Hey Joel, Your question fits more on the "general discussion" side of the sometimes* ambiguous boundary where it might be too in-depth for Stack Overflow, with too many possible answers, and yet is also a sort-of-specific issue which tends to get redirected to Stack. In the spirit of helping you out regardless, I'll be happy to assist here with some advice.
Cache invalidation is traditionally held up as a class of problem in computing which is devilishly hard to get right. The cardinal sin is to have the system perceive stale data as fresh, although it's acceptable to fetch stale data as long as it's identifiable as such and the path to fresh data is relatively cheap. The race condition you describe is definitely something to avoid, and I see one possible solution: The deletion of the memcache key by instance 1 is clearly meant to prepare the way for a put() to the datastore of the new entity, so we should make the composition of these two actions into one atomic action, so that instance 2 will fail to fetch from memcache and thereafter find the new value in datastore. This can be accomplished as follows: Rather than deleting the memcache key before put, a timestamp can be put in its place (or somehow associated with it), only to be deleted once the datastore entity is safely put. This means that instance 2, attempting to access the memcache key, would notice that it's currently being updated and do extremely small incremental sleeps until it finds the new version available in the datastore. Could you go into a little more detail about your system specifically? I feel this will help spur more concrete discussion of the trade-offs you can make in your specific situation. Regards, Nick Cloud Platform Community Support On Thursday, March 17, 2016 at 1:27:46 PM UTC-4, Joel Holveck wrote: > > By the way, feel free to say this should be on Stack Overflow if that's > more appropriate. I still don't have a good feel for what should be posted > here vs. elsewhere. > -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-appengine+unsubscr...@googlegroups.com. To post to this group, send email to google-appengine@googlegroups.com. Visit this group at https://groups.google.com/group/google-appengine. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/54d3c351-83d7-481d-a46d-0d3225a447ed%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
