Write/Read with High Replication Datastore + NDB

Question

So I have been reading a lot of documentation on HRD and NDB lately, yet I still have some doubts regarding how NDB caches things.

Example case:

Imagine a case where a users writes data and the app needs to fetch it immediately after the write. E.g. A user creates a "Group" (similar to a Facebook/Linkedin group) and is redirected to the group immediately after creating it. (For now, I'm creating a group without assigning it an ancestor)

Result:

When testing this sort of functionality locally (having enabled high replication), the immediate fetch of the newly created group fails. A NoneType is returned.

Question:

Having gone through the High Replication docs and Google IO videos, I understand that there is a higher write latency, however, shouldn't NDB caching take care of this? I.e. A write is cached, and then asynchronously actually written on disk, therefore, an immediate read would be reading from cache and thus there should be no problem. Do I need to enforce some other settings?

Answer 1

I suspect it might be because of the redirect that the NoneType is returned.

https://developers.google.com/appengine/docs/python/ndb/cache#incontext

The in-context cache persists only for the duration of a single incoming HTTP request and is "visible" only to the code that handles that request. It's fast; this cache lives in memory. When an NDB function writes to the Datastore, it also writes to the in-context cache. When an NDB function reads an entity, it checks the in-context cache first. If the entity is found there, no Datastore interaction takes place. Queries do not look up values in any cache. However, query results are written back to the in-context cache if the cache policy says so (but never to Memcache).

So you are writing the value to the cache, redirecting it and the read then fails because the HTTP request on the redirect is a different one and so the cache is different.

I'm reaching the limit of my knowledge here but I'd suggest initially that you try the create in a transaction and redirect when complete/success.

https://developers.google.com/appengine/docs/python/ndb/transactions

Also when you put the group model into the datastore you'll get a key back. Can you pass that key (via urlsafe for example) to the redirect and then you'll be guaranteed to retrieve the data as you have it's explicit key? Can't have it's key if it's not in the datastore after all.

Also I'd suggest trying it as is on the production server, sometimes behaviours can be very different locally and on production.

Answer 2

Pretty sure you are running into the HRD feature where queries are "eventually consistent". NDB's caching has nothing to do with this behavior.