Zitat von \"vicente.botet\" <vicente.botet@wanadoo.fr>:
the catch has no access to the transaction variable as it out of scope (already destructed). Some times it is interesting to increase the priority of a transaction to avoid starvation. In this case we need to be able to access the transaction.
wouldn\'t that then be a new transaction with an increased priority, after the first transaction failed? why do you keep the transaction object and restart it instead of just creating a new transaction object (possibly with an increased priority)?
BOOST_TRANSACT_ATOMIC(_) {
} BOOST_TRANSACT_BEFORE_RETRY(_) {_.increase_priority(); }.
since setting a priority would be resource manager specific, you\'d have to access the resource transaction: TxMgr::resource_transaction(TxMgr::active()).set_priority(...) (or a simpler shortcut defined by your library, but my point is that it can\'t be a member function of basic_transaction)
On Boost.STM we have developed some macros (see stm/branches/vbe/boost/stm/language_like.hpp) . Next follows the Transact adaptation of some of them.
#define BOOST_TRANSACT_BASIC_ATOMIC(TXMGR, TX)\\ for (boost::transact::basic_transaction<TXMGR> TX; \\ ! TX.committed() && TX.restart_if_not_inflight(); \\ TX.no_throw_commit()) try
#define BOOST_TRANSACT_BASIC_RETRY(TXMGR, TX) \\ catch (boost::transact::isolation_exception &i) {}
#define BOOST_TRANSACT_BASIC_BEFORE_RETRY(TXMGR, TX) catch (boost::transact::isolation_exception &i)
#define BOOST_TRANSACT_BASIC_RETHROW(TXMGR, TX, E) \\ catch (E&) {(TX).commit(); throw;}
#define BOOST_TRANSACT_BASIC_ABORT_ON_EXCEPTION(TXMGR, TX, E) \\ catch (E&) {(TX).force_to_abort();}
could you explain a situation in which you\'d use those other macros? to me, the macros are a simplified syntax for the most common use case, not a syntax which covers every possible use case you could think of. at some point I think the macros loose their benefit and you might just as well write the loop yourself. e.g. if you need to execute custom code on retry. if you think some of those macros would be widely used, please explain which ones and what the functions in the macros like no_throw_commit/force_to_abort etc. do. some of this seem resource-manager specific again.
BTW, what unwind does? Could you explain the problem it intend to solve.
unwinding nested transaction stack. an isolation exception thrown inside a nested transaction might e.g. make it necessary to rethrow it until the outermost transaction is destroyed. for example: do atomic{ loc<pers_type> pers_obj=...; do atomic{ //pers_obj is deleted by another transaction at this point cerr << pers_obj->value; //throws }commit(); }commit(); if the isolation exception would only cause the innermost transaction to be repeated, it would be repeated forever, since pers_obj is not coming back. via unwind() the resource manager throwing the exception can control up to which transaction on the stack the isolation_exception is rethrown. see exception.hpp.
There are a lot of other macros. I\'ll present all these and more as for example how to exit succesfully from a transaction block (return, break, continue or goto) at BoostCon.
justin mentioned a to-be-released paper about that some time ago, has this been published in the meantime?
Let me know if you want I add them to Boost.Transact;
if we can identify another common use case that is not resource manager specific, sure. if it is Boost.STM-specific, your library is the right place to define the macro.