I miss some Redis functionality in App Engine as well. Memcache is just an 
unreliable cache to hold some data for while... nothing more.

To make such calculations which iterate over large sets of data, I use 
backends with in-memory processing: loading part of the data from datastore 
into memory, spawn multiple threads (if applicable) and iterate over data. 
Ugly, strange, error-prone and sometimes slow, but it works.

A bomb-to-kill-an-ant solution would be using Google BigQuery. I don't like 
like the idea, but depending on your problem it can solve it for you.

You can try to use some MapReduce processing as well. But since I'm using 
Java (a not so loved language in App Engine, see servlet 3.0 
discussion<http://code.google.com/p/googleappengine/issues/detail?id=3091>) 
MapReduce (Mapper, actually <http://code.google.com/p/appengine-mapreduce/>) 
is too experimental to put in production (after the Conversion and Files 
API, I learned my lesson: never ever ever use an experimental API in App 
Engine).

Anyway, you have several options to try. I just recommend you to avoid 
storing large datasets on Memcache, since it's just a cache and can wipe 
your data at any time - invalidating your calculations.

On Tuesday, November 12, 2013 6:07:34 PM UTC-2, Mathieu Simard wrote:
>
> Since there is no appengine solution available such as the Redis atomic 
> list, I'm left wondering how to implement a cost effective rolling median.
> Has anyone come up with a solution that would be more convenient than 
> running a redis instance on Google Compute Engine?
>

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