[CAP] Basic Questions on CAP

[Art Botterell]

Bill -

On Nov 10, 2005, at 11/10/05 11:07 AM, you wrote:
> I am trying to understand the differences between a CAP based  
> implementation
> vs XML - is this a fair question or are we talking apples and oranges?

Apples and fruit, actually... CAP is a particular XML document type.

> Are users expected to poll servers or is there some other mechanism?

The CAP content standard doesn't specify any single transport  
mechanism.  Some folks implement an RSS-style polling arrangement,  
others a database-query poll, a few have experimented with XMPP push,  
and there's quite a bit of work being done on multicast push.

Personally I expect this to converge on a small set of transport of  
schemes, but right now we're in the thousand-flowers-blooming period.

> Specifically for the NOAA site - if it is all about me polling a  
> "feed" why
> choose CAP over XML or even RSS?

As noted above it isn't really an either/or... but if the question is  
"why CAP?" the answer is that CAP instantiates a bunch of social- 
science research on what works best in the form and contents of  
warning messages across all disciplines.

> For example, if I want to get NOAA Alert messages for Florida to  
> pass on to
> my customers, do I just hit their web page every 60 seconds forever?
>
> That would not seem the best approach for an Emergency Specific  
> Protocol.

Well, it depends.  Web-based polling is actually fairly efficient at  
the server end (at least it is if each query is answered with a  
cached text page rather that querying the underlying database on  
every hit.)  So it scales relatively well to provide large-scale  
"anonymous" access.

TCP/IP-based push mechanisms have less latency, but they require the  
sender or some intermediary to keep track of each and every  
recipient.  That's good if you're operating a private feed, but it  
gets tricky when we start talking about tens of thousands or more of  
consumers in a public warning application.

Multicast-based push (datacasting) can be very efficient, but it's  
hard to manage on large open networks, which is why it isn't  
generally supported over the open Internet.  Howver it's been very  
successful in broadcast and satellite applications.

> Pointers to a newbie FAQ would be appreciated

We're still learning what the frequently-asked questions are, but for  
the time being you might want to take a look at the CAP Cookbook at  
<http:/www.incident.com/cookbook/>.

Hope that helps!

- Art