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From Willem Jiang <>
Subject Re: Camel Exchange Patters
Date Tue, 14 Sep 2010 04:44:28 GMT
On 9/11/10 10:03 PM, Christian Müller wrote:
> Hello list!
> I read Claus, Jonathan and Hadrians book Camel in Action and I studied the
> Camel site [1]. I also hat a short conversation with Ade from Progress about
> the exchange pattern, but still I have the feeling I doesn't understand all
> aspects. May be you can help me to understand it correct. Here are my
> questions:
> - In the book and the Camel site only the exchange patterns InOut and InOnly
> are mentioned but org.apache.camel.ExchangePattern defines the following
> exchange patterns: InOnly, RobustInOnly, InOut, InOptionalOut, OutOnly,
> RobustOutOnly, OutIn, OutOptionalIn. Should we only use InOut and InOnly?
> - If I understood Ade correct, after each processing on a
> component/processor the message goes throught the pipieline before it
> receives the next component/processor. In the case of an InOut exchange, the
> pipeline will copy the out message body, headers and attachments into the in
> message. So that in the next component/processor can access these data from
> the in message again. If I use the InOnly exchange pattern, the
> component/processor will write the response into the in message and the
> pipeline has nothing to copy.
The pipeline always uses a new copy of exchange which will create a new 
inMessage copy as you described, if you are using InOnly exchange 
pattern,  pipeline still create a new Exchange and copy the in message 
from the first exchange.

The InOnly and InOut exchange patterns always take effect in the 
component side, current DefaultExchange will not check the exchange 
pattern and will create a new out message which copy from the exchange 
in message when you call the Exchange.getOutMessage().

 > From the end user perspective it looks like it
> doesn't matter, whether to use the InOnly or InOut exchange pattern.
> - The only one component I know which handle InOnly and InOut "really"
> defferently is the jms-component. It will only send a reply message if the
> exchange pattern InOut is used.
> - If I use a InOnly exchange for the following routes, I takes also more
> than 5 seconds until my templeate.send() method returns. I would expect that
> the call returns after the exchange was placed into the next sub route
> (after a few milliseconds). My key point here is not to improve the
> performance. Only to understand the exchange pattern correct, how the work
> and how they are used in the right way...
>                  from("direct:start")
>                  .to("direct:sub");
>                  from("direct:sub")
>                  .process(*new* Processor() {
>                               @Override
>                               *public* *void* process(Exchange exchange) *
> throws* Exception {
>                                     Thread.*sleep*(5000);
>                               }
>                          })
>                  .to("mock:result");
> - Do you have recommendations when to use InOnly and InOut?
There are some description about InOnly and InOut message echange 
pattern in the Camel in Action chapter 10, as the sync and async 
invocation. You may take a look.

> - If we write our own processor which modifies the in message, should they
> write the modified body into the out message, if the exchnage is out capable
> (and also copy all header and attachments we need for further processing)?
> Or should we always modify the in message because it requires less action?
You can modify the in message directly, or do some change on the out 
message and the pipeline will pick up the right modified message for 
you. The DefaultExchange will create a new out message based on the in 
message if you call the Exchange.getOutMessage(), so you don't need to 
copy the all the headers and attachments yourself.

> - The same question for our own type converters. I know the type converter
> is implemented in this way, that it will return the new (converted) object.
> But our type converters also have to modify the message header. Should they
> also check whether the exchange is out capable and than modify the out (if
> out capable) or in (if not out capable) message? Is this the way camel
> handels the converted object from the type converter?

I think you just need to make the change on the in message, if you want 
to avoid the addition copying (camel will copy the out message from the 
in message if you make the change on the out message of the exchange).
> [1]
> Thanks in advance for your help and taking time for my questions,
> Christian


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