Berkeley DB Reference Guide: Berkeley DB Replication

Ex_repquote: putting it all together

A replicated application must initialize a replicated environment, set up its communication infrastructure, and then make sure that incoming messages are received and processed.

To initialize replication, ex_repquote creates a Berkeley DB environment and calls DB_ENV->set_rep_transport to establish a send function. The following code fragment (from the env_init function, found in ex_repquote/ex_rq_main.c) demonstrates this. Prior to calling this function, the application has called machtab_init to initialize its environment ID to port mapping structure and passed this structure into env_init.

if ((ret = db_env_create(&dbenv, 0)) != 0) {
	fprintf(stderr, "%s: env create failed: %s\n",
	    progname, db_strerror(ret));
	return (ret);
}
dbenv->set_errfile(dbenv, stderr);
dbenv->set_errpfx(dbenv, prefix);
(void)dbenv->set_cachesize(dbenv, 0, CACHESIZE, 0);

dbenv->app_private = machtab;
(void)dbenv->set_rep_transport(dbenv, SELF_EID, quote_send);

flags = DB_CREATE | DB_THREAD |
    DB_INIT_LOCK | DB_INIT_LOG | DB_INIT_MPOOL | DB_INIT_TXN;

ret = dbenv->open(dbenv, home, flags, 0);

ex_repquote 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 -o command line argument). Applications can structure the details of this in different ways, but ex_repquote 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).

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 DB_ENV->rep_start:

if (whoami == MASTER) {
	if ((ret = dbenv->rep_start(dbenv, NULL, DB_REP_MASTER)) != 0) {
		/* Complain and exit on error. */
	}
	/* Go run the master application code. */
} else {
	memset(&local, 0, sizeof(local));
	local.data = myaddr;
	local.size = strlen(myaddr) + 1;
	if ((ret =
	    dbenv->rep_start(dbenv, &local, DB_REP_CLIENT)) != 0) {
		/* Complain and exit on error. */
	}
	/* Sleep to give ourselves a minute to find a master. */
	sleep(5);
	/* Go run the client application code. */
}

Note the use of the optional second argument to DB_ENV->rep_start in the client initialization code. The argument "myaddr" 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 Connecting to a new site for more information.

The final piece of a replicated application is the code that loops, receives, and processes messages from a given remote environment. ex_repquote runs one of these loops in a parallel thread for each socket connection; 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. The details may thus vary from application to application, but in ex_repquote the message-handling loop is as follows (code fragment from the hm_loop function, found in ex_repquote/ex_rq_util.c):

DB_ENV *dbenv;
DBT rec, control;	/* Structures encapsulating a received message. */
elect_args *ea;		/* Parameters to the elect thread. */
machtab_t *tab;		/* The environment ID to fd mapping table. */
pthread_t elect_thr;	/* Election thread spawned. */
repsite_t self;		/* My host and port identification. */
int eid;		/* Environment from whom I am receiving messages. */
int fd;			/* FD on which I am receiving messages. */
int master_eid;		/* Global indicating the current master eid. */
int n;			/* Number of sites; obtained from machtab_parm. */
int newm;		/* New master EID. */
int open;		/* Boolean indicating if connection already exists. */
int pri;		/* My priority. */
int r, ret;		/* Return values. */
int timeout;		/* My election timeout value. */
int tmpid;		/* Used to call dbenv->rep_process_message. */
char *c;		/* Temp used in parsing host:port names. */
char *myaddr;		/* My host/port address. */
char *progname;		/* Program name for error messages. */
void *status;		/* Pthread return status. */
for (ret = 0; ret == 0;) {
	if ((ret = get_next_message(fd, &rec, &control)) != 0) {
		/*
		 * There was some sort of network error;  close this
		 * connection and remove it from the table of
		 * environment IDs.
		 */
		close(fd);
		if ((ret = machtab_rem(tab, eid, 1)) != 0)
			break;

		/*
		 * If I'm the master, I just lost a client and this
		 * thread is done.
		 */
		if (master_eid == SELF_EID)
			break;

		/*
		 * If I was talking with the master and the master
		 * went away, I need to call an election; else I'm
		 * done.
		 */
		if (master_eid != eid)
			break;

		master_eid = DB_EID_INVALID;
		/*
		 * In ex_repquote, the environment ID table stores
		 * election parameters.
		 */
		machtab_parm(tab, &n, &pri, &timeout);
		if ((ret = dbenv->rep_elect(dbenv,
		    n, pri, timeout, &newm)) != 0)
			continue;

		/*
		 * If I won the election, become the master.
		 * Otherwise, just exit.
		 */
		if (newm == SELF_EID && (ret =
		    dbenv->rep_start(dbenv, NULL, DB_REP_MASTER)) == 0)
			ret = domaster(dbenv, progname);
		break;
	}

	/* If we get here, we have a message to process. */

	tmpid = eid;
	switch(r = dbenv->rep_process_message(dbenv,
	    &control, &rec, &tmpid)) {
	case DB_REP_NEWSITE:
		/*
		 * Check if we got sent connect information and if we
		 * did, if this is me or if we already have a
		 * connection to this new site.  If we don't,
		 * establish a new one.
		 */

		/* No connect info. */
		if (rec.size == 0)
			break;

		/* It's me, do nothing. */
		if (strncmp(myaddr, rec.data, rec.size) == 0)
			break;

		self.host = (char *)rec.data;
		self.host = strtok(self.host, ":");
		if ((c = strtok(NULL, ":")) == NULL) {
			dbenv->errx(dbenv, "Bad host specification");
			goto err;
		}
		self.port = atoi(c);

		/*
		 * We try to connect to the new site.  If we can't,
		 * we treat it as an error since we know that the site
		 * should be up if we got a message from it (even
		 * indirectly).
		 */
		if ((ret = connect_site(dbenv,
		    tab, progname, &self, &open, &eid)) != 0)
			goto err;
		break;
	case DB_REP_HOLDELECTION:
		if (master_eid == SELF_EID)
			break;
		/* Make sure that previous election has finished. */
		if (ea != NULL) {
			(void)pthread_join(elect_thr, &status);
			ea = NULL;
		}
		if ((ea = calloc(sizeof(elect_args), 1)) == NULL) {
			ret = errno;
			goto err;
		}
		ea->dbenv = dbenv;
		ea->machtab = tab;
		ret = pthread_create(&elect_thr,
		    NULL, elect_thread, (void *)ea);
		break;
	case DB_REP_NEWMASTER:
		/* Check if it's us. */
		master_eid = tmpid;
		if (tmpid == SELF_EID) {
			if ((ret = dbenv->rep_start(dbenv,
			    NULL, DB_REP_MASTER)) != 0)
				goto err;
			ret = domaster(dbenv, progname);
		}
		break;
	case 0:
		break;
	default:
		dbenv->err(dbenv, r, "DBENV->rep_process_message");
		break;
	}
}

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