"I have a mind like a steel... uh... thingy." Patrick Logan's weblog.

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Wednesday, December 24, 2003

Thoughts heading into 2004

Whatever else Jesus was, he was almost certainly a radical.

He challenged the mainstream religious authority.
He challenged the mainstream governing authority.

He challenged the mainstream attitudes toward those who are not in the mainstream.

My wish for us all is peace and true prosperity.
May we all take one radical step in that direction in 2004 and we'll be more than a billion steps closer.

Tuesday, December 23, 2003

REST and Linda for Distributed Coordination: Elaboration vs. Layering

Mark Baker makes an interesting distinction in my wRESTling with tuple spaces...

Patrick seems stuck with how to reconcile his position that generic abstractions are a good thing, but that systems should be built independent of the protocol. Note to Patrick; this is all well and good for transport protocols, but application protocols define the abstraction; for them, protocol independence requires that you disregard that abstraction.

This distinction of transport protocols vs. application protocols is exactly what I am wondering about REST. As I read the definition of REST, the architectural style being described is for a transport protocol rather than an application protocol. Not much is said really about the behaviors of the client or the server. Even when you bring HTTP per se into the definition of REST, Fielding makes a somewhat confusing statement about transport protocols...

HTTP is not designed to be a transport protocol. It is a transfer protocol in which the messages reflect the semantics of the Web architecture by performing actions on resources through the transfer and manipulation of representations of those resources.

Is it a transport protocol or not? Let's ignore that and pursue the part about "performing actions on resources" because that *does* seem to be about an application protocol. Fielding continues...

It is possible to achieve a wide range of functionality using this very simple interface, but following the interface is required in order for HTTP semantics to remain visible to intermediaries.

And so this is where I begin to have problems with REST, as I read it, as an application protocol for distributed system coordination. The problem is not that it is inappropriate, but rather that it is too vague.

I don't mean "vague" in a derogatory manner. What I mean is exactly what Fielding writes, i.e. it is possible to implement a wide range of functionality using this very simple interface.

How is this different from the tuple space interface? I have written, and many others better than I have written, that it is possible to implement a wide range of features using the very simple tuple space interface.

The difference is this: the HTTP interface is vague and the Linda interface is specific. Linda has precise, simple semantics. The possible range of behaviors exhibited in Linda-based systems benefit from being layered on *top* of those precise, simple semantics.

HTTP, on the other hand, has to be *elaborated* into something more specific in order to have a useful meaning as an application protocol. WebDAV is an example of such an elaboration.

Every web site that implements custom behavior using forms with GET or POST is an example of the open ended nature of HTTP per se. The architectural style of REST supports the HTTP transport protocol underlying these forms moving across the web, but the application protocol, that is, the behavior of the forms on the client and especially on the server is defined (at least in code) by each specific instance.

Distributed systems wishing to use HTTP, or more generally REST, to perform coordinated work will therefore require some more specifically defined application interface than that provided by REST, or that provided by HTTP.

WebDAV is one option as stated already, and it is proven to be viable in some specific cases. I don't believe the full range of systems that can be usefully built with WebDAV has been exhausted. By the same token, neither do I see a lot of evidence of that range being nearly as broad as that of Linda tuple spaces.

Vanessa Williams provides an elaboration of HTTP for a tuple space application protocol. As I understand REST this should therefore provide the application protocol of a tuple space on the architectural style of REST using the HTTP transport/application protocol mix. In this case the advantage of using REST and HTTP is supposed to be found in the hardware and software that would already be in place between the client and the server.

I think and hope this is fairly accurate. I am eager to be clued in further by Mark and others. I am still unsure that this advantage is significant over a less pure elaboration of HTTP, as in XML-RPC or the arguably more RESTian SOAP. I think there is a lot to be said for something else altogether as a transport for tuple spaces, in particular Jabber or perhaps Spread. The bottom line is the usage models of distributed systems coordination would benefit from a well defined, simple, axiomatic application protocol, but the best transport protocols *will* have to evolve just because the usage models themselves will have to evolve. For all but a handful of services (e.g. Google), they just may act nothing like today's web.

What Kinds of Queries?

Queries: reportedly, Adam Bosworth said at the XML conference...

"I don't know how we're ever going to truly optimize these queries."

Before we worry about that, though, we should consider how the average user is going to form those queries. The last twenty years have made great strides in forming SQL queries for numerical data analysis. The models underlying those queries fortunately support both the relatively non-technical users who wish to form them as well as the highly technical adminstrators who wish to optimize them.

What kinds of queries do these users wish to form? What are we trying to optimize? What relationships will these have with current typical business analysis?

Will we wait twenty years for the evolution to settle into a widely used winner?

These Changeable Things Avalon Has

Update: I almost forgot Doug Lea's thoughtful design and implementation of a collection package, which predates the java.util collections. That specific link will take you directly to his wisdom on designing type checked immutability. (Which makes me wonder if Microsoft has hired him yet too?!)

