CODEWITHSUNDEEP
javagoogle-app-enginecaching
honzajde
honzajde

Reputation: 2398

Reliable distributed cache on app engine (Java)

I need to keep some values in memory, sort of in-memory db. In terms of reliability, I am not affraid of system failure, I can live with that. However, I can not use memcache service, because the values can be evicted anytime. I need the values to be available on other machines, when application scales. I suppose that appengine will not make memory scale or will it (e.g. if I keep value in an ordinary Java collection)?

What I am trying to achieve here is a "pick a nickname" service. This works in two steps. First, user reserves a nickname. Then he registers the nickname. Nicknames are stored under one entity group (sic!). Therefore I need to avoid datastore contention.

As far as I understand from the https://developers.google.com/appengine/articles/scaling/memcache I can to a certain extent rely on that values in memcache should not be evicted on arbitrary resons. However, I have to count on that this will happen from time to time (e.g. on high memory levels). And this losses of value are very unpleasant to my users.

Upvotes: 1

Views: 254

Answers (2)

Paul Collingwood
Paul Collingwood

Reputation: 9116

Your application shares a single instance of Memcache, it is not local to a "machine" (or rather instance of your application).

So if you are running 2 instances and they both retrieve the same value from memcache they will both get the same value.

Running an "in memory" database is not feasible in the cloud - what memory is it you were planning to use, the memory in the instance that's about to shut down?

https://developers.google.com/appengine/articles/scaling/memcache

When designing your application, take the time to consider which datasets can be cached for future reuse. These could be commonly viewed pages or often read datastore entities, just to name a few. There may also be some data in your application which you would like to have shared among all instances of your app but does not need to be persisted forever. In such cases, memcache can improve the scalability of your app by providing a fast and efficient distributed storage system for transient data. Adding memcache logic to your server side code is often well worth the few extra lines of code.

Upvotes: 1

voscausa
voscausa

Reputation: 11706

You can use app engine NDB, when you use Python27. NDb is a datastore with auto caching and much more.

Other Machines ? You mean shared between instances of the same app.

Upvotes: 0

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