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From Roman Kalukiewicz <>
Subject Re: Components setting data on OUT
Date Mon, 26 Jan 2009 14:37:28 GMT
Why don't we talk about exchange properties here? My feeling here is
that properties should be used as user-headers, while headers are
always protocol headers. In fact it works this way right now: If I
want to keep some value through the whole flow I put it into

By current convention if I put something on a header it is sent as
protocol-specific header (JMS property, HTTP header), and out headers
are filled also with protocol headers (JSM properties of out emssage,
HTTP response headers). In this case headers shouldn't be propagated,
as there is no way to distinguish things propagated, from things
retrieved. And out headers ARE different than in headers.

It is a matter of naming, but currently headers are (what you call)
protocol/system headers, while properties are user-headers (work as
variables). Do we really need to extend it further? If someone mix
those two concepts then it is problem of documentation, but not lack
of functionality. I would just extend DSL a little to be able to
retrieve a property (instead using header()).

What do you think, guys? Maybe we should clearly communicate what
things are for and what are the consequences of using one or another.


PS. Pipeline should propagate all headers of course, but I believe an
endpoint is a place where we shouldn't guarantee that headers will be
propagated by stating it clearly.

2009/1/26 Claus Ibsen <>:
> On Sat, Jan 24, 2009 at 9:08 PM, William Tam <> wrote:
>>> What we have stored in Headers today in Camel is both:
>>> - user headers
>>> - and system headers (added by Camel itself).
>>> I am starting to be more and more convinced that we should separate the two.
>>> So any headers that a users has enforced to be set should be kept in
>>> one Map and the others that the components set internally (such as SQL
>>> number of rows returned, or whatnot we have, there are many) in
>>> another Map.
>> It means that a component would have to look for header in more than
>> one place.   Besides, the distinction of user vs system header is not
>> always clear.  For example, the operation name header for cxf endpoint
>> can be set by user but it is also created by cxf component.   I am
>> sure there are many more examples.  There is another header category:
>> protocol headers.  A protocol header is not really a user or system
>> header.  Protocol headers are header propagated from protocol like
>> HTTP, which we do want to preserve in message header.
>>> The user headers is always preserved and copied along in the routing.
>>> User can always clear/remove unwanted headers.
>>> The system headers should be short lived as they are not really
>>> useable. So they are "alive" in the next step (process) in the route,
>>> and when the pipeline invokes next route thereafter these information
>>> is cleared.
>>> Separating these will also make the routing/tracing a bit easier as
>>> Users can recognize their own headers instead its mixed with all the
>>> noise the Camel components add.
>> I wonder we can leverage/extend the HeaderFilterStrategy mechanism.
>> Currently, it is only used for filtering unwanted headers (in both
>> request and response direction) when we propagate headers between
>> Camel and external messages (like HTTP).   HeaderFilterStrategy is (or
>> will be) associated with an endpoint.  We could make
>> HeaderFilterStrategy available to the exchange object.  So, when an
>> endpoint creates an exchange, the exchange gets a header filter
>> strategy.  Then, pipeline can do something like this to filter
>> unwanted header: message.filterHeaders().   The header filter strategy
>> is highly customizable for each endpoint (can have a component wide
>> default) and it can be looked up from registry.
> Good pointers William.
> Yeah we can revist it after you have moved the header filters to the endpoint.
> Then we can check up upon how to leverage it as you suggest.
> --
> Claus Ibsen
> Apache Camel Committer
> Open Source Integration:
> Blog:

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