On Feb 7, 2009, at 9:37 PM, Antony Blakey wrote:
On 08/02/2009, at 12:05 PM, Damien Katz wrote:
But if the replication doesn't complete, like in the middle you
lose your connection, then the downstream db is in an inconsistent
state and will be until you regain the connection and complete the
replication.
Sure, that's OK. In my case there are potentially millions of
'downstreams'.
And its not just the downstream dbs that can't be used during
active or interrupted replication, it's the central db as well. If
you want to update it, you'll have to shut down access while
waiting for any in progress replications to complete, then perform
the update then reopen replication access.
I can't see why this needs to be the case. The fact that there are
peers that have an incomplete replication doesn't stop the master
taking updates - iff the updates to the master are multi-document
ACID.
Because during downstream replication it may get just some of the
documents from a commit that happens during replication. Replication
isn't all or nothing, and it doesn't know about transactions
In my case, replication can occur from any peer that isn't itself
replicating, and has completed a valid replication cycle.
The key thing being, even with multiple writers i.e. symmetric
peers, that replication occurs between nodes that are themselves
consistent e.g. conflicts are never replicated, they are only the
result of replication, and the node is locked until a replication
cycle completes (or is abandoned, which is a separate issue that I
have to address).
The thing is, to avoid all conflicts and inconsistent states, you must
read lock the source db during active replication and write lock the
target database pretty much until the replication is complete.
Otherwise, you'll will get inconsistent states.
Does each user get their own local instance of the database? If that's
the case, you don't need multi-document conflict detection. You just
lock the user out of the database during replication and lock out
replication while the user is using it. And it sounds like you already
have that.
Am I missing something?
.
Antony Blakey
--------------------------
CTO, Linkuistics Pty Ltd
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