If you're trying to create a write-through cache, you may want to look at this code:
http://code.google.com/p/objectify-appengine/source/browse/trunk/src/com/googlecode/objectify/cache/CachingAsyncDatastoreService.java It will remain transactionally locked to the datastore except in one condition: a DeadlineExceededException. As long as you don't get a hard cutoff, no amount of write contention can cause it to go out of sync. It doesn't require a write lock, but it does make heavy use of CAS. And transactions ignore the cache until commit, which is really the appropriate behavior. Jeff On Fri, Dec 2, 2011 at 3:16 AM, Crizegg <craig.qui...@gmail.com> wrote: > Looking into this solution, I've jumped farther down the rabbit hole > than anticipated. First off, memcache write failures are not very > common and neglecting to use compare and set was probably the main > source of my issues. > However memcache writes can still fail and regardless of whether they > throw errors, can cause stale data in race conditions. Also, since gae > transactions live solely within the datastore, you cannot solve this > problem elegantly with transactions. See here for more info: > > http://groups.google.com/group/appengine-ndb-discuss/browse_thread/thread/cd05c3c8ac7b27cf# > > On Dec 1, 2:36 pm, Crizegg <craig.qui...@gmail.com> wrote: > > Setting the cache is where my pain point lies. I see zero log messages > > indicating a memcache failed write on the python side. I can't think > > of a case where you WOULDN'T want to know memcache failed to overwrite > > stale data. Since the next update is going to read the stale data from > > memcache and write it to the datastore! > > > > > In Javaland there is an error handler you can set on MemcacheService > > > > @Jeff, this is the exact type of solution I'm after for Python. I'm > > thinking proxy hooks with backed off retries. > > > > Will post soon. > > > > On Nov 30, 10:43 am, "Brandon Wirtz" <drak...@digerat.com> wrote: > > > > > > > > > > > > > > > > > Yes, > > > > > But that is "Drawer failed to open" > > > > > Memcache and the drawer should error when theyfailto be a place that > may > > > or may not contain data. > > > > > Drawer is too busy an can't be bothered right now to check if it has > your > > > socks, is different than there are no socks. Drawer is suffering from > > > non-existence issues there is no drawer. Or if you are on MS. Drawer > only > > > allows you to read socks but you can't store new socks at this time so > stop > > > asking. > > > > > From: google-appengine@googlegroups.com > > > [mailto:google-appengine@googlegroups.com] On Behalf Of Jeff Schnitzer > > > Sent: Wednesday, November 30, 2011 5:13 AM > > > To: google-appengine@googlegroups.com > > > Subject: Re: [google-appengine] Re: What if I dpn't want memcache > tofail > > >silently? > > > > > FWIW, there are good reasons to ask when memcache fails - I mean a real > > > failure (say, timeout exception) rather than mere lack of data. > > > > > For example, if you are put()ing a new value in memcache and you want > to > > > make sure the old value is replaced, you really want to be able to > retry if > > > the put() fails. > > > > > In Javaland there is an error handler you can set on MemcacheService - > the > > > default is to log and ignore errors but you can set a strict error > handler > > > that propagates exceptions. Dunno what the equivalent for Python is. > > > > > Jeff > > > > > On Wed, Nov 30, 2011 at 2:03 AM, Brandon Wirtz <drak...@digerat.com> > wrote: > > > > > My socks are not in the drawer. > > > > > Return Error ("What has the Maid been doing all day?" > > > > > Just because the socks aren't in the drawer doesn't mean the drawer is > in > > > error. > > > > > From: google-appengine@googlegroups.com > > > [mailto:google-appengine@googlegroups.com] On Behalf Of Tim Hoffman > > > Sent: Tuesday, November 29, 2011 9:39 PM > > > To: google-appengine@googlegroups.com > > > Subject: [google-appengine] Re: What if I dpn't want memcache tofail > > >silently? > > > > > Hi > > > > > Its not generally considered an error if something is not in the > cache, (it > > > is after all a cache, not a guarunteed storage.) > > > > > Use a decorator to raise KeyError or whatever you feel is appropriate. > > > > > Rgds > > > > > Tim > > > > > -- > > > You received this message because you are subscribed to the Google > Groups > > > "Google App Engine" group. > > > To view this discussion on the web visithttps:// > groups.google.com/d/msg/google-appengine/-/A3p__F-3o1kJ. > > > To post to this group, send email to google-appengine@googlegroups.com > . > > > To unsubscribe from this group, send email to > > > google-appengine+unsubscr...@googlegroups.com. > > > For more options, visit this group athttp:// > groups.google.com/group/google-appengine?hl=en. > > > > > -- > > > You received this message because you are subscribed to the Google > Groups > > > "Google App Engine" group. > > > To post to this group, send email to google-appengine@googlegroups.com > . > > > To unsubscribe from this group, send email to > > > google-appengine+unsubscr...@googlegroups.com > > > <mailto:google-appengine%2bunsubscr...@googlegroups.com> . > > > For more options, visit this group athttp:// > groups.google.com/group/google-appengine?hl=en. > > > > > -- > > > > > I am the 20% > > > > > -- > > > You received this message because you are subscribed to the Google > Groups > > > "Google App Engine" group. > > > To post to this group, send email to google-appengine@googlegroups.com > . > > > To unsubscribe from this group, send email to > > > google-appengine+unsubscr...@googlegroups.com. > > > For more options, visit this group athttp:// > groups.google.com/group/google-appengine?hl=en. > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine" group. > To post to this group, send email to google-appengine@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=en. > > -- We are the 20% -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. 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