Simon Slavin wrote: > On 7 Oct 2009, at 7:20pm, Adam DeVita wrote: > > >> regarding this >> " The fault is that >> almost nobody does it right: they neglect to keep an 'unaltered >> central copy' and think they can cross-apply journals each time two >> databases talk to one-another. That does not work for various >> reasons." >> >> Would a central repository of journals that can be applied to local >> repositories be sufficient? I suppose I assume that running the same >> program on N workstations with the same set of journals should >> produce N >> identical results. >> > > SNIP > The problem comes when you apply multiple journals in a different > order. Start with each site with identical copies of a TABLE with > three clients: one managed by Mr. Green, one by Mr. Red, and one by > Mr. Black. 'G R B'. Then, in this order ... > > Mr. Green goes on holiday ... > Site A says that all Mr. Green's customers will be handled by Mr. Red. > UPDATE clients SET contact = 'red' WHERE contact = 'green' > > Mr. Red goes on holiday ... > Site B says that all Mr. Red's customers will be handled by Mr. Black. > > Then Mr. Green comes back from holiday, and Mr. Black goes on holiday > so ... > Site C says that all Mr. Black's customers will be handled by Mr. Green. > > Then they all synchronise databases. See if you can make them all end > up with the same data if they synch against each-other rather than a > central unaltered copy of the databases. Doesn't work: one site might > have 'B B B', another 'R B R'. You can do it only by luck ... by > happening to know in which order people went on holiday. However, if > you always synch against a central unaltered copy of the database you > can synch in any order. Once everyone has synchronised you distribute > a copy of the central database to everyone and they all have identical > data once more. That's the simplest setup. You can get more > complicated by having each site remember which journals they've played > back. >
Simon, Isn't this a variation of the DVCS problem? In other words, would it be correct in saying that the underlying issue is treating this as a database problem, rather than it being a versioning problem which happens to involve a database? I ask because there are two separate projects which involve this sort of issue which I have simply deferred for the time being. John _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
