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From Daniel Fagerstrom <daniel.fagerst...@swipnet.se>
Subject Re: Flowmaps: the wrong approach (was Re: [RT] (long) Rethinking Finite State Machine Approach to Flow Management)
Date Sun, 02 Dec 2001 23:28:03 GMT
Ovidiu Predescu wrote:
<snip/>
> I now believe we should have a system centered around logic, not
> around states and transitions.
I agree completely, IMHO, writing FSM:s is like goto-programming, very
small systems are easy to understand, but as soon as they grow, they
easily become a maintainance nightmare.

> The logic should be expressed in a
> language that supports continuations. The logic should drive what
> pages are presented to the user. These pages could be expressed in an
> XML markup language with appropriate elements for extracting data
> previously created in the program. These XML pages could be then
> processed through a pipeline, similarly with how they are processed
> today in the sitemap. However since incoming URLs are handled directly
> by the logic, there's no need for matchers in the sitemap.
>
>
>  HTTP request                         transformations
> --------------> logic -----> XML page -----------------> HTML/WML/... page
>
>
> The generated pages contain URLs that point back to continuation
> points in the logic.
>
> The biggest problem is the fact that the logic needs to be expressed
> in language that supports continuations. Since most people don't like
> the Lisp syntax, a language that abstracts out the continuations in
> higher level abstractions like send-response could be developed. This
> can probably be done by extending a familiar language, like
> Javascript, with these concepts.
<snip/>
> This is a huge paradigm shift from what we have right now, but I
> believe leads to easier ways to write Web applications. They become
> more like usual programs, instead of the complex beasts that we have
> today, with state scattered all over the place in the code.
>
> And yes, please read the papers I pointed to in my previous email, to
> understand what the heck I'm talking about. Here they are for your
> convenience:
>
>    http://youpou.lip6.fr/queinnec/Papers/webcont.ps.gz
>    http://www.cs.rice.edu/CS/PLT/Publications/esop2001-gkvf.ps.gz

I happen to like Lisp as well at its syntax ;) Still I wonder if it
would not be possible to continue in the great cocoon tradition of
SoC, and find a convenient description of webapp flow, without going
all the way to a full high level programming language. Now that you
have succeeded in exorcizing the mix of programming language
constructs and tags from JSP etc, it seem like a pity to let this mix
in again.

Anyhow, inspired of the interesting discussion and the articles that
you referd to, I started to think about how to use these concepts in
cocoon. If possible, without having to do a "huge paradigm shift".

--------------------------------------------------------------------
Flowmap
-------

First, to make it more concrete, I will try to express the main
example from, http://youpou.lip6.fr/queinnec/Papers/webcont.ps.gz, in
terms of xslt, cocoon components, a sitemap and a flowmap. For those
of you that not have read that article yet, the main example is a
small webapp:

1. On the first page it ask for the conversion rate between French
Francs and another currency.
2. Then it ask for an amount of Francs.
3. And on the third page it returns the result.

One of the coolest things about the implementation in the article, is
that it can take care of multiple questions at once. If you browse
through the three steps above, and then click on the "new window"
button in your browser. You can the go back to the first or second
screen, and fill in new data, without affecting the result in the
other browser, (even if you use the refresh button in it). This
behavior is very useful for "what if" kind of questions, where one can
evaluate several alternative scenarios in a convenient way.

Ok, here we go!
We start with a high level description of the application flow:

<!-- flowMap.xml -->
<fm:flowMap xmlns:fm="...">
  <fm:flow url="conversion">
    <fm:until test="/in/exchange/rate &gt; 0" id="rateTest">
      <fm:show src="cocoon://readRate.html" id="rate"/>
    </fm:until>
    <fm:show src="cocoon://readFrancs.html" id="francs"/>
    <fm:show src="cocoon://result.html" id="result"/>
  </fm:flow>
</fm:flowMap>

(The "id" attributes are not necesarry and are only used for making
reference easier) The flow map is either a part of the sitemap or
mounted from it. It will be executed by a "flowmap engine" on a
request for "cocoon://conversion". The children of "fm:flow" are
executed in sequence. Each child works as a pipeline. The flowmap
engine feeds the pipeline with an xml-document, that has "in" as root
element. This document contains two parts, a continuation, that is an
url to the next stage (or stages) in the flowmap, and description of
the current state.

The input to the first stage, "conversion#rateTest" could look like
this:

<in/>

Now, the first stage is an until-statement (a mistake from a
pedagogical point of view, I realize :) ), the test - an XPath
expression, will obviously not succeed on the current input
data. Therefore the body of the until-statement is executed. But
before we can do that we have to set the continuation. The next stage
after "conversion#rate" is "conversion#rateTest". We represent this
situation by creating the new input:

<in>
  <flow>
    <next>conversion?next=rateTest-23454</next>
  </flow>
</in>

Here the url "conversion?next=rateTest-23454" consists of two parts, one
that identifies the next stage to go to in the flowmap and one "23454"
that uniqely identifies the current state, which this far happens to
be empty. The current state is stored in a hash table with the url as
a key.

We need an implementation of "conversion#rate":

<!-- readRate.xsl -->
<html xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <head>
    <title>Conversion</title>
  </head>
  <body>
    <h1>Conversion from Francs</h1>
    <form action="{/in/flow/next}" method="post">
      <label for="/exchange/rate">rate</label>
      <input name="/exchange/rate" type="text" size="10"/>

      <label for="/exchange/currency">currency</label>
      <input name="/exchange/currency" type="text" size="10"/>

      <input type="submit" name="submit" value="Continue"/>
    </form>
  </body>
</html>

The main things to notice here is that, the form/@action will be
replaced with the current continuation from the input, in our case
"conversion?next=rateTest-23454", and that the names in the form
describes positions in a output xml-document, (this idea is taken from
the XForms draft).

