Thanks for your comments James,
It is good to know that BNF is supported in eclipse out of the box.
I'll try not to concentrate on the grammar too much and implement
from-to syntax. And when it works, we can discuss the final syntax.
Vadim.
James Strachan wrote:
2008/8/30 Vadim Chekan <[EMAIL PROTECTED]>:
Claus,
I'm glad you see the problem. Arguably, difficulties with routing
configurations is one of the the biggest obstacles on wider Camel adoption.
There is nothing more frustrating than knowing exactly what you want in
terms of EIP but spending many hours with so minor issue as syntax.
Interesting enough, I was thinking about it yesterday too and my conclusion
is such that it is very difficult to achieve smooth configuration using xml.
It is not the tool for the job. I mean it can do it (it does it currently)
and we can keep improving it but will we get the easiest possible way of
configuring camel routes?
I'll skip large philosophical discussion but in my opinion xml sometimes
becomes victim of its own success and programmers try to stretch its
application beyond the reasons because "xml is so great, I would add it to
my salad if I could" :)
Basically we are trying to define Abstract Syntax Tree in xml serialized
form. People don't usually think in terms of AST.
Totally agree BTW. FWIW I'd always envisioned we'd have multiple DSLs;
Java, XML, Groovy, Ruby, Scala and even a real DSL - hopefully all
using the same AST underneath...
http://activemq.apache.org/camel/maven/camel-core/apidocs/org/apache/camel/model/package-summary.html
allowing folks to create routes using one language and save them in
another DSL if folks wanted to etc.
We can give a whirl an idea of Domain Specific Language (DSL). I have some
under my belt and it is not too difficult. The result would look like this
(syntax can be any, but I'm trying to be java script alike):
routes {
route {
from 'queue:Incoming'
process {
bean 'org.my.company.camel' 'MyFunction'
aggregator()
}
if (header('myHeader') == 'value1') {
to 'queue:Outgoing'
} else {
to 'queue:Alternative'
}
}
}
Have you looked at the Scala DSL? Its pretty close to this already! :)
http://activemq.apache.org/camel/scala-dsl.html
http://activemq.apache.org/camel/scala-dsl-eip.html
I thought about juel (or others evaluators).
I see a problem that those languagas will dictate syntax which may be not
optimal for camel route definition domain.
Another and bigger problem is that external language will not detect camel
invalid constructions. For example creating serializer but not assigning it
data format would be invalid in camel but from Juel point of view it would
be perfectly ok.
Yeah - we need to make sure we validate as much of the AST up front as
we can along with giving great error messages.
Having our own syntax parser will let us detect any syntax deviations and
provide user with very clear and informative error messages for example "
'bean' or 'aggregator' are expected inside in 'process' ".
Another benefit we get is Backus-Naur Form (BNF) file which is human
readable and is very easy to use in documentation. As opposite to xml schema
which can be used for learning purposes but I would not risk claiming it a
recommended way.
Here are fragments of BNF as I see it:
route :
ROUTE
| from_list process_list to_list
;
...
process_list :
/* can be empty*/
| process
| process process_list /* works as pipiline */
;
process :
/* can be empty */
| BEAN LITERAL
| MULTICAST '{ process_list '}'
| PIPELINE '{' process_list '}'
| SPLIT LITERAL
| AGGREGATE expression
| AGGREGATE
| resequence
;
resequence :
BATCH RESEQUENCE
| BATCH RESEQUENCE expression
| BATCH RESEQUENCE NUMBER NUMBER
| BATCH RESEQUENCE NUMBER NUMBER '(' expression_list ')'
| STREAM RESEQUENCE resequence_options
| STREAM RESEQUENCE resequence_options '(' expression ')'
;
to_list :
to
| to to_list
;
to :
LITERAL
| IF '(' bool_expression ')' THEN to
| IF '(' bool_expression ')' THEN to ELSE to
;
If you guys think it worth a try I can mock up a prototype.
I think I would build an AST and then have another process which would
traverse AST and invoke RouteBuilder to build the actual route.
This way we preserve backward compatibility with existing ways of building
routes.
This route builder would be a separate plugin in order to be as little
invasive as possible.
AST can be used in future to build graph representation of camel routes
without building the actual routes. I recall James recent comment that
building route for display purpose only (without running it) added some
complexity.
Let me know what do you think folks.
I've always fancied a real DSL as well :). As you mention - the
hardest part of any real DSL is what language is used for
predicates/expressions. e.g. do we let any language be used...
xpath( /foo/bar )
or something?
But it'd be great to try make one. Incidentally if you do fancy
experimenting with this; have a look at the xtext library in eclipse.
I created a little prototype project here...
https://svn.apache.org/repos/asf/activemq/camel/ide/camel-eclipse/
which attempts to take an Antlr grammar for the DSL and auto-create
both a parser but also an eclipse based editor for the language. It
takes an Antlr grammer here...
https://svn.apache.org/repos/asf/activemq/camel/ide/camel-eclipse/camel.routing/src/camel.xtxt
which as you can see is kinda basic :)
Having both a nice textual DSL along with an IDE to edit it would rock :)
Its well worth looking at the Scala DSL which looks great; maybe they
could both look and feel kinda similar? FWIW the only thing I don't
like with the scala DSL is the use of _ which is a bit odd - but thats
a scala thing
