Google Cloud端点与Android - 缓存响应？

Question

I am using Google Cloud Endpoints (Java) as a backend for an Android app. The data on the backend is stored to Datastore .

The problem is that the Android client often doesn't receive correct data from the backend. After repeating the request, the correct data is eventually received.

Can you tell me what could cause this problem ? Is there some kind of response caching in the Cloud Endpoints library ? Or caching within Datastore ?

I have read about eventual consistency provided by Datastore, so I am trying to wait at least 10 seconds after updating the entities before the next request with query. Is it enough ?

Thanks a lot.

Answer 1

Straight from the docs at Structuring Data for Strong Consistency : If you always require the result of a Datastore query to be consistent, you will need to use an ancestor query as per the example code in the article:

Query query = new Query("Greeting", guestbookKey).setAncestor(guestbookKey);

You can't rely on a delay to obtain consistency as it depends on replication across data centers and the amount of time this takes is never, well, consistent :)

Answer 2

It sounds as though the way you structured your data model is not compliant with the way in which you want to use the data model. Without seeing an example of your data model I can't give you hints as to how to change it but here is a simple example that might help you.

Suppose I have a "Post" entity and a "CommentForPost" entity. Users can create many CommentForPost entries for each Post entry. Logically, the CommentForPost is a child of Post but if you don't make the Post an ancestor of all the CommentForPost entries then immediate consistency is not guaranteed. If user A creates a comment and then queries for it they may not see the entry right away.

On the other hand, if you make the Post entity the parent of all of the CommentForPost entries then you will guarantee immediate consistency because they are being saved as a single EntityGroup. If user A creates an entry and immediately queries for it (using the Post key as an ancestor) then they are for sure going to get accurate data back.

The limitation here is that you would only be able to create one CommentForPost entry per second. This is where the give and take lays. If you need a lot of updates within short periods of time then you can't guarantee consistency (eg 100 users are all adding CommentForPost entries at the same time). If you want to guarantee consistency then you can't have a lot of updates within a short period of time.

Make sense?