Berkeley DB Transactional Data Store Applications

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Opening the Databases

Next, we open three databases ("color," "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, address, 
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 opens 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.