18.04.2017 19:28, Adriano dos Santos Fernandes wrote: > Hi! > > In distributed systems there is a problem to use Firebird and maintains > read consistency. > > Imagine follow situations: > > 1) A server (not database server) receives a request and dispatch it to > others servers for extra processing and more than one server need to > access the same database in a consistent way. > > 2) A multi-threaded program (may be a future version of gbak) wants to > dispatch simultaneous queries to Firebird, so need to use more than one > attachment and transaction and then do not have transaction consistency. > > I propose a clause to SET TRANSACTION (and its TPB) so user can inform a > "base transaction number" (better words and syntax are welcome). > > SET TRANSACTION ISOLATION LEVEL SNAPSHOT FROM BASE 100 > > In this case, 100 must be an active transaction and the snapshot of the > new transaction will be the same snapshot used when transaction 100 was > started. > > New transaction will not get uncommitted changes of transaction 100. > > With this, a program (or a set of coordinated programs) may use same > Firebird database in multi-threaded / multi-process way preserving read > transaction consistency. > > New semantics may also be introduced to use non-active transaction as > base number and a policy to retain some garbage for a period so one can > do time-lapsed queries. This is, however, out of the scope of this proposal.
Some time ago there was discussion about sharing snapshots. As for me, it is useful feature. Not a "must have", but useful. With current implementation of database snapshots (private copy of TIP) it is not enough just to specify "base" transaction number to obtain its snapshot, especially for Classic. But, with new snapshots acounting (based on commit order, proposed by Nickolay) it will be enough to get just one number from the "base" transaction. Regards, Vlad ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot Firebird-Devel mailing list, web interface at https://lists.sourceforge.net/lists/listinfo/firebird-devel
