The event loop is the executing mechanism of a Verticle.
Java Thread distribution
In Vert.x, every verticle runs in its own thread and waits for incoming events (event-driven).
The Java main thread executes the Vert.x instance which in turn:
- deploys and undeploys verticles
- coordinates the startup and shutdown of all verticles
- provides the APIs so that verticles can talk to each other and with the outside world (e.g. the event bus)
A verticle thread:
- spawns once the verticle starts
- executes asynchronous code inside the verticle on incoming events (which can trigger other events)
- dies once the verticle stops
Event Loop inside a Verticle Thread
Since a verticle is only executed by one thread, this thread must handle all incoming events and their connected event handlers.
Most of the time this isn't a problem, especially if the event handler is compact and runs fast code. But once the thread grabs an event handler that contains blocking or heavy-lifting code, new events clog up the event queue, making the verticle unresponsive.
Therefore, Vert.x and Telestion use asynchronous code structures to move the blocking part away from the Verticle Thread into another "service" Thread which handles the resource-intensive task (e.g. the HTTP server in turn spawns another Thread in the background to efficiently handle incoming connections).