In watching this discussion about cacheing versus mirroring, I note that
people are tending to assume that certain behaviors are coupled. In my
experience, unless there is something intrinsic that couples them,
assuming coupling is a mistake. Several dimensions each of which could
be independent follow:
Push vs Pull - Did the origin server (treating such as well defined for
now), decided to put the information in the "sometype" replica, or did
the "sometype" replica decide to get / keep the information?
Expiration - Is the "sometype" replica responsible for expiring the
information based on its own clock, or will the information provider
notify it when the information should be considered invalid.
(Obviously, there are combinations.)
Full versus partial copying - This ne is slightly different in that I
doubt that one would ever build the system around "copy the entire
server". Even "full copying" would just be some subtrees. And one
could easily imagine cases where not even the entire tree from a given
point down was copied.
Scripts - There is a tendency to assume that script execution goes with
Push and originating execution. While that is the current state of
practical affairs, binding that into the terminology and taxonomy of the
system would seem to be a mistake. One can imaging, as the technology
gets more sophisticated, that partial databases and scripts could get
replicated in a demand fashion with distributed refresh and other
properties.
The point is that in trying to build the taxonomy we need to take into
account both what exists and what couplings are intrinsic versus
coincidental. The taxonomy should not mandate couplings that may well
not be needed in the future. This is closely related to the name of the
group. The point of the exercise is that there truly are a spectrum of
behaviors in many dimensions, and we need ways of talking about them.
There may be useful names for certain specific combinations, because
they are frequently built. But we should be careful not to confuse that
with the general elements of the system.
Yours,
Joel M. Halpern
PS: I think that actually many members of this group have a similar view
to what I am trying to say above, but I could not detect it in the
recent conversations.
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