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* replication.html
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*
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* Authors: Chad Berkley
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* Copyright: 2000 Regents of the University of California and the
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* National Center for Ecological Analysis and Synthesis
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* For Details: http://www.nceas.ucsb.edu/
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* Created: 2001 January 23
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* Version:
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* File Info: '$ '
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*
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*
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-->
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<HTML>
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<HEAD>
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<TITLE>Metacat</TITLE>
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<link rel="stylesheet" type="text/css" href="./default.css">
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</HEAD>
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<BODY>
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<table width="100%">
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<tr>
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<td class="tablehead" colspan="2"><p class="label">Replication</p></td>
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<td class="tablehead" colspan="2" align="right">
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<a href="./packages.html">Back</a> | <a href="./metacattour.html">Home</a> |
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<a href="./datafiles.html">Next</a>
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</td>
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</tr>
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</table>
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<p>Metacat has built in replication to allow different Metacat servers to
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share data between themselves. In this release, Metacat not only replicate
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XML documents but also data files. </p>
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<p> A new hub feature was added to Metacat too. Previous version of Metacat
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only replicate XML documents whose home server is itself. But hub Metacat can
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replicate both its documents and other's documents which replicated from
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other server.</p>
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<p>The replication scheme that Metacat uses is both push and pull. There are
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several triggers that can start a replication mechanism. </p>
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<ul>
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<li>Delta-T monitoring. At a set time interval a server checks each of the
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other servers in its list for updated documents</li>
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<li>INSERT trigger. Whenever a document is inserted, the server notifies
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the remote hosts in its list that it has a new file available.</li>
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<li>UPDATE trigger. Whenever a document is updated, the server notifies
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each server in its list of the update.</li>
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<li>File locking. When a local user tries to alter a document on a local
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server that belongs to a remote server, the local server must first
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obtain a lock on that file. Once the lock is obtained, the file can
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be updated, then it is force replicated out to each server in the list.
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The lock ensures that the remote copy is up to date and that an older
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file does not overwrite a newer one. Only a documents home server
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can give a lock for that file to be altered.</li>
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</ul>
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<p>Each server contains a list of servers to which it can replicate. One-way
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replication is enabled by the 'replicate' and 'datareplicate' flags in the
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list. The server list may look like the following.</p>
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<table border="1">
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<tr>
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<td><b>serverid</b></td>
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<td><b>server</b></td>
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<td><b>last_checked</b></td>
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<td><b>replicate</b></td>
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<td><b>datareplicate</b></td>
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<td><b>hub</b></td>
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</tr>
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<tr>
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<td>1</td>
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<td>localhost</td>
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<td>null</td>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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</tr>
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<tr>
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<td>2</td>
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<td>alpha.nceas.ucsb.edu:8080/berkley/servlet/replication</td>
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<td>2001-01-22 14:52:12.1</td>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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</tr>
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<tr>
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<td>3</td>
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<td>dev.nceas.ucsb.edu/Metacat/servlet/replication</td>
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<td>2001-01-23 9:10:02.5</td>
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<td>1</td>
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<td>1</td>
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<td>0</td>
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</tr>
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</table>
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<p>The server list is kept in a table in the database called xml_replication.
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Localhost must always be the first entry in the table and have a serverid of 1.
