The CQRS system interfaces are used from outside the system to initiate business use cases. For example, a client accesses the waiter system to order a burger.

These interfaces are used externally and ideally implemented using a technology-agnostic protocol.

For REST-like HTTP services, this implies that the command services implement HTTP methods that modify resources, such as POST, DELETE, PATCH, or PUT. The query services usually only implement resources queried by GET.

In our example, this means that the client POSTs a new meal order to a command service resource. Similarly, meal orders are retrieved via GET resources from query services.

These HTTP interfaces concern the external communication. Internally the application communicates via events that are published using an event hub.