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I'm trying to use appengine datastore to share a key between appengine modules.
I'm writing the key in one module, I can read it from that module.
Trying to read it from the second module doesn't work, and I get no such entity response.
What's the best way to share a key between modules? can I use datastore for that? from what I have read in the documentation data store is shared between services. but it doesn't seems to work for me.

There are eventual consistency issues you may be hitting. Datastore is distributed so it's possible one service hits a consistent version of the data and another service(module) hits another version that is stale. The datastore is definitely a way to share data between modules so it could either be a consistency problem where you won't be guaranteed to be able to read the data back right away unless you restructure your data or your query to be strongly consistent, or it could be some other problem like you have the wrong key in the other module or are somehow hitting a different namespace.
Here are a some resources on datastore consistency:
https://cloud.google.com/datastore/docs/concepts/structuring_for_strong_consistency
https://cloud.google.com/datastore/docs/articles/balancing-strong-and-eventual-consistency-with-google-cloud-datastore/
https://cloud.google.com/datastore/docs/best-practices
From the last link:
If you need strong consistency for your queries, use an ancestor
query. (To use ancestor queries, you first need to structure your data
for strong consistency.) An ancestor query returns strongly consistent
results. Note that a non-ancestor keys-only query followed by a
lookup() does not return strong results, because the non-ancestor
keys-only query could get results from an index that is not consistent
at the time of the query.