Event-driven architecture 

Event-driven programming is a programming style in which code executes in response to an event, such as a keypress or a mouse click. In short, the flow of a program is driven by events.

GUI programming is an example of event-driven programming. For example, X Windows (driving most of your Linux GUI) processes a series of XEvents. Every keypress, mouse button press or release, and mouse movement generates a series of events. If you are on Linux, there is a command called xev. Running it via Terminal spawns a window. When moving a mouse over the window or pressing some keys, you can see the events that are generated.

Here is a capture of the xev program on my Linux laptop: 

You can plug in a callback, which gets triggered upon the reception of such an event. For example, an editor program could use keypress events to update its state (resulting in its documents being edited). Traditional event-driven programming could create a complex callback flow, thereby making it hard to figure out the control flows in the code.

Event-driven architecture (EDA) helps in decoupling a system's modules. Components communicate using events, which are encapsulated in messages. A component that emits an event does not know anything about the consumers. This makes EDA extremely loosely coupled. The architecture is inherently asynchronous. The producer is oblivious of the consumers of the event messages. This process is shown in the following diagram:

Given one thread and an event loop, with the callbacks executing quickly, we have a nice architecture. How does all this relate to concurrency? There could be multiple event loops running on a pool of threads. Thread pooling is an essential concept, as we will see in the upcoming chapters. 

As we have seen, an event loop manages events. The events are passed on to an installed handler, where they are processed. The handler can react to an event in two ways: either it succeeds or it fails. A failure is passed to the event loop again as another event. The handler for the exception decides to react accordingly.