Berkeley DB Reference Guide: Berkeley DB Transactional Data Store Applications

Opening the databases

Next, we open three databases ("color" and "fruit" and "cats"), in the database environment. Again, our DB database handles are declared to be free-threaded using the DB_THREAD flag, and so may be used by any number of threads we subsequently create.

int
main(int argc, char *argv)
{
	extern char *optarg;
	extern int optind;
	DB *db_cats, *db_color, *db_fruit;
	DB_ENV *dbenv;
	pthread_t ptid;
	int ch;

	while ((ch = getopt(argc, argv, "")) != EOF)
		switch (ch) {
		case '?':
		default:
			usage();
		}
	argc -= optind;
	argv += optind;

	env_dir_create();
	env_open(&dbenv);

	/* Open database: Key is fruit class; Data is specific type. */
	db_open(dbenv, &db_fruit, "fruit", 0);

	/* Open database: Key is a color; Data is an integer. */
	db_open(dbenv, &db_color, "color", 0);

	/*
	 * Open database:
	 *	Key is a name; Data is: company name, cat breeds.
	 */
	db_open(dbenv, &db_cats, "cats", 1);

	return (0);
}

void
db_open(DB_ENV *dbenv, DB **dbp, char *name, int dups)
{
	DB *db;
	int ret;

	/* Create the database handle. */
	if ((ret = db_create(&db, dbenv, 0)) != 0) {
		dbenv->err(dbenv, ret, "db_create");
		exit (1);
	}

	/* Optionally, turn on duplicate data items. */
	if (dups && (ret = db->set_flags(db, DB_DUP)) != 0) {
		dbenv->err(dbenv, ret, "db->set_flags: DB_DUP");
		exit (1);
	}

	/*
	 * Open a database in the environment:
	 *	create if it doesn't exist
	 *	free-threaded handle
	 *	read/write owner only
	 */
	if ((ret = db->open(db, name, NULL,
	    DB_BTREE, DB_CREATE | DB_THREAD, S_IRUSR | S_IWUSR)) != 0) {
		dbenv->err(dbenv, ret, "db->open: %s", name);
		exit (1);
	}

	*dbp = db;
}

There is no reason to wrap database opens inside of transactions. All database creates are transaction-protected internally to Berkeley DB, and applications using transaction-protected environments can simply rely on files either being successfully re-created in a recovered environment or not appearing at all.

After running this initial code, we can use the db_stat utility to display information about a database we have created:

prompt> db_stat -h TXNAPP -d color
53162   Btree magic number.
8       Btree version number.
Flags:
2       Minimum keys per-page.
8192    Underlying database page size.
1       Number of levels in the tree.
0       Number of unique keys in the tree.
0       Number of data items in the tree.
0       Number of tree internal pages.
0       Number of bytes free in tree internal pages (0% ff).
1       Number of tree leaf pages.
8166    Number of bytes free in tree leaf pages (0.% ff).
0       Number of tree duplicate pages.
0       Number of bytes free in tree duplicate pages (0% ff).
0       Number of tree overflow pages.
0       Number of bytes free in tree overflow pages (0% ff).
0       Number of pages on the free list.

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