// A function that performs a series of writes to a
// Berkeley DB database. The information written
// to the database is largely nonsensical, but the
// mechanism of transactional commit/abort and
// deadlock detection is illustrated here.
void *
writerThread(void *args)
{
int j, thread_num;
int max_retries = 20; // Max retry on a deadlock
const char *key_strings[] = {"key 1", "key 2", "key 3", "key 4",
"key 5", "key 6", "key 7", "key 8",
"key 9", "key 10"};
Db *dbp = (Db *)args;
DbEnv *envp = dbp->get_env();
// Get the thread number
(void)mutex_lock(&thread_num_lock);
global_thread_num++;
thread_num = global_thread_num;
(void)mutex_unlock(&thread_num_lock);
// Initialize the random number generator
srand(thread_num);
// Perform 50 transactions
for (int i=0; i<50; i++) {
DbTxn *txn;
bool retry = true;
int retry_count = 0;
// while loop is used for deadlock retries
while (retry) {
// try block used for deadlock detection and
// general db exception handling
try {
// Begin our transaction. We group multiple writes in
// this thread under a single transaction so as to
// (1) show that you can atomically perform multiple
// writes at a time, and (2) to increase the chances
// of a deadlock occurring so that we can observe our
// deadlock detection at work.
// Normally we would want to avoid the potential for
// deadlocks, so for this workload the correct thing
// would be to perform our puts with autocommit. But
// that would excessively simplify our example, so we
// do the "wrong" thing here instead.
txn = NULL;
envp->txn_begin(NULL, &txn, 0);
// Perform the database write for this transaction.
for (j = 0; j < 10; j++) {
Dbt key, value;
key.set_data((void *)key_strings[j]);
key.set_size((u_int32_t)strlen(key_strings[j]) + 1);
int payload = rand() + i;
value.set_data(&payload);
value.set_size(sizeof(int));
// Perform the database put
dbp->put(txn, &key, &value, 0);
}
// countRecords runs a cursor over the entire database.
// We do this to illustrate issues of deadlocking
std::cout << thread_num << " : Found "
<< countRecords(dbp, txn)
<< " records in the database." << std::endl;
std::cout << thread_num << " : committing txn : " << i
<< std::endl;
// commit
try {
txn->commit(0);
retry = false;
txn = NULL;
} catch (DbException &e) {
std::cout << "Error on txn commit: "
<< e.what() << std::endl;
}
} catch (DbDeadlockException &) {
// First thing that we MUST do is abort the transaction.
if (txn != NULL)
(void)txn->abort();
// Now we decide if we want to retry the operation.
// If we have retried less than max_retries,
// increment the retry count and goto retry.
if (retry_count < max_retries) {
std::cerr << "############### Writer " << thread_num
<< ": Got DB_LOCK_DEADLOCK.\n"
<< "Retrying write operation." << std::endl;
retry_count++;
retry = true;
} else {
// Otherwise, just give up.
std::cerr << "Writer " << thread_num
<< ": Got DeadLockException and out of "
<< "retries. Giving up." << std::endl;
retry = false;
}
} catch (DbException &e) {
std::cerr << "db put failed" << std::endl;
std::cerr << e.what() << std::endl;
if (txn != NULL)
txn->abort();
retry = false;
} catch (std::exception &ee) {
std::cerr << "Unknown exception: " << ee.what() << std::endl;
return (0);
}
}
}
return (0);
}
