For receiving data from the server to the client we generally establish an HTTP connection. In case of getting a continuous update, we would previously(in some cases) use Polling or Long Polling on an HTTP request.
To have a persistent connection we use WebSockets so that we can send/receive data continuously without making a new connection.
Though WebSockets are excellent ways to communicate to servers when we need to both send and receive data to and from the server. But in lots of cases, we need to receive data from the server using a persistent connection, but there is no need to send data to the server. Like, a live dashboard, or a live analytics board. In these cases, WebSockets become an unnecessary overhead.
To solve this problem, a new way is introduced, named Server-Sent Events(SSE).
Here is the list of articles on SSE that will give you complete knowledge about SSE: