[undertow-dev] Predicate Handlers

[Stuart Douglas]

I think you may have missed the point.

Undertow uses handers, which are conceptually similar to valves, and you 
can still provide a custom handler to run any custom code you want.

The idea of predicates is that often you will only want a handler to be 
applied in a certain situation. So with the case of the compression 
handler there may a number of things that may determine if a request is 
compressible or not, including:
- content-type
- size
- request path
- cache control headers etc
- accept-encoding (although this has to be handled by the compression 
handler to make sure the correct encoding is selected)

Without something like predicates we have to figure out what all these 
criteria are and code them into our handler, and then provide so means 
of configuring them. With predicates the compression handler just 
worries about compression, and the predicate handler deal with if the 
request is compressible or not.

Also it is possible to provide custom predicates, so with this approach 
you can use custom code to decide if an existing handler should be run 
without having to modify or extend the handler.

Stuart


Bill Burke wrote:
> 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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>