[undertow-dev] Predicate Handlers

[Anil Saldhana]

Additionally, I think you will have to standardize on:
a) operators - words (or you support symbols also). [a and b]  vs [a && b]
b) order of processing. (Mis-configuration may have side effects)

On 05/12/2013 07:17 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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