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From Ovidiu Predescu <ovi...@apache.org>
Subject Re: [RT] Flowscript [was Re: [RT] Flowmaps]
Date Wed, 19 Jun 2002 23:49:20 GMT
On 6/18/02 8:35 AM, "Stefano Mazzocchi" <stefano@apache.org> wrote:

> Ovidiu Predescu wrote:
> [...]
>
>> First of all, I think the thanks should go to Christian Queinnec, who came
>> up with the idea, and to Christopher Oliver, who implemented continuations
>> in Rhino. I just put all the things together and implement the support in
>> Cocoon.
> 
> Once flow docos are in place, we'll make sure to give all credits to
> those who deserve it. For sure, you deserve lots of it.

Thanks!

>> 1. Stefano points out that flowmaps (I don't really like this term, a "map"
>> is more appropriate for state machines, how about flowscript instead?)
> 
> You are right. The concept of 'flowmap' came because of a mental
> parallel I had between a declarative approach and a procedural one...
> but after understanding that we don't need that, being the HTTP
> client/server model intrinsically declarative.
> 
> So +1 for 'flowscript'.

OK.

>> should not be considered as ordinary resources, but a separate concept. I
>> have no issue declaring them in a different way, just to emphasize this
>> point. So something like
>> 
>>   <map:flowscripts default="calculator">
>>    <map:flowscript name="calculator" language="javascript">
>>      <map:script src="calc.js"/>
>>    </map:flowscript>
>>   </map:flowscripts>
>> 
>> should be OK. 
> 
> Hmmm, call me picky, but what's wrong with
> 
>  <map:flowscripts default="calculator">
>   <map:flowscript name="calculator" src="calc.js"
> language="javascript"/>
>  </map:flowscripts>

Because my calculator implementation may be composed of multiple script
files. This is not the case in this simple example, but I may need to split
the implementation for clarity in multiple files. Having the ability to
specify multiple files by adding <map:script> as needed solves the problem.

>> Beside a different name for elements, the fundamental
>> difference between this syntax and Stefano's is that it allows multiple
>> files to contain the implementation of a flow script.
> 
> yes, this is also a parallel with any other sitemap concept, such as
> components, resources and views.

Yes.

> [...]
>
>> The transformer would probably work best when the continuation id is encoded
>> in the URL. When the id is encoded as a request parameter, there will be
>> some awareness required from the page designer to have this id enclosed
>> inside a form.
> 
> Many ask to make continuations ID as transparent as session IDs. Do you
> think this is possible/desirable?

I think it is both desirable and possible. I think the responsibility for
deciding this should be in the sitemap, and only in one place.

>> 3. (Stefano) Template languages other than XSP to be used with the flow
>> layer.
>> 
>> This should certainly be possible, I've started with XSP because it was very
>> easy to get implement. JSP and Velocity should be equally easy to implement
>> though. We just need somebody to do it ;)
> 
> Great. Please explain how then, because I think lack of knowledge and
> docos was the real issue (see how things changed with a single email)!

The only thing which needs to be implemented is the equivalent of the
jpath.xsl logicsheet. The idea of the logicsheet is very simple: to extract
data from a Java object model or DOM tree using XPath expressions. If you
look at the logicsheet, you'll get the idea.

> [...]
>
>> It
>> essentially allows you to have a page processed through a pipeline, and
>> continue the execution of the script. BTW, I think your sitemap snippet has
>> a bug in it, shouldn't it have the <map:match> names swapped?
>> 
>> Also in your callPipeline(), how does the output parameter get used?
> 
> That's the key issue: many people, in the past, pointed out that they
> would like to reuse the concept of cocoon pipelines to create stuff but
> send it somewhere else, not back to the client.
> 
> Currently, there is no way for a pipeline stage to call another pipeline
> *detaching* it from the client output stream and attaching it a
> different output stream. I would like the flowscript to be able to do
> that.

Yes, that's right! HP's SOAP server based on Cocoon had to do some really
nasty things to achieve this. I see what you mean, and it should be
possible.

