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<p>
Base API applications must implement a communication
infrastructure. The communication infrastructure consists of
three parts: a way to map environment IDs to particular sites,
the functions to send and receive messages, and the
application architecture that supports the particular
communication infrastructure used (for example, individual
threads per communicating site, a shared message handler for
all sites, a hybrid solution). The communication
infrastructure for ex_rep_base is implemented in the file
<code class="filename">ex_rep/base/rep_net.c</code>, and each part
of that infrastructure is described as follows.
</p>
<p>
Ex_rep_base maintains a table of environment ID to TCP/IP
port mappings. A pointer to this table is stored in a
structure pointed to by the app_private field of the <a href="../api_reference/C/env.html" class="olink">DB_ENV</a>
object so it can be accessed by any function that has the
database environment handle. The table is represented by a
machtab_t structure which contains a reference to a linked
list of member_t's, both of which are defined in
<code class="filename">ex_rep/base/rep_net.c</code>. Each member_t
contains the host and port identification, the environment ID,
and a file descriptor.
</p>
<p>
This design is particular to this application and
communication infrastructure, but provides an indication of
the sort of functionality that is needed to maintain the
application-specific state for a TCP/IP-based infrastructure.
The goal of the table and its interfaces is threefold: First,
it must guarantee that given an environment ID, the send
function can send a message to the appropriate place. Second,
when given the special environment ID <a href="../api_reference/C/reptransport.html#transport_DB_EID_BROADCAST" class="olink">DB_EID_BROADCAST</a>, the
send function can send messages to all the machines in the
group. Third, upon receipt of an incoming message, the receive
function can correctly identify the sender and pass the
appropriate environment ID to the <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a> method.
</p>
<p>
Mapping a particular environment ID to a specific port is
accomplished by looping through the linked list until the
desired environment ID is found. Broadcast communication is
implemented by looping through the linked list and sending to
each member found. Since each port communicates with only a
single other environment, receipt of a message on a particular
port precisely identifies the sender.
</p>
<p>
This is implemented in the quote_send, quote_send_broadcast
and quote_send_one functions, which can be found in
<code class="filename">ex_rep/base/rep_net.c</code>.
</p>
<p>
The example provided is merely one way to satisfy these
requirements, and there are alternative implementations as
well. For instance, instead of associating separate socket
connections with each remote environment, an application might
instead label each message with a sender identifier; instead
of looping through a table and sending a copy of a message to
each member of the replication group, the application could
send a single message using a broadcast protocol.
</p>
<p>
The quote_send function is passed as the callback to
<a href="../api_reference/C/reptransport.html" class="olink">DB_ENV->rep_set_transport()</a>; Berkeley DB automatically sends messages as
needed for replication. The receive function is a mirror to
the quote_send_one function. It is not a callback function
(the application is responsible for collecting messages and
calling <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a> on them as is convenient). In the sample
application, all messages transmitted are Berkeley DB messages
that get handled by <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a>, however, this is not always
going to be the case. The application may want to pass its own
messages across the same channels, distinguish between its own
messages and those of Berkeley DB, and then pass only the
Berkeley DB ones to <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a>.
</p>
<p>
The final component of the communication infrastructure is
the process model used to communicate with all the sites in
the replication group. Each site creates a thread of control
that listens on its designated socket (as specified by the
<span class="bold"><strong>-l</strong></span> command line argument)
and then creates a new channel for each site that contacts it.
In addition, each site explicitly connects to the sites
specified in the <span class="bold"><strong>-r</strong></span> and
<span class="bold"><strong>-R</strong></span> command line
arguments. This is a fairly standard TCP/IP process
architecture and is implemented by the connect_thread,
connect_all and connect_site functions in
<code class="filename">ex_rep/base/rep_msg.c</code> and supporting
functions in
<code class="filename">ex_rep/base/rep_net.c</code>.
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