[undertow-dev] Predicate Handlers

[Bill Burke]

IMO, you'd be better off providing a Valve architecture instead of 
maintaining an expression language.  I think you'll often hit a scenario 
where the expression language isn't adequate enough and a user will need 
to write code anyways.  Plus a config-file expression language can't be 
compiler checked.

On 5/12/2013 8:17 PM, Stuart Douglas wrote:
> A fairly common requirement for web requests is to enable or disable a
> certain action based on the state of the request. e.g:
> - disable compression if the response > 10Mb
> - If the user agent is X add header Y
> - If the path starts with /private disable caching
> etc
>
> To enable us to acomplish this Undertow has the concept of
> predicates[1], which as you would expect are a function object that
> takes the exchange as an argument and returns a boolean value.
>
> We also provide boolean operator predicates (and, or, not) that allow
> you to combine these predicates into arbitrarily complex values. The
> idea is that if a user want to configure a conditional action on the
> request they will use these predicates to specify the condition.
>
> To that end we need some way of representing them in the Wildfly
> configuration file, and so I have hacked up a basic syntax to configure
> them, and I would like some feedback. The configuration syntax will
> written directly into the standalone.xm and jboss-web.xmll to
> conditionally enable handlers.
>
> A basic example looks like:
>
> path["/MyPath"] or (method[value="POST"] and not
> requestHeadersPresent[value={Content-Type, "Content-Encoding"},
> ignoreTrailer=true] )
>
> The following boolean operators are built in, listed in order or
> precedence:
> - not
> - and
> - or
>
> They work pretty much as you would expect them to. All other tokens are
> taken
> to be predicate names. If the predicate does not require any parameters
> then the
> brackets can be omitted, otherwise they are mandatory.
>
> If a predicate is only being passed a single parameter then the
> parameter name can be omitted.
> Strings can be enclosed in optional quotations marks, and quotation
> marks can be escaped using \"
>
> Array types are represented via a comma separated list of values
> enclosed in curly braces.
>
> Predicate definitions themselves are loaded via a service loader
> mechanim, with the service implementation specifying the name and the
> parameter types.
>
> I would be interested to hear what people think. At the moment this is
> very simple, however a by product of this simplicity is that you need
> multiple predicates to handle similar things (e.g.
> requestHeadersPresent, responseHeadersPresent, requestHeaderContains,
> requestHeaderEquals etc).
>
> [1]
> https://github.com/undertow-io/undertow/tree/master/core/src/main/java/io/undertow/predicate
>
>
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-- 
Bill Burke
JBoss, a division of Red Hat
http://bill.burkecentral.com