[Gluster-devel] compound fop design first cut

[Jeff Darcy]

On December 8, 2015 at 12:53:04 PM, Ira Cooper (ira at redhat.com) 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.