Error opening container with DB_READ_UNCOMMITTED

Question

I am running into an exception condition when I try re-creating the Isolation sample found at [http://www.oracle.com/technology/documentation/berkeley-db/xml/gsg_xml_txn/cxx/isolation.html#dirtyreads] . Everything works fine with all of the settings in that example except when I try to pass the DB_READ_UNCOMMITTED flag to XmlManager.openContainer(); when I try to pass that in I get the following exception:
XmlException: DbSequence::open: Invalid argument
ConfigurationDatabase.cpp line 65
the numeric value of the flags parameter at this stage is 134217857, if that helps...
my sample code:
// BdbXmlWin32Console.cpp : Defines the entry point for the console application.
#include "DbXml.hpp"
#include
#include
#include "stdafx.h"
using namespace DbXml;
int _tmain(int argc, _TCHAR* argv[])
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 env handle
DB_INIT_TXN; // Initialize transactions
std::string envHome("C:\\dev\\db\\samples\\ReadUncommitted");
DbEnv myEnv(0);
XmlManager *myManager = NULL;
try {
myEnv.open(envHome.c_str(), env_flags, 0);
myManager = new XmlManager(&myEnv, 0);
u_int32_t containerFlags =
DB_CREATE | // If the container does not exist,
// create it.
DBXML_TRANSACTIONAL | // Enable transactions.
DB_READ_UNCOMMITTED; // Enable uncommitted reads.
std::string containerName = "myContainer.dbxml";
XmlContainer myContainer =
myManager->openContainer(containerName, containerFlags);
myContainer.close();
} catch(DbException &e) {
std::cerr << "Error opening database environment: "
<< envHome << std::endl;
std::cerr << e.what() << std::endl;
return (EXIT_FAILURE);
} catch(std::exception &e) {
std::cerr << "Error opening database environment: "
<< envHome
<< " or opening XmlManager or XmlContainer." << std::endl;
std::cerr << e.what() << std::endl;
return (EXIT_FAILURE);
// File to open
std::string file1 = "doc1.xml";
// Transaction handle
// Enable uncommitted reads for this transaction.
XmlTransaction txn = myManager->createTransaction(DB_READ_UNCOMMITTED);
// From here, you perform your container reads and writes as normal,
// committing and aborting the transactions as is necessary, as well as
// testing for deadlock exceptions as normal (omitted for brevity).
}Thanks!
Edited by: Lucas Vogel on Jan 11, 2009 7:45 PM

Answer 1

Lucas,
This appears to be a bug. That flag is not legal for one of the internal databases in the system and is not properly being cleared on open. That said, there are few cases where you'd really want to use that flag in BDB XML. What is your intent? Allowing readers to see not-yet-committed data is dangerous. Using MVCC is a better route for concurrent applications.
Regards,
George