>> It
>> should be the task of the sitemap pipeline where the file is written. Am I
>> missing something?
> 
> It's *not* the sitemap concern to know where the output stream is. If
> you look at how the sitemap is implemented, it's the sitemap caller's
> concern to provide the output stream. Servlets call the sitemap engine
> passing the ResponseOutputStream and the CLI calls the sitemap engine
> passing FileOutputStreams.
> 
> But the granularity is all or nothing. I would like to be able to call a
> pipeline 'detaching it' from the output stream that the container
> passed.
> 
> Why? simply because it allows very creative uses of cocoon pipelines and
> would instantly stop all those requests for 'forking pipelines' that
> keep bugging me in the back of my mind.

OK, I get it! That's indeed a nice feature to have.

>> 5. Ivelin points out the potential of stale continuations.
>> 
>> I think this is indeed a real issue. One way to avoid it is to have
>> continuations automatically expire after a time of inactivity, or do
>> deactivate them manually in the flow script. The first approach is something
>> I considered, and there are some hooks in the current code, but more support
>> code needs to be written. The second approach can be implemented today: you
>> can invalidate not only a single continuation, but the whole subtree of
>> continuations which start from it. You just need to keep a hold on the
>> continuation at the top of the tree, and invoke invalidate() on it.
> 
> Speaking of which, would it good to have a 'continuation' object in the
> FOM?

It is already there, together with the data object. Check-out the jpath.xsl
logicsheet to see how this works.

>> 6. Multi-page forms are not dominating, and continuation based compared to
>> even driven (GUI-like) programming approaches.
>> 
>> I used to program GUIs a lot, I think for certain things, event-driven is
>> better than using continuations. I do think however that continuations have
>> a place, especially for multi-page forms. Now I think it depends a lot on
>> the application, whether these forms are prevalent or not.
> 
> I agree and this is the reason why I think deprecating Actions is a bad
> thing (at least, today). We'll provide options and guidelines, users
> will decide what to do with the tools we provide.

OK.

>> 8. Vadim's idea about making the syntax easier.
>> 
>> I think the idea of having something as simple as:
>> 
>> <map:match pattern="calc/*">
>>   <map:flow method="calculator" continuation="{1}" />
>> </map:match>
>> 
>> makes a lot of sense. Using different matchers, the continuation id can be
>> extracted from the request parameters, cookies, you name it. The only
>> problem with it is that is a bit inflexible in terms of the URI design. If
>> I'd like to have calc/kont/*, there's no easy way to specify it.
>> 
>> So I'd say we can have two alternative ways of writing this: using the above
>> proposal and what is already implemented, for power users that want more
>> control. How about this?
> 
> Sorry, but I lost you here.

I think the idea is to have something which just works by default, with no
configuration necessary. In addition to this, more experienced developers
can use the current method, which is more complex. Vadim's proposal is for
the first method. What is already implemented in the current code is the
second method.

>> 9. Reinhard asks what happens if two different flow scripts have a function
>> with a similar name name.
>> 
>> Since the flow scripts have their own variable space (not now due to some
>> problems in the current implementation), there shouldn't be any problem.
>> Each flow script can use its own variable and function names, without
>> worrying about possible conflicts with other scripts.
> 
> Yes, and also I was thinking about having something like
> 
> <map:call flow="calculator" method="calculator"/>
> 
> that provides sort of namespacing and avoids collisions.

In the new scheme I'm working on, there's no potential for collisions, each
flow script has its own namespace, separated from the others.

> [...]
>
>> - automatic binding of JavaScript variables to form values. This would allow
>> you to declare something like:
>> 
>>   var username, password;
>> 
>>   // Send a page to collect the user name and the password
>>   sendPage("login.html");
>> 
>>   // When the user fills in the form and presses the submit button, the
>>   // script restarts here. The flow engine automatically binds the username
>>   // and password to the values submitted in the form.
>> 
>>   // make use of username and passwd here
> 
> I didn't know PHP had problems with this approach, but it smells a lot
> anyway. I'm always afraid of implicity behavior, expecially when it's
> user-triggerable.
> 
> This is not automatically a security hole, but it could become one since
> you are letting potential attackers one step closer to your door. And
> you are making things hidden so harder to spot.
> 
> I would be against anything so implicit.

No, no, I wanted to say something completely different. I replied to this
topic already, but I'll repeat it here to be clear.

