|
|
To enable transactions for your environment, you must initialize the transactional subsystem. Note that doing this also initializes the logging subsystem. In addition, you must initialize the memory pool (in-memory cache). Frequently, but not always, you will also initialize the locking subsystem. For example:
#include "db_cxx.h" ... int main(void) { u_int32_t env_flags = DB_CREATE | // If the environment does not // exist, create it. DB_INIT_LOCK | // Initialize locking DB_INIT_LOG | // Initialize logging DB_INIT_MPOOL | // Initialize the cache DB_INIT_TXN; // Initialize transactions std::string envHome("/export1/testEnv"); DbEnv myEnv(0); try { myEnv.open(envHome.c_str(), env_flags, 0); } catch(DbException &e) { std::cerr << "Error opening database environment: " << envHome << std::endl; std::cerr << e.what() << std::endl; return (EXIT_FAILURE); } try { myEnv.close(0); } catch(DbException &e) { std::cerr << "Error closing database environment: " << envHome << std::endl; std::cerr << e.what() << std::endl; return (EXIT_FAILURE); } return (EXIT_SUCCESS); }
You then create and open your database(s) as you would for a non-transactional system. The only difference is that you must pass the environment handle to the DbEnv::open() method, and you must open the database within a transaction. Typically auto commit is used for this purpose. To do so, pass DB_AUTO_COMMIT to the database open command. Also, make sure you close all your databases before you close your environment. For example:
#include "db_cxx.h" ... int main(void) { u_int32_t env_flags = DB_CREATE | // If the environment does not // exist, create it. DB_INIT_LOCK | // Initialize locking DB_INIT_LOG | // Initialize logging DB_INIT_MPOOL | // Initialize the cache DB_INIT_TXN; // Initialize transactions u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT; Db *dbp = NULL; const char *file_name = "mydb.db"; std::string envHome("/export1/testEnv"); DbEnv myEnv(0); try { myEnv.open(envHome.c_str(), env_flags, 0); dbp = new Db(&myEnv, 0); dbp->open(NULL, // Txn pointer file_name, // File name NULL, // Logical db name DB_BTREE, // Database type (using btree) db_flags, // Open flags 0); // File mode. Using defaults } catch(DbException &e) { std::cerr << "Error opening database and environment: " << file_name << ", " << envHome << std::endl; std::cerr << e.what() << std::endl; } try { dbp->close(0); myEnv.close(0); } catch(DbException &e) { std::cerr << "Error closing database and environment: " << file_name << ", " << envHome << std::endl; std::cerr << e.what() << std::endl; return (EXIT_FAILURE); } return (EXIT_SUCCESS); }
Never close a database that has active transactions. Make sure all transactions are resolved (either committed or aborted) before closing the database.