Leslie,
A few of the prior messages are proof that my memory is full of holes
these days, so take this with a grain of salt! This is very
unfinished stuff. Cached operations has been tested minimally
interactively, but hasn't been verified via test suites nor used in
any significant way.
As best as I can remember, it should work fine in a "single-user" mode
or in a read-only mode. The open problem for concurrent use is making
cached reads transaction safe in the presence of writes (Alex pointed
out some of the holes in the original scheme in earlier e-mails).
Another issue is maintaining the semantics of index operations within
a transaction, etc. This is going to be tricky to get right.
There may be some stylized usage modes, but it will take some careful
thought to make sure any of these work properly in the presence of
writes and transaction restarts, either concurrent or single-writer.
- Read-only: I can't see a problem with this
- Single-writer: but only if reading stale data is safe...
- Single-writer w/ guard: have a persistent 'locked' slot that acts
as a guard and is touched at the beginning of a transaction by readers
and written by writers at the beginning and end. So long as all
transactions have idempotent memory side effects (which should always
be the case), this should allow you to maintain transaction semantics
for the single-writer case. I'm not confident this actually covers
all cases though. If it does work, you should be able to write a
macro to simplify this; something like:
(with-guarded-instances (:reading instances :writing instances)
&body body)
should be sufficient.
There is also a 'with-cached-instances' macro there which caches reads
only during a transaction, which means that all slots are read at the
beginning. A write through policy on slots will capture write locks
appropriately. I'm also not sure if this covers all cases and will
require some thought as well.
I'm not up to getting the transaction semantics right anytime soon,
but we should be able to write some macros that cleanly encapsulate
any design patterns that we can verify.
Ian
On Sep 27, 2008, at 9:44 AM, Leslie P. Polzer wrote:
Are these supposed to work, or is this unfinished stuff?
--
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