We also need a sitemap fragment to see how readRate.xsl is suposed to
be called:

<!-- sitemap.xmap -->
<map:sitemap xmlns:map="http://apache.org/cocoon/sitemap/1.0">
  <map:pipelines>

    <map:pipeline>
      <map:match pattern="**.html">
        <map:generate type="flowMapGenerator"/>
        <map:transform src="{1}.xsl"/>
        <map:serialize/>
      </map:match>
    </map:pipeline>

  </map:pipelines>
</map:sitemap>

Here the "flowMapGenerator" feeds the current input to
e.g. readRate.xsl.

More interesting things happens when the user have filled in the form
and hits the submit button, this will create a request for
"conversion?next=rateTest-23454", and the flowmap-engine will respond
in the following manner:

1. Read the request parameters, in our case they might be:
/exchange/rate=1.4551&/exchange/currency=SEK.

2. Create an XML-document from the request parameters:
<exchange>
  <rate>1.4551</rate>
  <currency>SEK</currency>
</exchange>

3. Resume the state that is associated with the
   url, from the hashtable. It happens to be empty at this moment.

4. Combine the restored state with the current input. This can and
   needs to be done in many different ways, but for our current
   example, an insert/replace operation, is enough, and results in:
<in>
  <exchange>
    <rate>1.4551</rate>
    <currency>SEK</currency>
  </exchange>
</in>

5. And this is the new input to "conversion#rateTest", this time the
   test will succeed, and as a result, the flowmap engine continues to
   the next stage "conversion#francs", and sets the continuation to
   the stage after:
<in>
  <exchange>
    <rate>1.4551</rate>
    <currency>SEK</currency>
  </exchange>
  <flow>
    <next>conversion?next=result-54328</next>
  </flow>
</in>

So, here I will stop boring you with all the details. The last two
pages look as follows:

<!-- readFrancs.xsl -->
<html xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <head>
    <title>How many Francs?</title>
  </head>
  <body>
    <h1>Converting into <xsl:value-of
select="{/in/exchange/currency}"/></h1>
    <form action="{/in/flow/next}" method="post">
      <label for="/FRF">Francs</label>
      <input name="/FRF" type="text" size="10"/>

      <input type="submit" name="submit" value="Continue"/>
    </form>
  </body>
</html>

<!-- result.xsl -->
<html xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <head>
    <title>Conversion result</title>
  </head>
  <body>
    <h1>Conversion result</h1>
    <p>
      <xsl:value-of
        select="concat('If 1 FRF corresponds to ',/in/exchange/rate,' ',
                       /in/exchange/currency,' then ',/in/FRF,
                       ' FRF correspond to ',/in/exchange/rate * /in/FRF,
                       ' ',/in/exchange/currency,'.')"/>
    </p>
  </body>
</html>

result.xsl will be called with input like this:

<in>
  <exchange>
    <rate>1.4551</rate>
    <currency>SEK</currency>
  </exchange>
  <FRF>100</FRF>
  <flow/>
</in>

------------------------------------------------------------------------
More constructions
------------------

There are certainly need for more language constructions to make the
flowmap usable, some examples:

<fm:if test="XPath">
  Do something
</fm:if>

A possiblity to have several possible continuations:

<fm:switch>
  <fm:case link="l1" id="i1">
    Do something
  </fm:case>
  <fm:case link="l2" id="i2">
    Do something
  </fm:case>
  ...
  <fm:case link="ln" id="in">
    Do something
  </fm:case>
</fm:switch>

The switch statement will give the preceding statement the input:
<in>
  <flow>
    <next>flow?l1=i1-76456</next>
  </flow>
  <flow>
    <next>flow?l2=i2-09877</next>
  </flow>
  ...
  <flow>
    <next>flow?ln=in-65433</next>
  </flow>
</in>

Maybe the switch statement should be nestable with if statements, to
make it possible to describe that some of the links only are
available if certain conditions are fullfiled. An important example is
to only show the links that one is allowed to traverse.

It is useful to call other flows:
<fm:call src="cocoon://flow1"/>

To make flow calls possible one need to store a stack of
continuations from the calling flows, in the state.

Some kind of try, catch statement would probably simplify error
handling.

-------------------------------------------------------------------
State handling
---------------

The state handling described above, is to primitive for many
situations. It allows for the "what if"-scenarios mentioned in the
beginning, (I guess that is far from obvious from what I have said,
but the images in the beginning of the referred article explains the
situation quite well). This flexibility comes with a high cost, each
continuation, that is created is associated with an own copy of the
state. As long as the state is read-only all the copies can have
references to common parts, and thus take away most of the copying,
still the approach requires a lot of resources. Another problem is
garbage handling, when should an unaccesed continuation, be taken
away? (some ideas can be found in the referred articles).

In situations where one updates a data source with a large state, a
data base, for example, a "many world"-behavior is not desirable at
all. It would mean that the system have to handle several copies of
the database, or that the database must be able to take care of
multiple branches of the stored information.

This situation can be handled by restricting the creation of new
continuations, so that one copy of a continuation is allowed for each
stage in the flow.

---------------------------------------------------------------------

Thats enough for tonight.

Comments, ideas?

regards,
Daniel



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