Spring WebFlux + Server Sent Events (SSE)


What options we have to gather long processing data from the backend to send out to the UI?
Usually the followings:

  • open frequent connections – polling
  • keep an open connection – websocket

Server Sent Events

With Server Sent Events you’re able to send continuous updates from the server.
Imagine as a client subscription to a server for a stream.
Sometimes we don’t need a full bi-directional protocol (sending data back from client side), just a non-blocking data flow with the ability to process data row by row, while still receiving from the connection.
These are updates, time splitted data chunks from the server side to provide a fluent UI experience.



  • unidirectional protocol
  • simultaneous request limit (browser defaults: 6)


With reactive programming, the purpose of this type of data processing is the fluency and non-blocking nature.

There are more conceptual advantages, like: backpressure, split and delayed and limited processing of data.

It supports asynchronous and event-driven concept in programing with a fluent stream-like API.

The reactive API is present in Spring since version 5

It can be used with the specific content types in between:

  • backend-backend server-client (SOA/microservice calls)
  • backend-ui server client (server-browser calls)

Example Project

I picked some sample data about a movie database.
Our sample application is very simple: We stream the file line-by-line. Convert the data into a data model and producing the result as a stream to the endpoint.
The client side subscribed to the backend endpoint and processing the data line-by-line.
This way we have a stream from a datasource till the client visualization streamlined.

Backend stream logs

UI streaming processing and visualization

Besides the standard Flux stream we only needed to use the EVENT_STREAM Mediatype. While on the UI client side EventSource interface is required to capture the stream.

For more details see the Webflux + SSE example project: https://github.com/PazsitZ/webflux_sse