CODEWITHSUNDEEP
google-app-engine
Binarycrayon
Binarycrayon

Reputation: 1045

Is ndb async guaranteed to execute after application request has finished?

I am using ndb to write a profiling model that logs some data per application request. Each request calls a ndb request by ndb.put_async to log the data, while the client do not care about the result. In essence, I do not want the application request to wait for saving statistics data for profiling.

However, I was confused about the explanation from the official documentation. If an application request has finished before the ndb request finishes, would the ndb request still be guaranteed to finish? The documentation indicates that

if the request handler exists too early, the put might never happen

Under what criteria would this happen? Does this mean that regardless of whether a user care about the result, future.get_result needs to be called anyway just to make sure the ndb request is performed?

The original documentation (https://developers.google.com/appengine/docs/python/ndb/async) says:

In this example, it's a little silly to call future.get_result: the application never uses the result from NDB. That code is just in there to make sure that the request handler doesn't exit before the NDB put finishes; if the request handler exits too early, the put might never happen. As a convenience, you can decorate the request handler with @ndb.toplevel. This tells the handler not to exit until its asynchronous requests have finished. This in turn lets you send off the request and not worry about the result.

Upvotes: 8

Views: 703

Answers (2)

Jason A. Collins
Jason A. Collins

Reputation: 176

Thanks for the clarification. What about a general RPC (non-NDB) - e.g., incr_async() in memcache.Client()? Setting aside that this is a very, very fast RPC call, is it guaranteed that the RPC will complete?

I.e., which of the following is true:

(a) there is something in the infrastructure that will wait on all known RPCs before completing the request

(b) the request will complete and the async RPCs will also complete regardless of when the request completes

(c) the in-flight RPCs are formally cancelled

(d) something else?

Upvotes: 1

Takashi Matsuo
Takashi Matsuo

Reputation: 3436

If an application request has finished before the ndb request finishes, would the ndb request still be guaranteed to finish?

No.

Does this mean that regardless of whether a user care about the result, future.get_result needs to be called anyway just to make sure the ndb request is performed?

Basically yes, but you can use ndb.toplevel decorator for the convenience so that you don't have to wait for the result explicitly. That said, I don't think this is what you want.

Probably taskqueue is what you want. Please check it out.

Upvotes: 7

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