Publish and Subscribe (PubSub) refers to a data communication pattern. Instead of continually polling for resources, a client application using a PubSub style subscribes to a source and is updated only once data is changed. Applications that adopt a PubSub pattern are asynchronous, meaning they aren't actively communicating consistently. Instead of requesting data from a server and "pulling' it to the application, the application subscribes to events and is "pushed" data only once actions occur. These actions could be any event programmed to affect a change of server state. Cloud application updates, sensor readings, stimuli from an external user, and other scenarios could trigger such events.