On December 8, 2015 at 12:53:04 PM, Ira Cooper (ira@xxxxxxxxxx) wrote: > Raghavendra Gowdappa writes: > I propose that we define a "compound op" that contains ops. > > Within each op, there are fields that can be "inherited" from the > previous op, via use of a sentinel value. > > Sentinel is -1, for all of these examples. > > So: > > LOOKUP (1, "foo") (Sets the gfid value to be picked up by compounding, 1 > is the root directory, as a gfid, by convention.) > OPEN(-1, O_RDWR) (Uses the gfid value, sets the glfd compound value.) > WRITE(-1, "foo", 3) (Uses the glfd compound value.) > CLOSE(-1) (Uses the glfd compound value) So, basically, what the programming-language types would call futures and promises. It’s a good and well studied concept, which is necessary to solve the second-order problem of how to specify an argument in sub-operation N+1 that’s not known until sub-operation N completes. To be honest, some of the highly general approaches suggested here scare me too. Wrapping up the arguments for one sub-operation in xdata for another would get pretty hairy if we ever try to go beyond two sub-operations and have to nest sub-operation #3’s args within sub-operation #2’s xdata which is itself encoded within sub-operation #1’s xdata. There’s also not much clarity about how to handle errors in that model. Encoding N sub-operations’ arguments in a linear structure as Shyam proposes seems a bit cleaner that way. If I were to continue down that route I’d suggest just having start_compound and end-compound fops, plus an extra field (or by-convention xdata key) that either the client-side or server-side translator could use to build whatever structure it wants and schedule sub-operations however it wants. However, I’d be even more comfortable with an even simpler approach that avoids the need to solve what the database folks (who have dealt with complex transactions for years) would tell us is a really hard problem. Instead of designing for every case we can imagine, let’s design for the cases that we know would be useful for improving performance. Open plus read/write plus close is an obvious one. Raghavendra mentions create+inodelk as well. For each of those, we can easily define a structure that contains the necessary fields, we don’t need a client-side translator, and the server-side translator can take care of “forwarding” results from one sub-operation to the next. We could even use GF_FOP_IPC to prototype this. If we later find that the number of “one-off” compound requests is growing too large, then at least we’ll have some experience to guide our design of a more general alternative. Right now, I think we’re trying to look further ahead than we can see clearly. _______________________________________________ Gluster-devel mailing list Gluster-devel@xxxxxxxxxxx http://www.gluster.org/mailman/listinfo/gluster-devel
