I'm currently trying to figure the best solution for following problem using NServiceBus: I have GUI that user can use to search different things, but information about those things is spread in multiple services/databases. Lets say for example that user is searching for list of parks in a city, but each district of this city keeps only info of their parks in their own database (which they expose by web-services). I need NServiceBus to send message to each endpoint(district) what info user needs, wait for response, and then when it gets it from all endpoints (and only then) send it back to user(GUI). User is only interested in full information so Bus needs to know if every endpoint have send its response or not (it also needs to be in real time so Bus will assume that endpoint is offline and will send failure message if it will take too much time). Endpoints can change at any time so code needs to be easy to maintain. Best option will be that adding/removing endpoints can be done without changes in code.
Here are my thoughts about possible solution:
Publish/subscribe pattern lets me easily send message to multiple endpoints and add/remove endpoints at will by subscribing/unsubscribing without changing code of publisher. Problem: By definition publisher doesn't know how many subscribers are there (and what they are), so waiting for all of the subscribers to responds become difficult, if not impossible.
Request/response pattern lets me easily tell endpoints that i want answer and I will know if endpoint responded yet. Problem: Every time I need to add/remove new endpoint I need to change code of the sender. Also scalability may be a problem.
My question: Is there any way to combine those patterns? Or am I looking at this problem wrong way? Is there even a way that I can achieve all I want?
