Question

I’m developing a spring based web application that is deployed on Google app Engine.
I have a manager that store data in application scope. I’m using a spring bean (singleton) that hold a map and simply perform get and remove from the map,
however GAE is a distributed environment and this design has a problem since each application instance will have its own manager and request are not guaranteed to be made to the same application instance.
So I’ve looked around and found 2 possible solution:

1. use data store
2. use memcache

storing the data will cause a lot of read and writes and eventually I do not need the data to be saved.
second looked promising but Google mentioned that:

In general, an application should not expect a cached value to always be available.

and I need a guarantee that when I ask my manger to get a value it will return it.

is there any other solution?
did I miss anything?

Answer 1

A common solution is storing the values in a memcache, backed by the datastore.

First fetch the application-scope value from the memcache, and if the memcache returns zero-result (a cache-miss event), fetch the value from the datastore and put the fetched value on the memcache.

In this way, the next fetch to the memcache would return your application-scope data, reducing the need for a (relatively) costly read to the datastore.

App Engine memcache is a least-recently used cache, so values that are frequently read would suffer from few cache-miss event.

A more complex solution to have a application-scope value is to store the values in-memory at a resident backend, and have all your other instances request/update the value from/to this particular backend via a servlet handler.