On Thu, Mar 10, 2011 at 12:31 PM, Otis Gospodnetic <[email protected]> wrote: >> Options 1) and 2) will give you a snapshot on a table at a particular >> instance in time. You'll get the state of the row at the time the >> MapReduce job crosses that row. > > Hm, isn't this contradictory? That is, doesn't "snapshot of a table at a > particular instance in time" means that I'd get a snapshot of *all* rows at a > single point in time, and not a value of a row when the Export or Copy MR job > crosses it? >
Sorry for the sloppy phrasing. Being a distributed system, getting a consistent view on a table at a particular moment in time would be a little tough. The only thing we guarantee -- currently w/ some caveats, HBASE-2856 -- is a consistent at the row level only. > Also, it seems like all options are per-table, right? There is nothing other > than near real-time full-cluster replication that would back up all tables at > once? Right. > This is important when you have multiple tables storing data that depend on > each > other. Imagine tables A and B where table B depends on A. If you first back > up > A, then by the time I back up B, it may reference some data in A that my A's > backup doesn't contain. If you flip the order and first back up B, then by > the > time I back up A it may contain some extra data that B's backup doesn't refer > to. > Yes. > Simply put, the backup copies of these 2 tables won't be in sync. > > How do people deal with this? > If you want them in sync, you are into the world of cross-row, cross-table transactions. Replicating the tables will eventually be consistent with each other (Replication is edit-scoped, not table or even x-table scoped). > Would it make sense to document this sort of stuff on > http://hbase.apache.org/book/book.html ? > You mean the list of backup options? Yes. And their individual failings/constraints. (Otis, in this list you've made other useful 'lists' -- the reporting one for instance -- that I've put on my 'doc this' list, my list of things to add into the manual when have a minute). St.Ack
