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<title>Berkeley DB Reference Guide: Ex_rep: a replication example</title>
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<td><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Replication</dl></td>
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Ex_rep: a replication example
Ex_rep, found in the examples_c/ex_rep subdirectory
of the Berkeley DB distribution, is a simple but complete demonstration of a
replicated application. The application is a mock stock ticker. The
master accepts a stock symbol and an numerical value as input, and
stores this information into a replicated database; either master or
clients can display the contents of the database, given an empty input
line.
There are two versions of the application: ex_rep_mgr uses Replication
Manager, while ex_rep_base uses the Base replication API. This is
intended to demonstrate that, while the basic function of the
application is the same in either case, the replication support
infrastructure differs markedly.
The communication infrastructure demonstrated with ex_rep_base has the
same dependencies on system networking and threading support as does
the Replication Manager (see the <a href="../../ref/rep/intro.html">Introduction</a>). The Makefile created by the standard UNIX
configuration will build the ex_rep examples on most platforms. Enter
"make ex_rep_mgr" and/or "make ex_rep_base" to attempt to build
them.
The synopsis for both programs is as follows:
<pre>ex_rep_xxx [-MC] [-h home] [-m host:port] [-o host:port] [-n sites] [-p priority]</pre>
where "ex_rep_xxx" is either "ex_rep_mgr" or "ex_rep_base".
The options, to either version of the program, are as follows:
 
-M<ul compact><li>Configure this process as a master.</ul>
-C<ul compact><li>Configure this process as a client.</ul>
-h<ul compact><li>Specify a home directory for the database environment; by
default, the current working directory is used.</ul>
-m<ul compact><li>Listen on port "port" of host "host" for incoming connections.</ul>
-o<ul compact><li>Attempt to connect to another member of the replication group which is
listening on host "host" at port "port". Members of a replication group
should be able to find all other members of a replication group so long
as they are in contact with at least one other member of the replication
group.</ul>
-n<ul compact><li>Specify the total number of sites in the replication group.</ul>
-p<ul compact><li>Set the election priority. See <a href="../../api_c/rep_elect.html">DB_ENV-&gt;rep_elect</a> for more
information.</ul>
 
A typical ex_rep session begins with a command such as the
following, to start a master:
<blockquote><pre>ex_rep_mgr -M -p 100 -n 4 -h DIR1 -m localhost:6000</pre></blockquote>
and several clients:
<blockquote><pre>ex_rep_mgr -C -p 50 -n 4 -h DIR2 -m localhost:6001 -o localhost:6000
ex_rep_mgr -C -p 10 -n 4 -h DIR3 -m localhost:6002 -o localhost:6000
ex_rep_mgr -C -p 0 -n 4 -h DIR4 -m localhost:6003 -o localhost:6000</pre></blockquote>
In this example, the client with home directory DIR4 can never become
a master (its priority is 0). Both of the other clients can become
masters, but the one with home directory DIR2 is preferred. Priorities
are assigned by the application and should reflect the desirability of
having particular clients take over as master in the case that the
master fails.
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