CODEWITHSUNDEEP
google-app-engineasynchronousgoogle-cloud-datastore
bgenchel
bgenchel

Reputation: 4059

Synchronicity and the Datastore in Google App Engine

I seem to be having a consistency problem with some of my data; i'm writing a unit test to see if a certain model has been placed in the datastore. It fails in the unit test unless I put a 5 second sleep before the return of the storing function.

I've been reading about asynchronous functions in gae, thinking that perhaps I need something along the lines of a promise so that the function will wait before returning until the data has been placed into the datastore. However, all the documentation on asynchronous versions of functions in GAE seem to imply that its non async functions already sort of act like promises in that way.

What does it mean for a function like put() to return? It seems to not mean that the data has been appropriately stored. Is there a way to wait until the data has been stored?

EDIT: My problem wasn't simply dealing with consistency, but that I was unsure of whether the problem was a consistency issue at all, and wanted instead to ask specifically about how the return of call to put() related to what was happening under the hood of GAE.

I think this question is similar to that listed, but is still useful to remain up because it approaches the consistency issue from a different perspective. If other people need to find this information, but aren't entirely sure of the phrasing as I was, or follow a similar train of thought as me, they may be able to reach the information through this question. It's also written more explicitly, with less domain specific terminology.

That being said, I do see the issue in terms of end-goal informational content; I would understand if it's taken down.

Upvotes: 0

Views: 70

Answers (1)

thegeebe
thegeebe

Reputation: 625

https://cloud.google.com/appengine/docs/java/datastore/#Java_Datastore_writes_and_data_visibility

Data writes happen in two stages, commit and apply. Commit records the transactions to a majority of replicas, and apply does two things in parallel: 1) writes the data and 2) writes the indexes.

Your unit test query may be executing on a replica that has a stale version of the data. The write operation returns immediately after the commit phase but the apply phase happens asynchronously. Ancestor queries are guaranteed to be up-to-date, however, so try testing by getting on the object key.

Upvotes: 1

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