Question

I’m new to Google App Engine, and I’ve spent the last few days building an app using GAE’s Memcache to store data. Based on my initial findings, it appears as though GAE’s Memcache is NOT global?

Let me explain further. I’m aware that different requests to GAE can potentially be served by different instances (in fact this appears to happen quite often). It is for this reason, that I’m using Memcache to store some shared data, as opposed to a static Map. I thought (perhaps incorrectly) that this was the point of using a distributed cache so that data could be accessed by any node.

Another definite possibility is that I’m doing something wrong. I’ve tried both JCache and the low-level Memcache API (I’m writing Java, not Python). This is what I’m doing to retrieve the cache:

MemcacheService cache = MemcacheServiceFactory.getMemcacheService();

After deployment, this is what I examine (via my application logs):

1. The initial request is served by a particular node, and data is stored into the cache retrieved above.
2. The new few requests retrieve this same cache and the data is there.
3. When a new node gets spawned to serve a request (from the logs I know when this happens because GAE logs the fact that “This request caused a new process to be started for your application ..”), the cache is retrieved and is EMPTY!!

Now I also know that there is no guarantee to how long data will be in Memcache, but from my findings it appears the data is gone the moment a diff instance tries to access the cache. This seems to go against the whole concept of a distributed global cache no?

Hopefully someone can clarify exactly how this SHOULD behave. If Memcache is NOT suppose to be global and every server instance has its own copy, then why even use Memcache? I could simply use a static HashMap (which I initially did until I realized it wouldn’t be global due to different instances serving my requests).

Help?

Answer 1

Yes, Memcache is shared across all instances of your app.