<listitem>
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<para>OFFLINE: the region is offline and not opening</para>
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</listitem>
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<listitem>
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<para>OPENING: the region is in the process of being opened</para>
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</listitem>
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<listitem>
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<para>OPEN: the region is open and the region server has notified the master</para>
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</listitem>
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<listitem>
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<para>FAILED_OPEN: the region server failed to open the region</para>
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</listitem>
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<listitem>
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<para>CLOSING: the region is in the process of being closed</para>
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</listitem>
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<listitem>
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<para>CLOSED: the region server has closed the region and notified the master</para>
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</listitem>
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<listitem>
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<para>FAILED_CLOSE: the region server failed to close the region</para>
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</listitem>
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<listitem>
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<para>SPLITTING: the region server notified the master that the region is
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splitting</para>
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</listitem>
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<listitem>
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<para>SPLIT: the region server notified the master that the region has finished
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splitting</para>
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</listitem>
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<listitem>
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<para>SPLITTING_NEW: this region is being created by a split which is in
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progress</para>
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</listitem>
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<listitem>
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<para>MERGING: the region server notified the master that this region is being merged
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with another region</para>
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</listitem>
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<listitem>
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<para>MERGED: the region server notified the master that this region has been
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merged</para>
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</listitem>
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<listitem>
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<para>MERGING_NEW: this region is being created by a merge of two regions</para>
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</listitem>
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</itemizedlist>
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<figure>
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<title>Region State Transitions</title>
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<mediaobject>
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<imageobject>
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<imagedata align="center" valign="middle" fileref="region_states.png"/>
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</imageobject>
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<caption>
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<para>This graph shows all allowed transitions a region can undergo. In the graph,
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each node is a state. A node has a color based on the state type, for readability.
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A directed line in the graph is a possible state transition.</para>
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</caption>
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</mediaobject>
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</figure>
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<itemizedlist>
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<title>Graph Legend</title>
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<listitem>
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<para>Brown: Offline state, a special state that can be transient (after closed before
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opening), terminal (regions of disabled tables), or initial (regions of newly
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created tables)</para></listitem>
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<listitem>
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<para>Palegreen: Online state that regions can serve requests</para></listitem>
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<listitem>
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<para>Lightblue: Transient states</para></listitem>
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<listitem>
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<para>Red: Failure states that need OPS attention</para></listitem>
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<listitem>
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<para>Gold: Terminal states of regions split/merged</para></listitem>
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<listitem>
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<para>Grey: Initial states of regions created through split/merge</para></listitem>
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</itemizedlist>
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<orderedlist>
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<title>Region State Transitions Explained</title>
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<listitem>
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<para>The master moves a region from <literal>OFFLINE</literal> to
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<literal>OPENING</literal> state and tries to assign the region to a region
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server. The region server may or may not have received the open region request. The
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master retries sending the open region request to the region server until the RPC
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goes through or the master runs out of retries. After the region server receives the
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open region request, the region server begins opening the region.</para>
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</listitem>
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<listitem>
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<para>If the master is running out of retries, the master prevents the region server
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from opening the region by moving the region to <literal>CLOSING</literal> state and
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trying to close it, even if the region server is starting to open the region.</para>
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</listitem>
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<listitem>
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<para>After the region server opens the region, it continues to try to notify the
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master until the master moves the region to <literal>OPEN</literal> state and
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notifies the region server. The region is now open.</para>
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</listitem>
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<listitem>
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<para>If the region server cannot open the region, it notifies the master. The master
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moves the region to <literal>CLOSED</literal> state and tries to open the region on
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a different region server.</para>
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</listitem>
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<listitem>
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<para>If the master cannot open the region on any of a certain number of regions, it
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moves the region to <literal>FAILED_OPEN</literal> state, and takes no further
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action until an operator intervenes from the HBase shell, or the server is
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dead.</para>
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</listitem>
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<listitem>
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<para>The master moves a region from <literal>OPEN</literal> to
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<literal>CLOSING</literal> state. The region server holding the region may or may
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not have received the close region request. The master retries sending the close
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request to the server until the RPC goes through or the master runs out of
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retries.</para>
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</listitem>
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<listitem>
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<para>If the region server is not online, or throws
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<code>NotServingRegionException</code>, the master moves the region to
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<literal>OFFLINE</literal> state and re-assigns it to a different region
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server.</para>
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</listitem>
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<listitem>
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<para>If the region server is online, but not reachable after the master runs out of
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retries, the master moves the region to <literal>FAILED_CLOSE</literal> state and
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takes no further action until an operator intervenes from the HBase shell, or the
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server is dead.</para>
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</listitem>
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<listitem>
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<para>If the region server gets the close region request, it closes the region and
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notifies the master. The master moves the region to <literal>CLOSED</literal> state
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and re-assigns it to a different region server.</para>
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</listitem>
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<listitem>
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<para>Before assigning a region, the master moves the region to
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<literal>OFFLINE</literal> state automatically if it is in
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<literal>CLOSED</literal> state.</para>
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</listitem>
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<listitem>
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<para>When a region server is about to split a region, it notifies the master. The
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master moves the region to be split from <literal>OPEN</literal> to
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<literal>SPLITTING</literal> state and add the two new regions to be created to
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the region server. These two regions are in <literal>SPLITING_NEW</literal> state
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initially.</para>
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</listitem>
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<listitem>
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<para>After notifying the master, the region server starts to split the region. Once
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past the point of no return, the region server notifies the master again so the
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master can update the META. However, the master does not update the region states
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until it is notified by the server that the split is done. If the split is
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successful, the splitting region is moved from <literal>SPLITTING</literal> to
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<literal>SPLIT</literal> state and the two new regions are moved from
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<literal>SPLITTING_NEW</literal> to <literal>OPEN</literal> state.</para>
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</listitem>
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<listitem>
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<para>If the split fails, the splitting region is moved from
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<literal>SPLITTING</literal> back to <literal>OPEN</literal> state, and the two
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new regions which were created are moved from <literal>SPLITTING_NEW</literal> to
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<literal>OFFLINE</literal> state.</para>
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</listitem>
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<listitem>
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<para>When a region server is about to merge two regions, it notifies the master
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first. The master moves the two regions to be merged from <literal>OPEN</literal> to
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<literal>MERGING</literal>state, and adds the new region which will hold the
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contents of the merged regions region to the region server. The new region is in
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<literal>MERGING_NEW</literal> state initially.</para>
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</listitem>
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<listitem>
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<para>After notifying the master, the region server starts to merge the two regions.
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Once past the point of no return, the region server notifies the master again so the
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master can update the META. However, the master does not update the region states
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until it is notified by the region server that the merge has completed. If the merge
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is successful, the two merging regions are moved from <literal>MERGING</literal> to
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<literal>MERGED</literal> state and the new region is moved from
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<literal>MERGING_NEW</literal> to <literal>OPEN</literal> state.</para>
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</listitem>
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<listitem>
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<para>If the merge fails, the two merging regions are moved from
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<literal>MERGING</literal> back to <literal>OPEN</literal> state, and the new
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region which was created to hold the contents of the merged regions is moved from
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<literal>MERGING_NEW</literal> to <literal>OFFLINE</literal> state.</para>
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</listitem>
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<listitem>
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<para>For regions in <literal>FAILED_OPEN</literal> or <literal>FAILED_CLOSE</literal>
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states , the master tries to close them again when they are reassigned by an
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operator via HBase Shell. </para>
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</listitem>
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</orderedlist>
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</section>
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</section> <!-- assignment -->
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<section xml:id="regions.arch.locality">
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