[OAI-implementers] protocol comments, OAI 2.0
Fri, 25 Jan 2002 14:00:06 -0800
Themes: use SOAP, make it stateless, make it simpler.
SOAP is being implemented everywhere. Even AppleScript can make SOAP
calls. The last time I saw this many vendors use one standard was
Ethernet. So replace the custom XML protocol with SOAP.
A stateless protocol can be cached outside of the server,
so that is a very valuable thing to do. The current spec
has a defensive tone about load. That should not be necessary.
A few modifications will make the protocols stateless and
ListIdentifiers and ListRecords are stateful now. They should be
replaced with a "paged list" model, where the client requests a
starting element number and a number of results. This is an exact
match for the normal web results interface. It is also the
approach used in the LDAP virtual list control.
ListIdentifiers/Records also needs to be very clear about the
contents of successive calls for different parts of the
list. Consider a rapidly changing repository, like a newswire.
The results for the list may change between calls. The list
is not part of some transaction, where the contents don't change
for the duration of the session. If a client really needs a
consistant list, it can ask for the whole thing. Each request
for a portion of the list is independent and can be cached.
Datestamps are problematic in protocols, and should not be
used. Computers insist that you choose some time for that
day. Is it noon? One minute after midnight? How do you compare
that to a time on the same day? So don't allow datestamps in
protocols. Always use timestamps to the second, in UTC. Don't
allow timezones or less precision.
Of course, the metadata may contain date-only times.
Since deleted record items are not reliable, they are not all
that useful. After the robot is burned the first time, the
implementors will switch to polling the entire repository.
They are sort of useful as hints, but I can see serious
problems in some uses. In the newswire example, it is common
to have a limited time right to the news articles, perhaps
two weeks. So a reliable list of deleted items would grow
without bound. Not good.
Robots can use something like the HTTP if-modified-since request.
This would be a parameter for GetRecord. If the record has
been modified since the timestamp, return it. Otherwise, return
the info that it has not been modified. Implementors should try
and make this request fast.
I have not looked at the interaction between SOAP and HTTP caches.
It is possibled that they can be cached. If so, take some extra care.
A GET can be satisfied by a cache, but a POST cannot. Properly
setting the HTTP headers on responses means that a server can rely
on an external HTTP cache rather than managing an internal cache.
This is a big win.
The "Set" concept is optional and I don't see much motivation for it.
I don't see a request which allows multiple sets, so there is no
reason to have both sets and repositories. Except for listing them
(ListSets), which is a limited sort of directory service.
In SOAP-land, directory service is the job of UDDI, and it should
not be re-implemented differently in a protocol. So remove Sets.
A single SOAP server can still be registered for multiple repositories,
or corpora, or collections, or whatever they are called.
I have a few minor nits, like there is no reason to outlaw XML
features which are required in all parsers, like UTF-16 or decimal
entities. But switching to SOAP makes lexical issues moot.
Overall, the protocol is good. Using URIs, Dublin Core, HTTP/XML,
all those things are exactly right. The spec is clear and readable.
And I'm delighted that it doesn't use RDF.
Walter R. Underwood
Senior Staff Engineer
Inktomi Enterprise Search