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Requests, Commands and an Events
We use the term Request for a data object containing parameters that you want to dispatch to a handler. Brighter uses the interface IRequest for this concept.
We do not recommend deriving from IRequest but instead from the classes Command and Event which represent types of Request.
Why have both Command and Event? The difference is in how the Command Dispatcher dispatches them to handlers.
- A Command is an imperative instruction to do something; it only has one handler. We will throw an error for multiple registered handlers of a command.
- An Event is a notification that something has happened; it has zero or more handlers.
The difference is best explained by the following analogy. If I say "Bob, make me a cup of coffee," I am giving a Command, an imperative instruction. My expectation is that Bob will make me coffee. If Bob does not, then we have a failure condition (and I am thirsty and cranky). If I say "I could do with a cup of coffee," then I am indicating a state of thirst and caffeine-withdrawal. If Bob or Alice make me a coffee I will be very grateful, but there is no expectation that they will.
So choosing between Command or Event effects how the Command Dispatcher routes requests.
Some messaging frameworks encourage you to share an assembly containing your message definitions between autonomous components, often as interfaces. Occasionally we see users trying to use IRequest for this purpose.
We do not recommend this, instead preferring to keep to the Service Oriented Architecture (SOA) tenet of Share schema not type.
Between components that we wish to be independently deployable - which might after all be in different languages, or use different frameworks - you should share a schema that defines the shape of the message (for example AsyncAPI.
The only exception is where you have two apps that form part of a single service - such as a Task Queue that supports offloading work from a web API - as these tend to be a unit for Continuous Integration and not independently deployable, then sharing types may be appropriate.
Many of our samples share types for convenience, but this is not advice to do that outside of a Task Queue.