Greg Schecter writes about the Changeable class. Not knowing much about the implementation this seems like a reasonable concept. (EmeddedChangeableReader sounds like it might be a bit over the top, but I don't know.)

Compare this to Object.freeze in Ruby. The Ruby implementation appears to be simpler, but the overall lesson I think is this: what we have in the Changeable class is more evidence of Java-like languages struggling with their underlying dynamic tendencies.

Inside every statically type checked language there is a dynamic language struggling to get out.

The simple statically checked approach is to create a class without mutation, e.g. Brush, and then add mutation in the subclass, e.g. MutableBrush. Another approach is to simply throw an exception in a method that would otherwise cause a mutation without any system-wide designation in the code. See the Java Collections API for example. In particular the unmodifiableCollection static method and its siblings for List, Set, etc.

Then again there is the option of just allowing mutation for all instances even when it's not expected. One of the first actions I took in my first Smalltalk system (the Tek 4404) was to change Black to White. Oops. Fortunately although Smalltalk has a persistent image, it's not difficult to back out of the change.

Wanted: Comparison of XML-based Query and Table-based SQL

Jon Udell writes about XML for the Rest of Us wherein Adam Bosworth writes...

"The relational database is designed to serve up rows and columns," said BEA's Adam Bosworth in his keynote talk. "But our model of the world is documents. It's 'Tell me everything I want to know about this person or this clinical trial.' And those things are not flat, they're complex.

I agree with the idea of semi-structured searching and manipulations. But I don't expect anyone would deny we still need traditional (e.g. business) calculations and those will be well served by more structure than less. I'd like to see a more direct XML Query and SQL comparison. As it stands, I'm being led to believe I'll should use XML Query (i.e. something with XPath-like stuff in it) for doing non-numerical property-tree searches over data that has been or could be expressed in an XML text; but I should use SQL for doing relationally flat table-column calculations over data that has been entered into a relational database.

There is a merger out there somewhere that I'm not seeing, or maybe just an example of what such a merger might look like.

Useful InfoPath Session at PDC

Watching this PDC session on InfoPath is worth the time. Better than previous demonstrations and white papers I've read, especially if you looking for somewhat technical information for InfoPath in the context of a modern Microsoft-based IT shop.

Monday, December 22, 2003

My First Computer: the IBM 5100

A collection of first computer stories relayed by Dan Gillmor.

My first computer was an IBM 5100. Flip the switch up, you're running BASIC. Flip the switch down, you're running APL.

I'd hardly call it a portable computer though. The box was not even as mobile as those "luggable" computers like the Kay Pro from a few years later.

What's On Public Radio

http://www.publicradiofan.com has a listing of, well, audio links to what's on public radio right now.

Teddy Bear's Picnic on DVD in February

Great news from Harry Shearer...

Now, some real Xmas cheer: many of you have been asking when you can buy my film "Teddy Bears' Picnic" on DVD. The answer, apparently, is this February. Be warned, you may have some trouble finding it, since, as the result of a monthlong battle lost this week, the cover art on the box will more closely resemble Porky's 4.

This is merely proof that, while it's now easier to make a movie outside the Hollywood mainstream, to get it seen one still has to run the usual gauntlet of Visigoths. These particular barbarians, in a nice twist, are Canadian, and they appear to believe that the target audience for this movie is teenage boys. I'd share the art with you, but I promised a low-impact blast, so take my word for it--the bikini-clad blonde who dominates the cover art appears nowhere in the film.

If you'd like to send a greeting to the primate who insists on this approach, his address is: rmanis@thinkfilmcompany.com. Yes, thinkfilm. When your business is irony, even that comes back to haunt you.

More on tuple spaces

Phil Windley expresses an interest in tuple spaces and points to an item that spells out in some detail why I am still unsure of what ReST really means at a deep semantic, implementation, and performance level. I wrote about this in a couple of (admittedly superficial and this is more of the same) items in April 2003.

Whether using "pure" HTTP, or SOAP, XML-RPC, or Jabber, (or SOAP over Jabber, XML-RPC over SMTP, SOAP over BEEP over HTTP, or... even RSS, RSS-Data, and polling or cloud APIs as part of the transport) the key is to distinguish transports from actions.

A tuple space (or XML space, where the tuples are represented as XML text) for distributed, asynchronous computing can be (and should be) implemented on top of multiple transports. The important aspect for applications are the actions (tuple space semantics). The implementation and performance of the transports can (and should) evolve independent of the simple semantics of tuple space actions.

Above the simple actions, independent of the transport implementations, the simple tuple space actions can be combined into various kinds of databases, queues, exchanges and marketplaces that we really want to focus on evolving in the first place.

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About Me

Portland, Oregon, United States
I'm usually writing from my favorite location on the planet, the pacific northwest of the u.s. I write for myself only and unless otherwise specified my posts here should not be taken as representing an official position of my employer. Contact me at my gee mail account, username patrickdlogan.