From: John Alton Tamplin (no email)
Date: Tue Feb 04 2003 - 09:51:21 EST
Henrique de Moraes Holschuh wrote:
>On Tue, 04 Feb 2003, Phil Howard wrote:
>
>
>>I wonder how well that method of replication works when both nodes
>>cannot reach each other, and both are doing updates. And I wonder
>>
>>
>They don't. If they cannot reach each other, at most one of them must allow
>updates.
>
>
Which, as far as I know, is how all commercial databases also do
replication. Only those servers involved in a quorum (even if it is
because the primary is treated as having more weight) can accept updates
-- the other replicas can only be read-only. Trying to allow arbitrary
updates during disconnected operation and then merge the results into a
consistent and deterministic state is not just hard, but impossible.
For example, if you allow flags to be set differently on the same
message on each node, there is no way to resolve the two updates without
discarding at least some of the work of one of the sessions. If you
carefully constrain the updates that can be performed during
disconnected operation (and for consistency that means even not in
disconnected operation), then you can transform the problem from
impossible to merely extremely difficult.
-- John A. Tamplin Unix System Administrator Emory University, School of Public Health +1 404/727-9931
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