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From Stefano Mazzocchi <stef...@apache.org>
Subject data goes in, data goes out
Date Wed, 28 Nov 2001 22:41:27 GMT
I'm restarting Jeremy's thread about WritableSource in order to achive a
more architecture-oriented view of the problem.

It is obvious that, in order to be a useful piece of a successful CMS,
Cocoon must be equally able to drive both data flows: inward and
outward.

For the outward flow, Cocoon beats the crap out of almost any other
solution (IMNSHO). 

For the inward flow, well.... let's be honest: it more or less sucks.

ok, ok, maybe I'm kinda rude but this is the truth: there are frameworks
outthere that give you much better tools to handle the data that flows
in.

Cocoon is currently a publishing framework, where publishing means
"outward-flow oriented", but all of us would like to see a better
synergy (or equilibrium) between the two flows, in order to make web-app
building easier, but without loosing the technological advancements that
Cocoon introduced.

Is this possible?

Giacomo and I are holding back a big RT about the next generation of
pipeline component assembling (a.k.a. flowmap, sitemapNG or more
generally "statemap"). We spent a few days talking about it and we
didn't come up with something solid enough to propose as a solution...

... but we *did* identify some general rules and general concepts that
might help triggering the discussion about this.

Anyway, this is not what this mail is about.

The question, today, gets deeper: is the current Cocoon architecture
asymmetric in respect of data flow?

First (and obvious) thought is to *reverse* the terminology:

 generators -> transformers -> serializers

becomes

 deserializer -> transformers -> accumulator

Is this something good?

No, I don't think so: what would be the difference between a generator
and a deserializer in terms of interfaces? I can't see one.

And what about accumulators? well, they'd store information, but would
need to give some information back. If this is in form of SAX events, an
accumulator is a transformer, if this is a binary stream, it's a
serializer.

But one concept here is good: the concept that the opposite of a
generator is an accumulator (I'm an electronic engineer, you know :), in
electrical terms, a battery, a place where energy (here, data) is stored
for future use.

In software terminology, an accumulator is normally called a "store",
"archive", "cabinet", "safe" and so on with terms that give you an idea
of "something" that allows you to place information that will
persistently remain there.

So, does this mean we need a "Store"-behaving component in the cocoon
pipelines?

No, such a store-behaving component would not be distinguishable from
another transformer.

So, I think I just proved that the current pipeline architecture is
*not* inherently biased toward outward data flow, despite the component
names that seem to inspire the opposite.

                            - o -

The above is an important result: it states that Cocoon doesn't require
core architecture modifications. I already knew this, otherwise I would
not be +1 on a final Cocoon 2.0 release. But it's good to let you guys
know about this as well.

So, now that we have an architecture that is, at least, in theory,
general enough to handle the problem, how do we attack it?

The "accumulator" concepts gives interesting ideas:

 1) it must receive SAX events (it would force concern overlap if the
binary adaptation was performed by the store itself, so
"parsing"/"deserialization" should be performed before, at XML
generation stage)
 2) it must throw data out (otherwise we would not be able to know what
happened)
 3) it should be SAX events (otherwise, we would not be able to process
the data further)

Thus, a "store" is behaviorally equivalent to a "transformer".

                           - o -

Now, how can this be handled? should we reverse the Source interface and
come up with a "Destination" (sort of InputStream/OutputStream
parallel)?

If this XML contained data, we could come up with an automatic
relational mapping between the arriving XML and the SQL update query on
a RDBMS. It could be pretty straightforward.

At the same time, how would you do such a thing on an article stored on
a native XML DB?

As Jeremy pointed out, XUpdate is a choice. I must tell you that I don't
like XUpdate that much, but I admit it's a choice.

Or we could use CVS, file systems, relational BLOBs, defragment the
document into LDAP nodes, FTP, POST it somewhere else, email it to your
mom, fax it to your brother, print it on a piece of paper for your
favorite human slave to subsequently crawl...

... up to you.

Now, the real question is: do we need any special protocol handlers for
these things?

In all honesty, I say that it's, again, up to you: you might find it
easier to come up with something like

  Destination d = DestinationFactory.create("dbxml://host/db/path/");

then serialize the input data into the source stream and have the
protocol handler automagically generate the XUpdate for you.

In this, case, admittedly, we need the "reversed source".

But the above could be done by hand using the DBXML API, or simply
calling a special xupdate wrapper around it (as an avalon component?).

Why am I telling you this?

Well, from an architecture design point of view, I can't see any
limitation in what we already have, but the "reversed source".

Jeremy named it "WritableSource" (which is admittedly an ossimoron).

Berin proposed to look at Monitors (which is not exactly a name that
reminds me of a storage location, BTW)

Anyway, this is, at this point, more an Avalon discussion than a Cocoon
one, since this is a general thing.

As for helping Jeremy to implement a Cocoon 2.0 version of FP, my
suggestion would be to aim at creating a general XUpdateTransformer that
wraps round the existing DB:XML API to make code persistent in a native
XML DB solution.

Having the ability to go both ways from a native xml db would instantly
turn Cocoon into a CMS, or, at least, in a toolkit to create your
customized CMS (which is what I'd really like to have, rather than a
fixed CMS)

Hope this helps.

-- 
Stefano Mazzocchi      One must still have chaos in oneself to be
                          able to give birth to a dancing star.
<stefano@apache.org>                             Friedrich Nietzsche
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