Chapter 10. Transactions

Quince's transaction class provides an RAII wrapper for SQL's BEGIN, COMMIT and ROLLBACK commands.

You can construct a transaction object while another transaction object, for the same database [19] , exists on the same thread. Then the newer transaction is said to be nested. Nested transactions (to any depth) wrap SQL's SAVEPOINT, ROLLBACK TO, and RELEASE SAVEPOINT commands.

Every DBMS offers a range of options about its synchronization behaviour (isolation level, shared-cache locking). To the extent that that control is available to quince applications, it is via the backend libraries, not via quince itself. Quince's transaction class assumes that the synchronization behaviour has already been configured to your taste, or more accurately: it has no awareness of it. If the DBMS does the right thing, or the wrong thing, or is efficient, or inefficient, then that is what an application using the transaction class will see.

Starting, committing, and rolling back a transaction

Code for a typical transaction looks like this:

    transaction txn(db);

    // ... statements to be executed in one transaction ...


If the body runs all the way through, then txn.commit(); will be executed and the transaction will be committed. If txn falls out of scope without being committed, then the transaction will be rolled back. So if you are part way through a transaction and you want it rolled back, throw an exception that will not be caught within the transaction's scope.

Caution: the transaction will be rolled back if you bypass the commit() by any means, including break, continue or return. That can lead to bugs, e.g. you've finished the work you wanted to do, you think you're returning in triumph, and then it all gets rolled back as if it were failure. To avoid confusion, I suggest that you never use break, continue or return in a way that would exit a transaction's scope.

commit() is only allowed on the transaction that is currently the innermost one for its database. If you try to commit an outer transaction (not necessarily the outermost) while an inner transaction exists, or commit a transaction that was created on another thread, then commit() throws a non_current_txn_exception.


When an operation would result in deadlock, the DBMS reports it to quince, and quince throws deadlock_exception. Usually that is not an error, but it is a sign that the transaction needs to be rolled back and tried again. Here is some code you can use:

for (;;) {
    try {
        transaction txn(db);

        // statements to be executed in one transaction ...

    catch (const deadlock_exception &) {


Quince transactions pass on the subtle behaviour of the SQL constructs that they wrap, so you might need to know what is going on at the SQL level. The following table provides translations.


SQL for an outermost transaction

SQL for a nested transaction

transaction txn constructed


SAVEPOINT save_point_nnn



txn destructed without having been committed


ROLLBACK TO save_point_nnn

txn destructed, whether committed or not

RELEASE SAVEPOINT save_point_nnn

(nnn stands for a number that quince allocates, to identify each nested transaction uniquely within an outermost transaction.)

[19] Transactions for different databases are unrelated to each other, and do not count as nested within each other.