Message Driven vs Event Driven
Reactive Systems rely on asynchronous message-passing to establish a boundary between components that ensures loose coupling, isolation and location transparency . This boundary also provides the means to delegate failures as messages. Employing explicit message-passing enables load management, elasticity, and flow control by shaping and monitoring the message queues in the system and applying back-pressure when necessary. Location transparent messaging as a means of communication makes it possible for the management of failure to work with the same constructs and semantics across a cluster or within a single host. Non-blocking communication allows recipients to only consume resources while active, leading to less system overhead.
A Message is some data sent to a specific address. In Message Driven systems, each component has a unique address other components can send messages to. Each of these components, or recipients, awaits messages and reacts to them.
An Event is some data emitted from a component for anyone listening to consume.
In traditional programming models, Component A invoking a method in Component B was coupled in time. If Component B was busy or if it was slow when handling the method invocation, then Component A had to wait.
In Message Driven systems, Component A produces a message indicating it must be delivered to the address of Component B. Component A then send the message and gets control back immediately instead of waiting for Component B to complete the handling of the message. Components on a message driven system often have a queue where incoming messages can be stored in case of a load spike. Message passing is a building block for space decoupling .
In Event Driven systems, components announce a location where they expose their events. So a Component A emitting events will use a well-known location to publish them but not know which components are consuming the events. This well-known location is implemented using an ordered queue. Sometimes the queue is indexable so consumers can keep track of the events already consumed and the events pending.
As discussed above, in Message Driven systems, each component send items to a fixed recipient. In Event Driven systems, on the other hand, each component produces items of data with a fixed sender and shares them with any consumer. This comparison, though, can be misleading: the term 'Message Driven' refers to a building block on a system and 'Event Driven' refers to a higher level property of a system. So, using Message Driven tools we can build an Event Driven system.
Command — a message sent to produce a change on the state. The receiving component, if the command is valid, will update its state.
Event — a message emitted by a component when its state changed. The event includes the necessary data to evolve the older state to the newer state. The event is a message sent to the publishing infrastucture where consumers may later retrieve it.
Query — a message sent to a component to obtain some information from it. A Query is a special type of message because it must include a sender address so the target component can reply.
Reply — the response to a query. A component handling a Query message will produce a Reply message and send it to the sender address specified in the query message.