[Modeling-users] Future directions: replication and auto-increments

[Ernesto R. <er...@si...>]

Hi,

when talking with some Python people, I heard about future requirements 
about multi-master replication. This is what you need if you have data 
split over several offices and you have no 100% uptime links. 
Multi-master replication is available in mysql, but Postgresql still is 
lacking this feature. I think Oracle as it also.

The problems you can have with multi-master replication are the update 
collisions. But these may not be very usual in real scenarios, depending 
how data is organized.

The first problem of all is that we can't use auto-increment (serial) 
fields. So we have to use our own function to produce this. For this to 
work, the original autoincremen field is defined as large integer, 
something like 64 bits, where a part is reserved as a local counter and 
the other part as station identifier which created the object, e.g. 16 
bit station identifier + 48 bit counter. There are other, more 
complicated, algorithms, like those which create a global unique id 
(GUID) bases on the Ethernet address and time, and distributed 
scenarios, where there is a process which gives new hi-numbers to 
stations (which in turn can fragment the bits used, say 16 into 8+8 and 
give their own low part hi number to other stations, producing a 
hierarchical system).

Concluding, IDs should not be created by database systems, but by the 
middleware in a nuclear operation.

With best regards, Erny