Let's say I have a stream with 3 applications - a source, processor, and sink.
I need to retain the order of my the messages I received from my source. When I receive messages A,B,C,D, I have to send them to sink as A,B,C,D. (I can't send them as B,A,C,D).
If I have just have 1 instance of each application, everything will run sequentially and the order will be retained.
If I have 10 instances of each application, the messages A,B,C,D might get processed at the same time in different instances. I don't know what order these messages will wind up in.
So is there any way I can ensure that I retain the order of my messages when using multiple instances?
No; when you scale out (either by concurrency in the binder or by deploying multiple instances), you lose order. This is true for any multi-threaded application, not just spring-cloud-stream.
You can use partitioning so that each instance gets a partition of the data, but ordering is only retained within each partition.
If you have sequence information in your messages, you can add a custom module using a Spring Integration Resequencer to reassemble your messages back into the same sequence - but you'll need a single instance of the resequencer before a single sink instance.