[undertow-dev] Predicate Handlers

[Stuart Douglas]

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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