What is a data stream or stream?

In Reactive programming, we write programs to work on a sequence of events. For instance, in a spreadsheet, we can observe the following events in a sequence:

1. The user enters a value 1 into cell A1. When the user inputs data into cell A1, the value in cell A3 is updated to 1.
2. The user enters a value 2 into cell A2. When the user inputs data to cell A2, A3 is updated to 3.

In the Reactive World, this sequence of events happening over time is known as a stream, events stream, or data stream. The following diagram shows how a sequence of events forms an events stream. It also shows how a Publisher sends events to an event stream and how a Subscriber receives events from that event stream: 

A stream or data stream is a sequence of ongoing events ordered in time.

In RP, the Publisher sends events to a Stream, and the Subscriber consumes those events from the Stream.

To react to events, we should monitor them. In RP, the process of monitoring events is known as listening to events or subscribing to events.

We can also define RP using this data stream:

RP is a programming paradigm to do programming with asynchronous data streams.