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K. Ari Krupnikov wrote:
> User A GETs a resource and edits it. User B GETs a resource and edits
> it. User A PUTs the modified resource back. User B PUTs her version of
> the modified resource back, unaware of A's edits. A's edits are lost
> without anyone noticing. What I want to happen is B to get a 409
> "Conflict" or some such.
I'm not sure how well REST provides answers to the problem of moving
resources around. Your problem is that the server, user A, and user B all
have copies of the same information, and the server has to decide whether to
accept or reject user B's version after user A has already done a checkin.
One easy solution is to add a sequence number or checkout date to each XML
document:
<pilot-record seq="123">
<name>Ari Krupnikov</name>
<license-type>private - aeroplane</license-type>
<types>
<type>single engine land</type>
</types>
<ratings>
<rating>IFR single engine</rating>
</rating>
</pilot-record>
When I send this back, the server checks my sequence number (or date)
against the highest sequence number checked back in for Ari's data record;
if the last checkin was, say, 111, then my information goes in; if the last
checkin was 125, it rejects (at which point my client can check out the
newest version and perhaps try to merge my previous changes into it).
Of course, it is easy for the client to cheat by simply bumping up the
sequence number and resubmitting, but then again, the client can arbitrarily
change information anyway -- it's just a matter of letting people know when
there might be a conflict.
If you wanted something more secure, you could figure out a system of hashes
and signatures against the original document accompanied by a list of
changes, but that's probably getting into silly territory again unless this
is a defence application or something similar.
As far as REST goes, the URL on the server always points to the server's
current version of the information (the last checkin it accepted) -- that's
about it.
All the best,
David
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