Hey Jeff,
That is more-or-less the best I've come up with as well. My flow looks like:
1) produce a transaction marker entity with a deterministic key-name
from the involved "accounts",
2) Start transaction:
2.a) Load the involved entities (including the marker).
2.b) If I get a marker, abort.
2.c) Do the stuff that needs doing....
3) At some later (possibly much later) time, cleanup the markers.
In some cases, when I've got revision numbers on the entities, I
improve the process by cleaning the marker up after the transaction.
But if it is valid for an account to be updated multiple times,
provided some condition is met, I leave the marker. Note that nearly
all of my processing is task-based, so tasks could run multiple times,
or rerun with a delay, etc.... That is why I leave the markers for a
while.
Robert
On Fri, Jan 20, 2012 at 00:22, Jeff Schnitzer <[email protected]> wrote:
> I'd like to guarantee a transaction runs once, and once only. Let's
> say I'm simulating a bank transfer of $5 from AccountA to AccountB.
>
> The normal transaction is not idempotent:
>
> Repeat until success {
> * start txn
> * debit $5 from AccountA
> * credit $5 to AccountB
> * commit txn
> }
>
> If this inadvertently runs multiple times, I will have very unhappy
> customers. Maybe I should do this:
>
> Create TxnID entity instance
> Repeat until success {
> * start txn
> * load TxnID (if null, return success immediately)
> * debit $5 from AccountA
> * credit $5 to AccountB
> * delete TxnID
> * commit txn
> }
>
> With the pain of adding a third entity to my XG transaction (and a
> cron that prunes really old, abandoned TxnIDs) it seems that I this
> does the trick. Or am I missing something? Is this the best way to
> accomplish once-only transactions? Is there an alternative approach?
>
> Thanks in advance,
> Jeff
>
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