Question

I’m using JDO to access Datastore entities. I’m currently running into issues because different processes access the same entities in parallel and I’m unsure how to go around solving this.

I have entities containing values and calculated values: (key, value1, value2, value3, calculated)

The calculation happens in a separate task queue.
The user can edit the values at any time.
If the values are updated, a new task is pushed to the queue that overwrite the old calculated value.

The problem I currently have is in the following scenario:

1. User creates entity
2. Task is started
3. User notices an error in his initial entry and quickly updates the entity
4. Task finishes based on the old data (from step 1) and overwrites the entire entity, also removing the newly entered values (from step 3)
5. User is not happy

So my questions:

• Can I make the task fail on update in step 4? Wrapping the task in a transaction does not seem to solve this issue for all cases due to eventual consistency (or, quite possibly, my understanding of datastore transactions is just wrong)
• Is using the low-level setProperty method the only way to update a single field of an entity and will this solve my problem?
• If none of the above, what’s the best way to deal with a use case like this

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background:

At the moment, I don’t mind trading performance for consistency. I will care about performance later.

This was my first AppEngine application, and because it was a learning process, it does not use some of the best practices. I’m well aware that, in hindsight, I should have thought longer and harder about my data schema. For instance, none of my entities use ancestor relationships where they would be appropriate. I come from a relational background and it shows.

I am planning a major refactoring, probably moving to Objectify, but in the meantime I have a few urgent issues that need to be solved ASAP. And I’d like to first fully understand the Datastore.

Answer 1

Obviously JDO comes with optimistic concurrency checking (should the user enable it) for transactions, which would prevent/reduce the chance of such things. Optimistic concurrency is equally applicable with relational datastores, so you likely know what it does.

Google’s JDO plugin uses the low-level API setProperty() method obviously. The log even tells you what low level calls are made (in terms of PUT and GET). Moving to some other API will not on its own solve such problems.