[undertow-dev] Predicate Handlers

[Stuart Douglas]

This would not be a specific config file, but rather just a line in 
standalone.xml:

<handlers>
   <compression enabled="maxContentSize(10000) and not 
responseHeaderContains[name=Content-Type, value='application/zip']" >
     <bzip enabled="pathMatch[*.js] or pathMatch[*.css]" />
     <deflate>
   </compression>
</handlers>

Stuart

Anil Saldhana wrote:
>
> On May 13, 2013, at 6:55 PM, Andrig Miller<anmiller at redhat.com>  wrote:
>
>>  From the Andiamo perspective, I would not want to see an external DRL config file.  That violates one of our very first Andiamo tenants.
>>
>> Andy
>
> Ok - if you look at Stuart's original email, it does say Wildfly config file for specifying predicates. If you assume that to be in the domain model, the specification of the predicates via DRL, can be done in the domain model. I am just wondering if the use case is large enough to warrant a rules engine. If yes, then drools core will save time.
>
> But from Stuart's response, use case is not large enough to require rules engine. I withdraw my suggestion. :)
>
>
>> ----- Original Message -----
>>> From: "Anil Saldhana"<asaldhan at redhat.com>
>>> To: "Stuart Douglas"<sdouglas at redhat.com>
>>> Cc: undertow-dev at lists.jboss.org
>>> Sent: Monday, May 13, 2013 5:28:03 PM
>>> Subject: Re: [undertow-dev] Predicate Handlers
>>>
>>> You definitely need the predicates. Thinking along First Order logic,
>>> I am wondering if this is just an use case for rules definition and
>>> processing - something drools has been doing for years.
>>>
>>> Maybe all you need is Drools core lite (or whatever you want to call
>>> the core rules processing framework) and one DRL external config
>>> file.
>>>
>>> On May 13, 2013, at 5:41 PM, Stuart Douglas<sdouglas at redhat.com>
>>> wrote:
>>>
>>>> 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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