12 jul 2014 kl. 14:48 skrev Andres Freund <andres@xxxxxxxxxxxxxxx>: > On 2014-07-12 14:37:02 +0200, Martin Gudmundsson wrote: >>> It's possible to do it to a streaming replication sync standby, but also >>> to another BDR node. The logical decoding facility added in 9.4 allows >>> logical replication solutions to use the same mechanism as streaming rep >>> does. >>> >> >> Sounds interesting, but it does not sound like it’s being bi-directional in that case? > > It is bidirectional if you configure a bdr node in > synchronous_standby_names. OK, I will play around with this a bit. Thanks for the hint. > Note that streaming replication's synchronous mode probably doesn't give > the guarantees you're thinking it does: > a) A sent commit might succeed locally on the primary, but the system > might fail to send it to the synchronous standby. I.e. postgres' > synchronous replication of normal commits is *not* 2-safe. > b) Even if a transaction is replicated to the standby it's *not* > guaranteed to be applied. All that's guaranteed is that it has been > shipped to the synchronous standby Very good info! I’m not sure that is so clear in the documentation but I have to revisit it. >> Ideally I’m looking for a solution where I can run an application and it does not really matter on what node a transaction executes, or if it changes data or not. > > I'd like that too, but I don't think that's going to happen. There's > just too many caveats (CAP theorem et al) to make that generally > useful/applicable. We are currently using solutions that works like that, DB2 Datasharing on z/OS is one example. But it’s not built around replication, but global filesystems and central locking facilities. That approach also have it’s issues, with performance bottlenecks etc. > I think we (as in postgres) will probably get the ability to run > individual transactions in such a mode, but you surely wouldn't want to > run every transaction in it. That sounds like o good trade off. Looking forward to it. > > Greetings, > > Andres Freund > > -- > Andres Freund http://www.2ndQuadrant.com/ > PostgreSQL Development, 24x7 Support, Training & Services
