Re: Supporting Partial Replication

On Thu, Dec 06, 2001 at 02:45:14PM +1100, Steven Legg wrote:
| 
| 
| Folks,
| 
| A while ago I promised to write up my thoughts on changes to the LDUP
| architecture to support partial replication. Well this is part one of
| that write up, which discusses changes to the architecture to make it
| more amenable to replication topologies involving partial replicas.
| 
| 
| Consider the following replication topology:
| 
|   S1 ====== S2
|     \      /
|      \    /
|       \  /
|        S3
| 
| Servers S1 & S2 hold full copies of replication area R1. S3 holds
| replication area R2, a subset of R1. R2 could be a subtree of R1, a
| sparse replica or a fractional replica. The exact details don't matter
| at this stage. It is enough to recognize that R2 is a subset of the
| information in R1.
| 
| Suppose that there are two successive update operations, U1 & U2, performed
| at S2, where U1 affects information in R1 but wholly outside of R2 and U2
| is wholly within R2 (and thus also within R1). The CSN alloted to U1 is
| less than the CSN alloted to U2.
| 
| Suppose S3 and S2 establish replication sessions to exchange updates.
| S3 has no changes to send. S2 will send U2 because it is within the scope
| of the replication agreement S3 has with S2, but will not send U1.
| 
| S3 and S1 then establish replication sessions. S1 has no changes to send.
| S3 sends U2 since the CSN for U2 is more recent than the CSN corresponding
| to S2 in S1's update vector. S1 will set the CSN corresponding to S2 in
| its update vector to be the CSN for U2.
| 
| Now, if S2 establishes a replication session with S1 it will send no
| updates. In particular, it won't send U1 because the CSN corresponding to
| S2 in S1's update vector is already greater than the CSN for U1. In fact,
| S1 will never receive U1, so the requirement for all replicas to converge
| will not be satisfied. In general, the current LDUP architecture only
| works if the replication topology has no cycles, or where there are
| cycles, if the replicas in each cycle have replication agreements for
| exactly the same area of replication.
|

Hold on...  I'm deleting the rest of this message because you've lost me
here.  I thought that (a) we had a separate CSN vector for each other server
and that (b) that CSN vector was to the level of attribute and entry.  Thus,
I don't see the problem.

Ryan