Updating files through BPM

Hi ,

I have a question where I need to append two files and send them as one file at the receiver by using a coorelation.

Lets Say we have two files -File A and File B both in delete mode .Both are required to be appended and sent as one file ,say AB on the basis of a correlation on a unique value .(e.g division number ).

Acc to the project requirement ,After sending a file A ,If once again a new file of A (e.g A(1)) is updated before the second file B reaches the BPM ; We should append the latest file( A(1) ) with B ,and send it across the receiver .(In this case the result file should become A(1)B ).To achive this we are using a container operation in assign mode and hoping that every new message of same type is overwritten.

But everytime we are updating the file A it opens a new instance of BPM although the coorelation Id is same .

When we send file B with that coorelation ID as of file A,it closes both the instances and creates two file , AB and A(1)B.

We wish that the BPM discards the old file(A) and uses the updated file( A(1) ) in the same instance and puts it on the receiver .

I would like to know if XI behaves in this way only or we are missing something .

Thanks and Regards ,

Neha.