Are there any issues with single-items conditional writes?

[kappolo]

Hello,

I know this may sound as a silly question, but does your library support conditional writes on single objects, provided that those writes are included in a transaction?
Since you are storing temporary images of the objects not yet committed in a temporary table, I was wondering how this will affect the case of conditional writes.
What if within the same transaction a conditional write fails, I therefore retrieve the current state of the object and I retry to write? Would you envision any issue with that?

Thanks

[Yanacek]

The library doesn't currently support user-specified conditional writes. Behind the scenes, it's doing conditional writes everywhere to drive the transaction, so we didn't add the complexity of mixing in user-specified conditions with those. However, you can achieve the same functionality that a conditional write would give you by:

1. Start a transaction
2. Call GetItem for your item (on the tx object)
3. Evaluate the condition you want to look for in your own code
4. Call PutItem or UpdateItem with your changes (again, on the tx object)
5. Commit the transaction

If the transaction had been rolled back, that means that a different transaction was modifying those items concurrently, and none of your writes in that transaction went through.

[kappolo]

Thanks for the explanation!

[Yanacek]

No problem. It's a good question that we've gotten more than once, so we published a blog post describing this in more detail: http://java.awsblog.com/post/Tx13H2W58QMAOA7/Performing-Conditional-Writes-Using-the-Amazon-DynamoDB-Transaction-Library