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:
Notice in the following example that you create your environment handle using the db_env_create() function before you open the environment:
#include <stdio.h>
#include <stdlib.h>
#include "db.h"
int
main(void)
{
int ret, ret_c;
u_int32_t env_flags;
DB_ENV *envp;
const char *db_home_dir = "/tmp/myEnvironment";
envp = NULL;
/* Open the environment */
ret = db_env_create(&envp, 0);
if (ret != 0) {
fprintf(stderr, "Error creating environment handle: %s\n",
db_strerror(ret));
return (EXIT_FAILURE);
}
env_flags = DB_CREATE | /* Create the environment if it does
* not already exist. */
DB_INIT_TXN | /* Initialize transactions */
DB_INIT_LOCK | /* Initialize locking. */
DB_INIT_LOG | /* Initialize logging */
DB_INIT_MPOOL; /* Initialize the in-memory cache. */
ret = envp->open(envp, db_home_dir, env_flags, 0);
if (ret != 0) {
fprintf(stderr, "Error opening environment: %s\n",
db_strerror(ret));
goto err;
}
err:
/* Close the environment */
if (envp != NULL) {
ret_c = envp->close(envp, 0);
if (ret_c != 0) {
fprintf(stderr, "environment close failed: %s\n",
db_strerror(ret_c));
ret = ret_c;
}
}
return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
}
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 db_create() function, 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 <stdio.h>
#include <stdlib.h>
#include "db.h"
int
main(void)
{
int ret, ret_c;
u_int32_t db_flags, env_flags;
DB *dbp;
DB_ENV *envp;
const char *db_home_dir = "/tmp/myEnvironment";
const char *file_name = "mydb.db";
dbp = NULL;
envp = NULL;
/* Open the environment */
ret = db_env_create(&envp, 0);
if (ret != 0) {
fprintf(stderr, "Error creating environment handle: %s\n",
db_strerror(ret));
return (EXIT_FAILURE);
}
env_flags = DB_CREATE | /* Create the environment if it does
* not already exist. */
DB_INIT_TXN | /* Initialize transactions */
DB_INIT_LOCK | /* Initialize locking. */
DB_INIT_LOG | /* Initialize logging */
DB_INIT_MPOOL; /* Initialize the in-memory cache. */
ret = envp->open(envp, db_home_dir, env_flags, 0);
if (ret != 0) {
fprintf(stderr, "Error opening environment: %s\n",
db_strerror(ret));
goto err;
}
/* Initialize the DB handle */
ret = db_create(&dbp, envp, 0);
if (ret != 0) {
envp->err(envp, ret, "Database creation failed");
goto err;
}
db_flags = DB_CREATE | DB_AUTO_COMMIT;
ret = dbp->open(dbp, /* Pointer to the database */
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 */
if (ret != 0) {
envp->err(envp, ret, "Database '%s' open failed",
file_name);
goto err;
}
err:
/* Close the database */
if (dbp != NULL) {
ret_c = dbp->close(dbp, 0);
if (ret_c != 0) {
envp->err(envp, ret_c, "Database close failed.");
ret = ret_c
}
}
/* Close the environment */
if (envp != NULL) {
ret_c = envp->close(envp, 0);
if (ret_c != 0) {
fprintf(stderr, "environment close failed: %s\n",
db_strerror(ret_c));
ret = ret_c;
}
}
return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
}
Never close a database that has active transactions. Make sure all transactions are resolved (either committed or aborted) before closing the database.