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<title>Berkeley DB Reference Guide: Ex_rep_base: putting it all together</title>
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<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Replication</dl></b></td>
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<p align=center><b>Ex_rep_base: putting it all together</b></p>
<p>Beyond simply initializing a replicated environment, an application
that uses the Base replication API must set up its communication
infrastructure, and then make sure that incoming messages are received
and processed.</p>
<p>To initialize replication, ex_rep_base creates a Berkeley DB environment and
calls <a href="/api_c/rep_transport.html">DB_ENV-&gt;rep_set_transport</a> to establish a send function.  (See
the main function in <b>ex_rep/base/rep_base.c</b>, including its
calls to the create_env and env_init functions in
<b>ex_rep/common/rep_common.c</b>.)</p>
<p>ex_rep_base opens a listening socket for incoming connections and opens
an outgoing connection to every machine that it knows about (that is,
all the sites listed in the <b>-o</b> command line argument).
Applications can structure the details of this in different ways, but
ex_rep_base creates a user-level thread to listen on its socket, plus
a thread to loop and handle messages on each socket, in addition to the
threads needed to manage the user interface, update the database on the
master, and read from the database on the client (in other words, in
addition to the normal functionality of any database application).</p>
<p>Once the initial threads have all been started and the communications
infrastructure is initialized, the application signals that it is ready
for replication and joins a replication group by calling
<a href="/api_c/rep_start.html">DB_ENV-&gt;rep_start</a>.  (Again, see the main function in
<b>ex_rep/base/rep_base.c</b>.)</p>
<p>Note the use of the optional second argument to <a href="/api_c/rep_start.html">DB_ENV-&gt;rep_start</a> in
the client initialization code.  The argument "local" is a piece of
data, opaque to Berkeley DB, that will be broadcast to each member of a
replication group; it allows new clients to join a replication group,
without knowing the location of all its members;  the new client will
be contacted by the members it does not know about, who will receive
the new client's contact information that was specified in "myaddr."
See <a href="/ref/rep/newsite.html">Connecting to a new site</a> for more
information.</p>
<p>The final piece of a replicated application is the code that loops,
receives, and processes messages from a given remote environment.
ex_rep_base runs one of these loops in a parallel thread for each
socket connection (see the hm_loop function in
<b>ex_rep/base/rep_msg.c</b>).  Other applications may want to queue
messages somehow and process them asynchronously, or select() on a
number of sockets and either look up the correct environment ID for each
or encapsulate the ID in the communications protocol.</p>
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