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From Ovidiu Predescu <ovi...@apache.org>
Subject Re: Flow wishlist :)
Date Mon, 02 Dec 2002 23:50:54 GMT
Peter,

If you ask me, you should never use the sitemap to describe any 
processing other than page generation. For everything else, just use 
the flow layer to describe the interactions between pages, and business 
logic you express in Java or a functional language (like Scheme) 
implemented in Java if you really want to. Trying to put the rules that 
describe your application in the sitemap is more cumbersome than the 
approach I suggested above.

I implemented an earlier version of the control flow layer in Scheme, 
to take advantage of the continuations concept this language supports. 
The current control flow layer is designed to support other languages 
as well, so if you're interested, you can add support for Scheme. But 
again, you should keep in mind the control flow layer is designed to 
write your application logic elsewhere than in the sitemap.

I hope this explanation helps a bit in clarifying your thoughts.

Best regards,
Ovidiu

On Monday, Dec 2, 2002, at 08:05 US/Pacific, Hunsberger, Peter wrote:

>>> Hmm, I guess I've been exploring this long enough that I thought it
>>> was somewhat intuitively obvious, sorry...  (Someone has also 
>>> remarked
>> that this sounded somewhat like a capability that was in C1.)
>>
>> yeah, you could implement what you want using dynamically added PI
>> (processing instructions) for the C1 reactor, but it would end up 
>> being
>> very messy anyway.
>
> When it was described to me it didn't really sound like what I 
> wanted...
>
>>> I'll skip the background for all this, but basically our requirements
>>> are a bit extreme; 1000's of variations on 100's of UI "screens"; 
>>> each
>>> UI variation can change over time, and it must be possible to audit
>>> the history of changes and see data in the form that the UI was at 
>>> the
>>> time the data was originally created.  As such, we need to be able to
>>> drive ALL UI construction, validation and workflow dynamically out of
>>> a database.
>>
>> Ok, let's start from these requirements.
>>
>>> Cocoon is a good starting point for this, but as we work with Cocoon
>>> we end up coding presentation rules and presentation flow rules in 
>>> the
>>> sitemap.
>
>> Well, that's what it was designed for. The design of the sitemap was
>> *explicitly* meant to be static and remains so. So far, nobody was 
>> able
>> to come up with an example where dynamically generated pipelines were
>> the *only* way to solve a functional requirement.
>>
>> Anyway, I'll be strongly against adding dynamic pipeline generation
>> capabilities, either in the sitemap or in the flowmap.
>
> Let me be clear that I'm not looking for dynamic pipeline generation. 
> The
> mapping of URI to generator is well defined for everything that what 
> we want
> to do.  The selection of transformer is a little less so; for example, 
> the 1
> result vs. multiple results example we talked about earlier. That's 
> still a
> static sitemap, but the transformer is chosen at run time; so the
> understanding of dynamic vs. static sitemap should be clear: dynamic 
> sitemap
> would mean building the sitemap every time it's run.  I can't imagine 
> anyone
> that would want that?
>
>>> Basically, we've got a requirement for a rules based evaluation of
>>> context data.  I don't want to code this in the sitemap language and 
>>> I
>>> don't want to hard code it in Java, I really want dynamic rules
>>> evaluation.
>>
>> Look: I really don't get what you mean by this. Sorry, I'm slow
>> sometimes: can you show me an *explicit* example of your functional
>> needs? otherwise I don't feel I can be much helpful if we keep this
>> level of abstraction without me understanding where you want to go.
>
> I'm not sure I can explain this via e-mail much more than I have. 
> Maybe this
> needs some background in rule based systems or expert systems design; I
> don't know how much you may have encountered such things?
>
> Let me use an example I've given previously on the list:  Patient 
> privacy
> rules are such that it's possible for a researcher to be doing research
> using patient data and not be allowed to know the identity of the 
> patient.
> It's also possible that there is such a small patient population for a 
> given
> treatment protocol that a  combination of very few searches would be 
> needed
> to uniquely identify a patient.  For example, for a given  protocol 
> there
> may be only a single patient born between the years 1980 and 1985 
> living in
> Tennessee. Thus, the rules might be that we allow a search by birth 
> date if
> the user hasn't previously done a search by geography (or vice-versa). 
>  We
> have to evaluate each action in the context of previous actions on the 
> same
> data.  So as a researcher uses the system he builds up this trail of 
> history
> data that starts to follow him around and accumulate; they have done 
> action
> X in the distant past, action Y more recently, then action Z just now:
> therefore, in the current context (of having just done Z) action Z is 
> not
> allowed or it is (because actions X and Y are still considered 
> relevant).
> This history data doesn't just come from a single source; it can come 
> from
> external systems so we really want to use some generic format to 
> process it,
> as such XML is well suited.
>
> In-other-words, for us, the decision on what action to take at any 
> given
> point is dynamically evaluated (like most action handlers), but the
> decisions are just based on one heck of a lot of complex processing 
> (and not
> just simple form field evaluation), this processing falls into the 
> general
> CS pattern of expert systems processing and more specifically rules 
> based
> expert systems (among other things). As a result, in our particular 
> case,
> functional programming meets our needs better than most other 
> solutions.
>
>>> We know a
>>> functional programming model can provide this.  Just so happens we
>>> have such capabilities at hand via XSLT if we can feed it rules
>>> selectors described as XML.
>>
>> Sounds like the good old 'golden-hammer' antipattern to me. But I 
>> won't
>> comment further until I understand your requirements.
>
> Perhaps so, if it wasn't so easy to get the data into XML format we 
> might be
> looking at wiring in LISP or Haskell (or whatever) processing on the 
> data,
> but as it sits XSLT is an obvious way to go to meet this need. I will
