I'm pretty sure the memcached clusters (if there are more than one) are not synchronized. First of all, that would be way too slow. Second, the talk I referenced specifically mentions that when the apps are being migrated from a DC, the memcache writes return success but are in fact noop, because synchronizing memcache data does not make sense. So I would expect that there's no synchronization going on during regular operation as well.
Anyway, I really hope that there's only one memcached cluster active at a time. On 14 May 2011 22:05, rekby <timofey.koo...@gmail.com> wrote: > I thing HR-applications have more than one syncronised memcache - by > me test save in HR-memcache in 3 times slower, than Master/Slave > application. > > On May 14, 6:56 pm, Sergey Schetinin <ser...@maluke.com> wrote: >> So, I was watching the presentation on the HR datastore from the IO >> 2011 >> (http://www.google.com/events/io/2011/sessions/more-9s-please-under-th... >> ) and one thing caught my attention: the slides were showing the >> frontend instances running in more than one datacenter at the same >> time. So I understand that the memcached can lose data at any time and >> if the app is migrated between the datacenters all of the data in the >> cache are lost, however, running the app in two or more datacenters at >> a time each DC having a separate memcached cluster, that changes the >> properties quite significantly. >> >> For example let's consider an app that uses some memcached key to keep >> a cursor where to write to the datastore, atomically incrementing it >> after each write. Such an app can detect if such a key is not present >> in the cache and determine what the cursor is by doing a query on the >> database and then add it to the cache atomically. If we add the >> possibility that there's another DC running the app w/ an independent >> memcached instance, such an app would just corrupt its own data. >> >> I hope I explained my concern well enough, and I would love to hear an >> answer from someone on the App Engine team. >> >> Thank you. >> >> -Sergey >> >> --http://self.maluke.com/ > > -- > 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. > > -- http://self.maluke.com/ -- 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.
