DB_LOCK_DEADLOCK: Using BerkeleyDB Xml in a threaded environment

Question

Hello,
I'm having problems running a Berkeley DB application in a threaded environment. In summary, this is what I'm doing:
I implemented the following class:
ref class MyTestClass
private:
     DbEnv* env;
     XmlManager* man;
     unsigned int ctr;
public:
     MyTestClass()
          ctr = 0;
          env = new DbEnv(0);
          env->set_cachesize(0, 64*1024, 1);
          env->set_lk_max_lockers(1000);
          env->set_lk_max_locks(1000);
          env->set_lk_max_objects(1000);
          env->open("c:\\temp\\SampleDb",
DB_CREATE | DB_INIT_LOCK | DB_INIT_LOG |
DB_INIT_MPOOL | DB_INIT_TXN | DB_THREAD | DB_PRIVATE, 0);
          man = new XmlManager(env, DBXML_ALLOW_AUTO_OPEN);
     void MyTestWriter()
          while(true)
               DbXml::XmlQueryContext *ctx;
               DbXml::XmlTransaction *txn;
               DbXml::XmlResults *res;
               try
                    ctx = new XmlQueryContext(man->createQueryContext(XmlQueryContext::LiveValues, XmlQueryContext::Lazy));
                    txn = new XmlTransaction(man->createTransaction());
                    res = new XmlResults(man->query(*txn, "for $v in collection('test.dbxml')/sessions return insert nodes <A/> into$v", *ctx, DB_RMW));
                    txn->commit(DB_TXN_SYNC);
               catch(XmlException& e)
               finally
                    delete res;
                    delete txn;
                    delete ctx;
     void DeadlockUnblocker()
          while(true)
               int ret;
               env->lock_detect(0, DB_LOCK_DEFAULT, &ret);
               Sleep(5000);
Basically I create a shared MyTestClass object and then spawn 4 threads: 3 of them execute the MyTestWriter method and 1 executes the DeadlockUnblocker method.
What happens is that the 3 threads block even before completing the first write. After 5 seconds the DeadlockUnblocker is executed and 1 thread is unblocked and throws a "DB_LOCK_DEADLOCK: Locker killed to resolve a deadlock" which is trapped by my catch block. However, the other threads are still hanging on the execution and the entire flow of the application is stopped.
Anybody can tell me what I'm doing wrong ??
Thanks
Matteo

Answer 1

Matteo,
First, the C++ API works best if you avoid new/delete of Xml* objects. There are relatively few cases where that's necessary. Using scoped objects ensures their destruction.
As for your hang the best tool to use is "db_stat -CA" in the environment directory at the time of the hang to find out what is going on. You'll have to not use DB_PRIVATE in the DbEnv::open() flags for it to work. While a deadlock thread is reasonable if you expect a lot of deadlocks (and concurrent write will do that) it is best to use DbEnv::set_lk_detect() to get immediate detection.
Regards,
George