Most of the configuration activities that you need to perform for your transactional DB application will involve the locking and logging subsystems. See Concurrency and Managing DB Files for details.
However, there are a couple of things that you can do to configure your transaction subsystem directly. These things are:
Configure the maximum number of simultaneous transactions needed by your application. In general, you should not need to do this unless you use deeply nested transactions or you have many threads all of which have active transactions. In addition, you may need to configure a higher maximum number of transactions if you are using snapshot isolation. See Snapshot Isolation Transactional Requirements for details.
By default, your application can support 20 active transactions.
You can set the maximum number of simultaneous
transactions supported by your application using
the
DbEnv::set_tx_max()
method. Note that this method must be called
before the environment has been opened.
If your application has exceeded this maximum value, then any attempt to begin a new transaction will fail.
This value can also be set using the
DB_CONFIG file's
set_tx_max parameter. Remember that
the DB_CONFIG must reside in your
environment home directory.
Configure the timeout value for your transactions. This value represents the longest period of time a transaction can be active. Note, however, that transaction timeouts are checked only when DB examines its lock tables for blocked locks (see Locks, Blocks, and Deadlocks for more information). Therefore, a transaction's timeout can have expired, but the application will not be notified until DB has a reason to examine its lock tables.
Be aware that some transactions may be inappropriately timed out before the transaction has a chance to complete. You should therefore use this mechanism only if you know your application might have unacceptably long transactions and you want to make sure your application will not stall during their execution. (This might happen if, for example, your transaction blocks or requests too much data.)
Note that by default transaction timeouts are set to 0 seconds, which means that they never time out.
To set the maximum timeout value for your transactions,
use the
DbEnv::set_timeout()
method. This method configures the entire
environment; not just the handle used to set the
configuration. Further, this value may
be set at any time during the application's
lifetime.
This value can also be set using the
DB_CONFIG file's
set_txn_timeout parameter.
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_THREAD | // Free-thread the env handle
DB_INIT_TXN; // Initialize transactions
std::string envHome("/export1/testEnv");
DbEnv myEnv(0);
try {
// Configure a maximum transaction timeout of 1 second.
myEnv.set_timeout(1000000, DB_SET_TXN_TIMEOUT);
// Configure 40 maximum transactions.
myEnv.set_tx_max(40);
myEnv.open(envHome.c_str(), env_flags, 0);
// From here, you open your databases, proceed with your
// database operations, and respond to deadlocks as
// is normal (omitted for brevity).
...