> observe that it's currently even harder for us to find LISP or Haskell
> developers than XSLT developers (we keep looking)...
>
>>> In-other-words: currently, sitemap has access to context via URI,
>>> parameters, generators, etc.  Based on this, sitemap spits out a
>>> decision on what transform to use.
>>
>> That's one way of looking at it. Another is that your functional logic
>> could be directly included in the generator.
>
> Well yes, and that's sort of what I am proposing.  However, let me note
> that, taken to the extreme, your  statement is equivalent to saying 
> that
> everything can be done with a single generator.  Even I'm not proposing
> that, though for us I'll end up reducing the number of required 
> generators.
>
>>> What I want instead is to feed an XSLT this same set
>>> of context as XML and have the XSLT pick the subsequent transform to
>>> use. The advantages to me are; 1) I can code in XSLT instead of
>>> sitemap language;
>>> 2) I can optimize the entire chain of events since the transform 
>>> picking
>>> XSLT can pass on the context to the next transform (standard 
>>> transform
>>> chaining); 3) I get a functional programming model (not an advantage 
>>> to
>>> some, I know).
>>
>> I don't get it: you say that your requirements are so horrible that 
>> you
>> need to keep all your rules into a database (which is a questionable
>> sentence right there, but I don't have details to judge it). Then you
>> say that a sitemap becomes a mess. Result: you want to write a XSLT
>> stylesheet that uses extensions to connect to a database to obtain
>> dynamically generated pattern-matching rules to transform an XML
>> representation of your request context into a directly-digestible 
>> output?
>
> The rules don't go into the database, the rule selectors go into the
> database. I don't think any extensions should be needed; the context 
> data
> will be created using standard generators and possibly aggregation, 
> though
> as we proceed we're finding that our generators inherit from each 
> other and
> aggregation isn't needed; each generator picks up what is needed
> automatically. Likely, for the other cases we'll end up using 
> composition in
> the generators and eliminate aggregation in the sitemap.
>
>> how that is goint to be any better than a sitemap+flowmap is *very* 
>> hard
>> to see from where I stand.
>
> Better is a relative thing.  As we've sort of concluded, in some cases 
> your
> development requirements get messy no matter what way you go.  When 
> you're
> building systems for research it's often a case of picking the less of 
> two
> evils...
>
>> moreover, it sounds like an optimal solution to kill your webapp
>> performances: that stylesheets becomes your bottleneck. So either you
>> write your own xslt engine (or extend an existing one) to be able to
>> optimize those database-extracted rules, or you're goint to have
>> *serious* scalability issues right there.
>
> Evaluating all these rules is going to be a performance issue no 
> matter how
> we go.  Unfortunately, the new government privacy requirements force 
> some of
> this on us (we've still got a couple of years before they all go into
> effect).  Similarly, the complexity of the research environment forces 
> some
> of this on us.  That's part of the "challenge" of working in 
> research...
>
>> In both cases, big implementation PITA.
>>
>> End result? your people might know the XSLT syntax, but one thing is 
>> to
>> know the syntax of a language, another is to be able to read a
>> stylesheet that includes hard-core functional programming connected to
>> external datasource via extensions.
>>
>> If you think that is going to be easier for you to find people able to
>> read/write/maintain those hard-core stylesheets than it will be to 
>> find
>> people that can learn the sitemap syntax and a read a few lines of
>> javascript, I think you have some thick walls to crash into in your
>> future :)
>
> As we've discussed this is going to be messy either way.  If it was 
> just a
> few lines of JavaScript needed to do all the complex evaluation of the
> current context data then it wouldn't be an issue.  However, to code 
> the
> processing we need would mean many 1000's of lines of JavaScript.
> Now-a-days we've got good XSLT editors, schema validators etc. so the  
> job
> of creating the XSLT isn't as hard as it used to be.  Functional 
> programming
> is always an issue (sigh)...
>
> So far this is all working, the XSLT's keep getting smaller as we 
> generalize
> things out and discover new generic processing patterns.  To me that's 
> a key
> sign that we are on the right track (in the past I've also I've seen 
> C++
> code get smaller as it gets generalized but gains more function.)
>
> <small snip/>
>
>>> However, I could also see
>>> how there might be situations where the serialization decision might
>>> be part of the new thingy, and thus the blocks discussion and how to
>>> hand off service calls becomes relevant.
>>
>> It is *NOT* a transformer decision to drive the serialization process.
>> It's against both SoC and IoC! There is nothing planned for Cocoon
>> Blocks that will allow this to happen and as soon as I have to vote
>> around here, you'll get my -1 on anything that makes possible for one
>> pipeline component to modify dynamically the pipeline execution,
>> including choosing a serializer.
>
> I'm not asking for the transformer to drive the serialization 
> decision; we
> definitely want to separate those decisions!  What I was saying is 
> that if
> you have a generalized way of extending the sitemap then the decision 
> on
> where to plug in the serialization becomes an issue.  It's the 
> resources
> discussion: a resource might do generation and transformation, or it 
> might
> do transformation and serialization, or whatever. In my case, the 
> question
> of whether blocks will allow this doesn't matter a whole lot, since 
> for the
> most part I think we can behave mostly as a pure transformer.  
> However, I
> could possibly see a case where I want our new thingy to behave more 
> like a
> generic resource and take over more of the otherwise standard sitemap
> processing.
>
> SOC shouldn't mean that the only place you can separate the 
> transformation
> and serialization decisions is in the master sitemap, some other
> component/block might also have a good way of separating these 
> decisions and
> handling them...
>
>
>
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