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<title>Berkeley DB Reference Guide: Ex_repquote: a TCP/IP based communication infrastructure</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Replication</dl></h3></td>
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<p>
<h3 align=center>Ex_repquote: a TCP/IP based communication infrastructure</h3>
<p>All Berkeley DB replication 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 get 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 is
implemented in the file <b>ex_repquote/ex_rq_net.c</b>, and each part
of that infrastructure is described as follows.</p>
<p>Ex_repquote maintains a table of environment ID to TCP/IP port mappings.
This table is stored in the app_private field of the <a href="../../api_c/env_class.html">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 <b>ex_repquote/ex_rq_net.c</b>. Each member_t contains
the host and port identification, the environment ID, and a file
descriptor. The table is maintained by the following interfaces:</p>
<blockquote><pre>int machtab_add(machtab_t *machtab, int fd, u_int32_t hostaddr, int port, int *eidp);
int machtab_init(machtab_t **machtabp, int priority, int nsites);
int machtab_getinfo(machtab_t *machtab, int eid, u_int32_t *hostp, int *portp);
void machtab_parm(machtab_t *machtab, int *nump, int *priorityp, u_int32_t *timeoutp);
int machtab_rem(machtab_t *machtab, int eid, int lock);
</pre></blockquote>
<p>These interfaces are particular to this application and communication
infrastructure, but provide 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_c/rep_transport.html#DB_EID_BROADCAST">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_c/rep_message.html">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>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>In ex_repquote's case, the send function (slightly simplified) is as
follows:</p>
<pre><blockquote>int
quote_send(dbenv, control, rec, lsn, eid, flags)
DB_ENV *dbenv;
const DBT *control, *rec;
const DB_LSN *lsn;
int eid;
u_int32_t flags;
{
int fd, n, ret;
machtab_t *machtab;
member_t *m;
<p>
machtab = (machtab_t *)dbenv->app_private;
<p>
/*
* If this is a broadcast, call a separate function to
* iterate through the table of environment (a/k/a
* machine) IDs and call quote_send_one on each.
* (This function is not reproduced here, but can be
* seen in ex_rq_net.c.)
*/
if (eid == DB_EID_BROADCAST) {
n = quote_send_broadcast(machtab, rec, control, flags);
if (n < 0)
return (DB_REP_UNAVAIL);
return (0);
}
<p>
/* Find the fild descriptor, fd, associated with this EID. */
fd = 0;
if ((ret = pthread_mutex_lock(&machtab->mtmutex)) != 0)
return (0);
for (m = LIST_FIRST(&machtab->machlist); m != NULL;
m = LIST_NEXT(m, links)) {
if (m->eid == eid) {
fd = m->fd;
break;
}
}
if (pthread_mutex_unlock(&machtab->mtmutex) != 0)
return (-1);
<p>
if (fd == 0)
return (DB_REP_UNAVAIL);
<p>
/* We have a file descriptor; write the data over it. */
ret = quote_send_one(rec, control, fd, flags);
<p>
return (ret);
}
<p>
int
quote_send_broadcast(machtab, rec, control, flags)
machtab_t *machtab;
const DBT *rec, *control;
u_int32_t flags;
{
int ret, sent;
member_t *m, *next;
if ((ret = pthread_mutex_lock(&machtab->mtmutex)) != 0)
return (0);
sent = 0;
for (m = LIST_FIRST(&machtab->machlist); m != NULL; m = next) {
next = LIST_NEXT(m, links);
if ((ret = quote_send_one(rec, control, m->fd, flags)) != 0) {
(void)machtab_rem(machtab, m->eid, 0);
} else
sent++;
}
if (pthread_mutex_unlock(&machtab->mtmutex) != 0)
return (-1);
return (sent);
}</blockquote></pre>
<p>The quote_send_one function has been omitted as it simply writes the
data requested over the file descriptor that it is passed. It contains
nothing specific to Berkeley DB or this communication infrastructure. The
complete code can be found in <b>ex_repquote/ex_rq_net.c</b>.</p>
<p>The quote_send function is passed as the callback to <a href="../../api_c/rep_transport.html">DB_ENV->set_rep_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_c/rep_message.html">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_c/rep_message.html">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_c/rep_message.html">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 <b>-m</b> 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
<b>-o</b> command line argument. This is a fairly standard TCP/IP
process architecture and is implemented by the following functions (all
in <b>ex_repquote/ex_rq_net.c</b>).</p>
<blockquote><pre>int get_connected_socket(machtab_t *machtab, char *progname, char *remotehost,
int port, int *is_open, int *eidp): Connect to the specified host/port, add the
site to the machtab, and return a file descriptor for communication with this
site.
<p>
int listen_socket_init(char *progname, int port): Initialize a socket for
listening on a particular part.
<p>
int listen_socket_accept(machtab_t *machtab, char *progname, int socket,
int *eidp): Accept a connection on a socket and add it to the machtab.
int listen_socket_connect</pre></blockquote>
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