Transactions in REST?

There are a few important cases that aren't answered by this question, which I think is too bad, because it has a high ranking on Google for the search terms :-)

Specifically, a nice propertly would be: If you POST twice (because some cache hiccupped in the intermediate) you should not transfer the amount twice.

To get to this, you create a transaction as an object. This could contain all the data you know already, and put the transaction in a pending state.

POST /transfer/txn
{"source":"john's account", "destination":"bob's account", "amount":10}

{"id":"/transfer/txn/12345", "state":"pending", "source":...}

Once you have this transaction, you can commit it, something like:

PUT /transfer/txn/12345
{"id":"/transfer/txn/12345", "state":"committed", ...}

{"id":"/transfer/txn/12345", "state":"committed", ...}

Note that multiple puts don't matter at this point; even a GET on the txn would return the current state. Specifically, the second PUT would detect that the first was already in the appropriate state, and just return it -- or, if you try to put it into the "rolledback" state after it's already in "committed" state, you would get an error, and the actual committed transaction back.

As long as you talk to a single database, or a database with an integrated transaction monitor, this mechanism will actually work just fine. You might additionally introduce time-outs for transactions, which you could even express using Expires headers if you wanted to.

Consider a RESTful shopping basket scenario. The shopping basket is conceptually your transaction wrapper. In the same way that you can add multiple items to a shopping basket and then submit that basket to process the order, you can add Bob's account entry to the transaction wrapper and then Bill's account entry to the wrapper. When all the pieces are in place then you can POST/PUT the transaction wrapper with all the component pieces.