What technology would be best for quick notifications of orders in a real-time ordering system?

For quick notifications in a real-time ordering system, utilizing WebSockets is the most effective option. WebSockets provide a full-duplex communication channel over a single, long-lived connection, allowing for instant message transmission between the server and client. This means once the connection is established, the server can immediately push updates or notifications to the client as soon as events (like new orders) occur without the need for the client to constantly poll the server for updates.

In contrast, other technologies listed have limitations in the context of real-time notifications. API Gateway is typically used for creating and managing APIs, but it does not inherently support real-time capabilities. While REST APIs are great for managing requests and responses over HTTP, they generally require the client to request information, creating a lag between the event happening and the notification being received. HTTP/2 improves the efficiency of HTTP and allows for multiplexing multiple requests, but it still does not support real-time push capabilities like WebSockets do.

Thus, for real-time updates, WebSockets stand out as the best technology, enabling immediate and efficient communication in a dynamic ordering system.