You are building a streaming application for the following use cases. Your goal is to have the best performance cost for each use-case. For each use case, decide which is the lowest processing guarantee that should be fulfilled for correct processing.

Remember, the higher the guarantee, the higher the cost of processing. So the lowest guarantee is defined as the the guarantee with the lowest cost.

For this exercise assume: Exactly-once > At-least-once > At-most-once.

• You are a bank and all customer transactions are streamed from a central message queue through some normalization steps before they are persisted in the user's account-history log.
• You are an online retailer that analyzes the content of their users' shopping carts to get an idea of which articles are frequently bought together.
• You are an internet company that processes ad clicks and bills customer's based on the ad-click-event. Each click event is analyzed and the billing information is written to a key-value store with the event's id as the key.