The automatic binding happens under the strict control of the programmer.
What I was thinking to have is the ability to map a request parameter to a
script variable, _if_ the programmer requests so for a particular variable.
How the mapping is done is completely under the control of the programmer.

>> - automatic validation of form values on both server and client side. This
>> should allow the same piece of JavaScript validation code to be executed
>> both on the server and client side. More on this later.
>> 
>> 11. With respect to how to put a back button on a page, I think I replied to
>> this some time ago. The idea is to use the <jpath:continuation select="1"/>
>> which selects the previous continuation.
> 
> I agree with Torsten: I'd like to keep the flow layer detached from the
> jpath logicsheet. (I would also use -1 as well to indicate backwards)

Again, I was only lazy to come up with a new logicsheet, so I reused things
in the same logicsheet.

The counting starts from a node in the tree and goes upwards, that was the
reason why the number is positive. To me this makes more sense than using
negative numbers, but I have no strong opinions.

>> 13. Piroumian asks how to cancel a continuation.
>> 
>> As I mentioned in several ocasions, sendPage() returns the saved
>> continuation. If you want to cancel the continuation just invoke on it the
>> invalidate() method. It will cancel all the receiver and all the
>> continuations that inherit from it.
> 
> Cool. Ovidiu, we *definately* need more docs on what you wrote. It's
> simply too undocumented right now... don't need to be XML-ized edited or
> anything, just bare raw but with juicy info inside, we'll make them
> better ourselves.

Yes, I know it's undocumented. I spent a lot of time describing the ideas on
emails, but didn't actually takes those and put in a document. I'll try to
come up with some docs, but there are simply too many things to work on at
the same time ;)

>> 14. Another question Piroumian has is how to validate against business
>> logic. Since JavaScript is tightly integrated with Java, you can call Java
>> logic from the flow script to do the business logic validation. This way you
>> can invoke whatever logic you want. If this is too generic, then provide
>> some use case and I'll try to sketch a solution.
> 
> I think you should provide a real-life example of use of a java data
> model to show how java can be called from the flowscript.

This too is on my TODO list ;)

>> 16. Sylvain: should flow scripts be inherited by sub-sitemaps?
>> 
>> I'm not sure of the security implications of allowing them to be inherited:
>> what should be the behavior if a developer alters definitions from an
>> inherited flow script?
> 
> My perception is that flowscripts are not components and should not be
> inherited.

I think the same too, but I'll keep my eyes opened for counterexamples.

>> 17. The proposal to use map:call with "function(arg, arg, ...)".
>> 
>> I think this is more like a syntactic sugar than real value. I don't think
>> passing arguments via map:param would confuse people, the semantic is still
>> the same as function invocation.
> 
> All right, let's remove this.

OK.

>> 18. Access to continuations.
>> 
>> I think there is a bit of confusion of terms in this discussion. The
>> continuation id is passed in the HTTP request in some way or another: as
>> part of the URL, as a request parameter etc. This encoding should be visible
>> to the sitemap only. I'd very much like to have it hidden from the view
>> layer as well, so if we can avoid putting explicit URLs in page templates,
>> that would be great! This reduces the dependency between the URL design in
>> the sitemap and the view generation.
>> 
>> A continuation is a real object that lives on the server side, which is
>> represented by its id in the request. A continuation object is returned by
>> sendPage(). A continuation id is fairly much useless in a flow script, what
>> you really need is the continuation object itself, which contains this
>> information and more.
> 
> Yes, there is confusing between the 'continuation object' and the
> 'continuation id'.
> 
> Many don't like the fact that you have to explicitly pass along the
> 'continuation id', unlike sessions where the 'session id' is (generally)
> transparently passed back and forth (which was one of the biggest values
> of the Servlet API)

Yes, I should think of a better solution.

> [...]
>
>> 20. My job - thanks Stefano for bringing it up. Here's my shameless plug :)
>> If you think of using Cocoon for building Web apps or Web services, and you
>> need support in doing so, I am providing consultancy for doing it, anytime,
>> anywhere.
> 
> Good luck for all this!

Thanks!

Regards,
-- 
Ovidiu Predescu <ovidiu@apache.org>
http://www.geocities.com/SiliconValley/Monitor/7464/ (Apache, GNU, Emacs...)



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