ovidiu 02/04/05 12:34:35
Added: src/scratchpad/schecoon/webapp/docs continuations.xml
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xml-cocoon2/src/scratchpad/schecoon/webapp/docs/continuations.xml
Index: continuations.xml
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.0//EN"
"dtd/document-v10.dtd">
<document>
<header>
<title>Advanced Control Flow</title>
<authors>
<person name="Ovidiu Predescu" email="[EMAIL PROTECTED]"/>
</authors>
</header>
<body>
<s1 title="A different approach">
<p>Web applications are essentially event-driven
applications. Such applications have to react to events
generated from the client browser, and they respond to these
perhaps by changing their internal state and generating a
response.</p>
<p>The result is that even a simple application that needs to
collect some information from the user using more than one page,
has to somehow maintain the input accumulated so far from the
user. This input is a characteristic of the application
state. Another characteristic of the application state is where
the program processing is.</p>
<p>But let's look at an example. Suppose we want to write a very
simple calculator, which collects the numbers to be added, as
well as the operator, each in a separate page. It would be very
nice if we could write something like this:</p>
<source><![CDATA[
function calculator()
{
var a, b, operator;
sendPage("getA.html");
a = request.getParameter("a");
sendPage("getB.html");
b = request.getParameter("b");
sendPage("getOperator.html");
operator = request.getParameter("op");
try {
if (operator == "plus")
sendPage("result.html", a + b);
else if (operator == "minus")
sendPage("result.html", a - b);
else if (operator == "multiply")
sendPage("result.html", a * b);
else if (operator == "divide")
sendPage("result.html", a / b);
else
sendResult("invalidOperator.html");
}
catch (Exception ex) {
sendResult("error.html", "Operation failed: " + ex.toString();
}
}
]]></source>
<p>In this example, the <code>calculator</code> function is
called to start the calculator application. We'd like the
<code>sendPage</code> function to be a special function, that
takes as arguments an HTML file to be sent as response, and some
optional data that needs to be placed dynamically in it. We
would like <code>sendPage</code> to send the response page and
then block the executing thread, until the user clicks on a link
in the response page, which sends a request back to the
server. This request resumes the processing at the point it was
left, right after the call to <code>sendPage</code>.</p>
<p>This approach looks very powerful, since the flow of pages
within the application can be described as a normal
program. Using this approach you no longer have to think of your
Web application as a finite state machine, which transitions
from one state to another, and in the process generates response
pages.</p>
<p>A big disadvantage of the approach above is that we need to
maintain a thread alive until the user hits the link on the
response page. This is clearly very expensive!</p>
<p>It would be very nice if we can capture the state of the
application, its stack of function calls, which includes local
variables, the global variables and the program counter, and
save them into an object. If this object would give us the
ability to restart the processing from the point stored in it,
this would be what we need!</p>
<s2 title="What are continuations?">
<p>A continuation is exactly the type of object that we need.
Think of a continuation as an object that, for a given point
in your program, contains a snapshot of the stack trace,
including all the local variables, and the program
counter. You can not only store these things in the
continuation object, but also restore the execution of the
program from a continuation object. This means that the stack
trace and the program counter of the running program become
the ones stored in a continuation.</p>
<p>Continuations are powerful concepts from the world of
functional languages, like <link
href="http://www.schemers.org/";>Scheme</link>, but they are
becoming popular in other languages as well.</p>
</s2>
</s1>
</body>
</document>
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