On 07/05/2008, at 3:57 PM, Toby Corkindale wrote:
On Wed, May 07, 2008 at 03:30:12PM +1000, Adam Clarke wrote:
The solution suggested in "Restful Web Services" is to POST to a
"factory"
resource which creates you with a transaction resource. e.g. "POST
/transactions/account-transfer" returns "Location:
/transactions/account-transfer/11a5", where the 11a5 is a unique
transaction identifier.
Then "PUT /transactions/account-transfer/11a5/accounts/checking/
11", where
11 is the account identifier. The body carries the transaction
details, in
the example the balances are adjusted absolutely, i.e.
"balance=150". A
similar PUT is sent for the other account.
Once the required components of the transaction have been PUT it is
possible to rollback by DELETEing the transaction resource or
commit it by
putting "committed=true" to the resource.
While seeming a bit fiddly, it does keep the state on the client
and allows
the client to make (at least some of) the commit / rollback
decision rather
than (only) the server.
I've read parts of RESTful Web Services, but not that bit.. I'll
have to go
back and look.
I wonder how one goes about implementing such a transaction on the
server
side.. One would not want to lock DB rows indefinitely, waiting for
the client
to finally complete the transaction. But if one just recorded the
queries and
then executed them all (internally) at the end, then other risks
exist, eg:
I haven't done this before, but I have thought about it a bit. I think
I would handle this as a two-phase commit. PostgreSQL has "PREPARE
TRANSACTION" which allows you to start a transaction and assign it a
"transaction_id" for use with a subsequent "COMMIT TRANSACTION". I
would also use Multi-Version Concurrency Control (MVCC) rather than
any kind of blocking locks to minimise the impact of the longer
transaction lifetime.
This would at least keep a good deal of the hard work in the DB.
Cheers
--
Adam
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