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<h2 class="title" style="clear: both"><a id="envopen"></a>Opening a Transactional Environment and
<span>Database</span>
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<p>
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.
<span>
For example:
</span>
</p>
<pre class="programlisting">#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);
} </pre>
<p>
You then create and open your database(s) as you would for a non-transactional system.
<span>
The only difference is that you must pass the environment handle to
the
<span>
<tt class="methodname">DbEnv::open()</tt> method,
</span>
and you must open the database within a transaction.
Typically auto commit is used for this purpose. To do so, pass
<tt class="literal">DB_AUTO_COMMIT</tt> to the database open command.
Also, make sure you close all your databases before you close
your environment.
For example:
</span>
</p>
<pre class="programlisting">#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
<b class="userinput"><tt>u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
Db *dbp = NULL;
const char *file_name = "mydb.db";</tt></b>
std::string envHome("/export1/testEnv");
DbEnv myEnv(0);
try {
myEnv.open(envHome.c_str(), env_flags, 0);
<b class="userinput"><tt>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 </tt></b>
} catch(DbException &e) {
std::cerr << "Error opening database and environment: "
<< file_name << ", "
<< envHome << std::endl;
std::cerr << e.what() << std::endl;
}
try {
<b class="userinput"><tt>dbp->close(0);</tt></b>
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);
} </pre>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Never close a database that has active transactions. Make sure
all transactions are resolved (either committed or aborted)
before closing the database.
</p>
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