How do you implement server-sent events (SSE) in Spring WebFlux?

Table of Contents

• Introduction
  • 1. What are Server-Sent Events (SSE)?
  • 2. Setting Up SSE in Spring WebFlux
    • Step 1: Add Dependencies
  • 3. Create a Server-Sent Event Controller
    • Step 2: Implementing an SSE Endpoint
  • 4. Setting the Content-Type for SSE
  • 5. Handling Client-Side SSE
  • 6. Handling Reconnection
  • 7. Closing the SSE Stream
• Conclusion

Introduction

Server-Sent Events (SSE) is a standard for allowing a server to push real-time updates to a web client over a single HTTP connection. Unlike traditional HTTP requests, where the client asks for data, SSE allows the server to send data to the client whenever there is new information available. This is particularly useful for applications like live feeds, notifications, stock market updates, and other real-time applications.

Spring WebFlux, being built for reactive programming, provides great support for SSE, making it easy to implement real-time streaming of events. In this guide, we'll walk through the process of setting up Server-Sent Events in a Spring WebFlux application.

1. What are Server-Sent Events (SSE)?

Server-Sent Events (SSE) is a mechanism that enables the server to push data to the browser over an HTTP connection. Unlike WebSockets, which allow bi-directional communication, SSE is one-way communication from the server to the client.

SSE has the following characteristics:

• The server sends data to the client when new data is available.
• The communication is over a single long-lived HTTP connection.
• Data is sent in text/event-stream format.
• It supports reconnection in case the connection is lost.

SSE is widely supported in modern browsers, making it a great choice for real-time data streaming.

2. Setting Up SSE in Spring WebFlux

To implement SSE in Spring WebFlux, you need to define an endpoint that streams data to the client. Spring WebFlux handles this using the **Mono** or **Flux** type to represent the stream of data.

Step 1: Add Dependencies

If you're using Spring Boot, include the spring-boot-starter-webflux dependency in your pom.xml or build.gradle file.

For Maven:

For Gradle:

3. Create a Server-Sent Event Controller

In Spring WebFlux, we can use the **Flux** type to stream a continuous flow of data to the client. The Flux will emit a sequence of events, which will be transmitted to the client over an open HTTP connection.

Here’s how you can implement an SSE endpoint in a Spring WebFlux controller.

Step 2: Implementing an SSE Endpoint

In this example:

• The streamSse() method returns a **Flux<String>**, which represents an asynchronous stream of data.
• The **Flux.interval(Duration.ofSeconds(1))** creates a stream of events emitted every second.
• The map() function transforms each event emitted by Flux into a message like "SSE Event 1", "SSE Event 2", etc.

This will create an SSE endpoint at /sse that pushes updates to the client every second.

4. Setting the Content-Type for SSE

For SSE, the content type must be set to text/event-stream. Spring WebFlux automatically sets the correct content type when returning a **Flux** in a controller method that streams data. This is important because the client (usually a browser) will expect the response in the SSE format.

However, if you're manually setting the response headers, you can do it as follows:

5. Handling Client-Side SSE

On the client-side, you can use the EventSource API to receive the SSE stream.

Here’s an example of how to set up the client to consume the SSE stream:

• The EventSource object is used to establish a connection to the /sse endpoint.
• Every time the server sends a new event, it is received by the client’s onmessage handler, which appends the event data to the webpage.
• If there is an error (e.g., network issue or server shutdown), the onerror handler will be triggered.

6. Handling Reconnection

By default, the EventSource API automatically handles reconnections if the connection is lost. The client will attempt to reconnect after a certain amount of time. You can also customize the behavior on the server-side by using the retry field in the SSE response.

Example of sending a custom retry interval in the SSE stream:

This tells the client to retry connecting after 1000 milliseconds (1 second) in case the connection is lost.

7. Closing the SSE Stream

You can close an SSE stream on the server-side by completing the Flux stream, for example, after a certain condition is met (such as a timeout or an event trigger). This can be done by emitting Flux.empty() or any other mechanism for ending the stream.

This example ensures that the stream ends after 10 events.

Conclusion

Server-Sent Events (SSE) in Spring WebFlux provide a powerful and efficient way to stream data from the server to the client in real-time. Using Flux to stream events and the EventSource API on the client-side, you can easily implement real-time features in your applications. SSE is particularly useful for applications that require continuous updates, such as live feeds, chat applications, or real-time dashboards.

By combining Spring WebFlux’s reactive capabilities with SSE, you can create highly scalable and efficient real-time applications that handle thousands of concurrent users with minimal resources.