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From Rakesh Sharma <rakesh_sharm...@hotmail.com>
Subject Event Driven Use Case
Date Sat, 02 Feb 2013 08:12:05 GMT
I am implementing a flow(route) using camel that requires interaction with
multiple external systems via RestFul service calls. These service calls
provide only an ACK back. The actual response is sent by external system by
making another service call back to my application. My app is running in
tomcat container on multiple machines fronted by a load balancer. The
subsequent steps of my route are dependent on the response of the external
service call. Having a load balancer complicates it further as external
service response may not come to the same machine where the original service
call was made from. I also need to timeout the exchange processing in case
external service response does not come back within a specified time
interval. I am thinking of using following strategy to solve this problem:

1. Break big route into multiple small routes. The last step of each small
route will make the service call to the external system. Essentially, I will
have as many routes as number of calls to external systems. The endpoint of
every small route (other than the initial one) will be the service end-point
where external service is going to come back with its response for the
request that was sent by the last step of the previous route.
2. Serialize the exchange just before making the service call and save it in
DB with UUID as key. 
3. Create a consumer on SEDA endpoint "seda:{uuid}" with a timeout. It is
being created only to track if external service provided a response back
within the expected time interval.
4. Make the service call to external system with UUID as part of the
request.
5. Retrieve the saved exchange state(using UUID) and remove the entry from
DB as soon as external service responds(response has the same UUID that was
passed with the request) back by invoking the next route endpoint. The route
has the previous state(retrieved from DB) and response from the external
system. It can proceed further now by merging the previous state and the
external service response. We know that response did not come within
expected interval if DB does not have an entry for the UUID extracted from
the service response.
6. The SEDA consumer will timeout and will make a check if DB still has the
entry. If entry exists we know that external service response has not come
within the expected interval. The entry from DB will be removed.

This is a very heavy traffic applications and I am not sure if I am doing it
in right way. Any suggestion will be highly appreciated. Using messaging
system is not an option due to many factors outside of my control.

Thanks,
Rakesh



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