On Mon, Dec 12, 2011 at 9:52 AM, Paul Davis <[email protected]> wrote: > On Sun, Dec 11, 2011 at 7:19 PM, Randall Leeds <[email protected]> > wrote: >> On Sun, Dec 11, 2011 at 04:00, Alex Besogonov <[email protected]> >> wrote: >>> I wonder, why there are no unique instance IDs in CouchDB? I'm >>> thinking about 'the central server replicates 2000000 documents to a >>> million of clients' scenario. >>> >>> Right now it's not possible to make replication on the 'big central >>> server' side to be stateless, because the other side tries to write >>> replication document which is later used to establish common ancestry. >>> Server can ignore/discard it, but then during the next replication >>> client would just have to replicate all the changes again. Of course, >>> the results would be consistent in any case but quite a lot of >>> additional traffic might be required. >>> >>> It should be simple to assign each instance a unique ID (computed >>> using UUID and the set of applied replication filters) and use it to >>> establish common replication history. It can even be compatible with >>> the way the current replication system works and basically the only >>> visible change should be the addition of UUID to database info. >>> >>> Or am I missing something? >> >> I proposed UUIDs for databases a long, long time ago and it's come up >> a few times since. If the UUID is database-level, then storing it with >> the database is dangerous -- copying a database file would result in >> two CouchDB's hosting "the same" (but really different) databases. If >> the UUID is host-level, then this reduces to a re-invention of DNS. In >> other words, all DBs should already be uniquely identified by their >> URLs. >> >> Regarding your second paragraph, replicating couches _could_ try to >> establish common ancestry only by examining a local checkpoint of >> replication, but the couch replicator looks for the log on both >> couches to ensure that the database hasn't been deleted+recreated nor >> has it crashed before certain replicated changes hit disk, as a double >> check that the sequence numbers have the expected shared meaning. >> >> It seems like maybe you're wondering about whether couch could >> generate snapshot ids that are more meaningful than the sequence >> number. For a single pair of couches the host-db-seq combo is enough >> information to replicate effectively. When there's more hosts involved >> we can talk about more powerful checkpoint ids that would be shareable >> or resolvable to find common ancestry between more than two >> replicating hosts to speed up those scenarios. My intuition always >> says that this leads to hash trees, but I haven't thought about it >> deeply enough to fully conceive of what this accomplishes or how it >> would work. >> >> -R > > I did have a shimmering of an idea for this awhile back. Basically we > do both host and db uuid's and the information we use to identifiy > replications is a hash of the concatenation. > > That way we can copy db's around and not muck with things as well as > error out a bit. Though this still has a bit of an issue if we copy > the host uuid around as well. Though we migth be able to look for a > mac address or something and then fail to boot if the check fails > (with an optional override if someone changes a nic).
A couch URL is its unique identifier. A database URL is its unique identifier. This sounds like a too-clever-by-half optimization. IMHO. -- Iris Couch
