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Re: Problem with aggregation and stateless DAP

James Gallagher wrote:

I talked with Benno about this at AGU (I think) and forgot to relay that conversation to this thread. What I remember was that Benno argued for use of an ETag and/or Last-Modified along with Expires headers. This has the benefit that the DAP stays stateless and it would work.

However... It seems to me that state is ultimately an optimization. I'd like to reopen the discussion and make sure that we get a cogent statement about the situation written somewhere. The answer might be in my email somewhere, but I was going to write a quick summary on the wiki and after scanning the messages for a bit, I couldn't. So...

John: Can you send a description of your idea about adding state and
Benno: Can you send me a description of your idea about using ETag, et cetera?

I was not advocating a solution, I just pointed out that the Last-Modified time would be sufficient "state" if the client specified it in a request.
I would state the problem slightly differently from John:

The problem is to deal with datasets that change faster between the client's dds request and its subsequent dods requests. If the client is behind a sufficiently slow link, it won't be able to request a dds and then request the dods file before the dataset changes. And if it is so silly as to assume that the dds has not changed ...

ETags are one HTTP/1.1 method for handling multiple versions -- but I have no experience with them.

John's problem is particularly nasty because the server deletes data and then essentially reuses the (OpenDAP) url. Someone pointed out that as a data provider he would never do that -- once he uses a url to serve data, he never changes its meaning. So for him, a rapiding changing dataset is only getting longer, and there is no major problem with a server serving to a client that thinks the dataset is shorter as long as the client always asks for a particular coordinate range in the dods request, a reasonable strategy for dealing with the more common problem.


-- Dr. M. Benno Blumenthal address@hidden International Research Institute for climate and society The Earth Institute at Columbia University Lamont Campus, Palisades NY 10964-8000 (845) 680-4450

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