[tahoe-lafs-trac-stream] [tahoe-lafs] #1869: pluggable backends: serialize backend operations on a shareset

[tahoe-lafs]

#1869: pluggable backends: serialize backend operations on a shareset
-------------------------+-------------------------------------------------
     Reporter:           |      Owner:
  davidsarah             |     Status:  new
         Type:  defect   |  Milestone:  1.11.0
     Priority:  normal   |    Version:  cloud-branch
    Component:  code-    |   Keywords:  cloud-backend storage shareset
  storage                |  cache design-review-needed
   Resolution:           |
Launchpad Bug:           |
-------------------------+-------------------------------------------------
Changes (by daira):

 * keywords:  cloud-backend storage shareset cache => cloud-backend storage
     shareset cache design-review-needed
 * owner:  daira =>


Comment:

 Okay, I think I understand how this will work now. We'll have a
 !WeakValueDictionary of !DeferredLocks, indexed by storage index, in each
 backend object. The lock object will be passed on to !ShareSet,
 !BucketReader, and !BucketWriter objects for that SI, and will be used to
 lock the remote operations. (Most of this can be done generically without
 only minimal support in the backend-specific objects.)

 A subtle complication is that because the locks are associated with
 sharesets, the {{{get_sharesets_for_prefix}}} method on a backend needs to
 have a rather weak specification:

 {{{
     def get_sharesets_for_prefix(prefix):
         """
         Return a Deferred that fires with an iterable of IShareSet objects
         for all storage indices matching the given base-32 prefix, for
         which this backend holds shares.

         Separate locks are taken for each shareset, and nothing prevents
         sharesets matching the prefix from being deleted or added between
         listing the sharesets and taking these locks. Callers must be able
         to tolerate this.
         """
 }}}

-- 
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1869#comment:4>
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