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The server field must always point to the other server's replication servlet,
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hence the servlet/replication on the end of both of the sample servers. Note
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that any port numbers (if your servlet engine is not running on port 80) must
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also be included. The replicate flag is set to 1 if you want this server to
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copy XML documents TO the remote host. If replicate flag is set to 1 and
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datareplicate is set to 1, this server can copy data file TO the remote host
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too. If this server is a hub to the remote host, the hub flag should be set to
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1. (Note that both servers (the local host and the remote host) must have each
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other in their respective tables or replication will not take place.)</p>
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<b>Example:</b>
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<table border="1">
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<tr>
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<td>host</td>
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<td>replication table</td>
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</tr>
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<tr>
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<td>snoopy.nceas.ucsb.edu</td>
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<td>
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<table border="2">
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<tr>
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<td><b>server</b></td>
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<td><b>last_checked</b></td>
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<td><b>replicate</b></td>
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<td><b>datareplicate</b></td>
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<td><b>hub</b></td>
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</tr>
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<tr>
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<td>localhost</td>
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<td>null</td>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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</tr>
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<tr>
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<td>alpha.nceas.ucsb.edu:8080/berkley/servlet/replication </td>
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<td>2001-01-22 14:52:12.1</td>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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</tr>
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<tr>
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<td>dev.nceas.ucsb.edu/Metacat/servlet/replication</td>
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<td>2001-01-23 9:10:02.5</td>
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<td>1</td>
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<td>1</td>
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<td>0</td>
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</tr>
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</table>
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</td>
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</tr>
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<tr>
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<td>alpha.nceas.ucsb.edu</td>
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<td>
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<table border="2">
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<tr>
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<td><b>server</b></td>
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<td><b>last_checked</b></td>
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<td><b>replicate</b></td>
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<td><b>datareplicate</b></td>
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<td><b>hub</b></td>
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</tr>
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<tr>
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<td>localhost</td>
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<td>null</td>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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</tr>
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<tr>
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<td>snoopy.nceas.ucsb.edu:8080/berkley/servlet/replication</td>
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<td>2001-01-21 11:33:12.7</td>
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<td>0</td>
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<td>1</td>
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<td>0</td>
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</tr>
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<tr>
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<td>dev.nceas.ucsb.edu/Metacat/servlet/replication</td>
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<td>2001-01-23 10:22:02.5</td>
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<td>1</td>
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<td>0</td>
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<td>0</td>
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</tr>
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</table>
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</td>
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</tr>
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<tr>
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<td>dev.nceas.ucsb.edu</td>
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<td>
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<table border="2">
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<tr>
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<td><b>server</b></td>
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<td><b>last_checked</b></td>
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<td><b>replicate</b></td>
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<td><b>datareplicate</b></td>
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<td><b>hub</b></td>
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</tr>
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<tr>
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<td>localhost</td>
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<td>null</td>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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</tr>
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<tr>
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<td>snoopy.nceas.ucsb.edu:8080/berkley/servlet/replication</td>
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<td>2001-01-21 11:33:12.7</td>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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</tr>
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<tr>
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<td>alpha.nceas.ucsb.edu:8080/Metacat/servlet/replication</td>
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<td>2001-01-22 12:15:32.5</td>
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<td>1</td>
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<td>1</td>
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<td>1</td>
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</tr>
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</table>
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</td>
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</tr>
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</table>
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<p>Our three servers, snoopy, alpha and dev are all set up to replicate
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between themselves. Snoopy is a one way replicator. Meaning that it only
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pushes XML documents and data file to dev but does not pull back from it. This
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is achieved by dev and alpha setting snoopy's 'replicate' value to 0 indicating
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that they do not want to send their files to snoopy(Even in in Alpha,
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'datareplicate' is set to 1 for snoopy but nothing will be sent to Snoopy from alpha).
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Alpha and dev have a two-way replication agreement since each of them have a 1
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in their 'replicate' value for the other.</p>
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<p>Snoopy will replicate both XML documents and data file to dev because it
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setting dev's 'replicata' and 'datareplicate' is 1. Alpha only replicate
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XML documents to dev and this is caused by it setting dev's 'datareplicate' 0.
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</p>
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<p>Dev is a hub of alpha because it setting alpha's 'hub' value to be 1. Moreover,
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dev set alpha's 'replicate' and 'datareplicate' value 1. So dev will replicate
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XML documents and data file whose home server is dev or snoopy(replicated
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from snoopy) to alpha</p>
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<P>Note: if 'replicate' value is 0, the value for 'datareplicate' and 'hub'
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has no sense.</P>
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<p>There is an html control panel for controling replication. After
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<a href="./Metacatinstall.html">installing</a> Metacat, you can access
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it by going through the Metacat servlet context you have setup and calling up
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replControl.html. For instance, if you setup a Metacat servlet instance
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called 'Metacat' you would probably type
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http://server.domain.com:8080/Metacat/replControl.html. The control panel
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is an easy interface for adding/removing/altering servers and starting the
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delta-T handler. It will also allow you to 'force replicate' your server list.
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This is useful if you want to initialize the state of one Metacat server
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from an existing state of another (i.e. copy all of the data from an existing
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server).</p>
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<br>
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<a href="./packages.html">Back</a> | <a href="./metacattour.html">Home</a> |
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<a href="./datafiles.html">Next</a>
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</BODY>
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</HTML>
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