Exploring the integration of Server-Sent Events in Angular

Question

Exploring the integration of Server-Sent Events in Angular

I'm currently working on incorporating Server-Sent Events (SSE) into my testing application. The server side configuration has been completed, and I am utilizing the endpoint (api/v1/sse/document). The aim here is that whenever a scan is performed, I expect to see the results reflected in both my testing client and main application simultaneously. However, the issue I'm facing is that the updates only show up in my testing client after manually refreshing the page. Despite writing some code, I haven't been successful in achieving automatic event updates in my testing client.

Below is the code snippet that I have written:

sse.service.ts

[document-list.component.ts]

 // Include component functions and methods here...

In addition, this is the service responsible for handling document-related operations:

[a.service.ts]

 // Add service functionality and methods here...

Even though there are no error messages displayed in the console, and a 200 response is received from the endpoint, the events are not automatically propagated to my testing client. Instead, I have to refresh the page each time to observe the changes. Any insights or suggestions on resolving this issue would be greatly appreciated.

angular typescript events event-handling server-sent-events

Answer 1

Answer №1

The issue arises from treating the SSE endpoint like a regular GET endpoint. By doing this, updates will only be received when explicitly requested, such as by refreshing the page.

This is illustrated in the code snippet below:

public getServerSentEvent(url: string): Observable<MessageEvent> {
  const token = this.aService.config.authorization;
  // WARNING - SSE does not support Headers in its API
  const headers = new HttpHeaders({
    Authorization: token,
  });

  return new Observable<MessageEvent>((observer) => {
    // You are making an HTTP GET request instead of creating an SSE Event Source
    this.http.get(url, {headers, responseType: "text"}).subscribe({
      //...
    })
  })
}

In essence, you are using a conventional HTTP GET to interact with your SSE endpoint and attempting to mimic SSE behavior, resulting in failure.

To address this, you should establish a proper EventSource for handling SSE events:

// Establishes SSE event source and manages SSE events
protected createSseEventSource(): void {
  // Close existing event source if any
  if (this.eventSource) {
    this.closeSseConnection();
    this.eventSource = null;
  }
  // Create new EventSource instance with your desired SSE URL
  this.eventSource = new EventSource(this.yourSSEurl);

  // Handle default event
  this.eventSource.onmessage = (event: MessageEvent) => {
    this.zone.run(() => this.processSseEvent(event));
  };

  // Implement custom event handlers...
  /*
    enum SSE_EVENTS [
      NewDocument: 'NewDocument',
      //...
    ]
  */
  Object.keys(SSE_EVENTS).forEach(key => {
    this.eventSource.addEventListener(SSE_EVENTS[key], event => {
      this.zone.run(() => this.processSseEvent(event));
    });
  });

  // Handle connection opened event
  this.eventSource.onopen = () => {
    this.reconnectFrequencySec = 1;
  };

  // Handle errors
  this.eventSource.onerror = (error: any) => {
    this.reconnectOnError();
  };
}

Refer to my previous answer: Handling SSE reconnect on error, which provides a comprehensive SSE service implementation with all necessary functionalities.

Answer 2

The issue arises from treating the SSE endpoint like a regular GET endpoint. By doing this, updates will only be received when explicitly requested, such as by refreshing the page.

This is illustrated in the code snippet below:

public getServerSentEvent(url: string): Observable<MessageEvent> {
  const token = this.aService.config.authorization;
  // WARNING - SSE does not support Headers in its API
  const headers = new HttpHeaders({
    Authorization: token,
  });

  return new Observable<MessageEvent>((observer) => {
    // You are making an HTTP GET request instead of creating an SSE Event Source
    this.http.get(url, {headers, responseType: "text"}).subscribe({
      //...
    })
  })
}

In essence, you are using a conventional HTTP GET to interact with your SSE endpoint and attempting to mimic SSE behavior, resulting in failure.

To address this, you should establish a proper EventSource for handling SSE events:

// Establishes SSE event source and manages SSE events
protected createSseEventSource(): void {
  // Close existing event source if any
  if (this.eventSource) {
    this.closeSseConnection();
    this.eventSource = null;
  }
  // Create new EventSource instance with your desired SSE URL
  this.eventSource = new EventSource(this.yourSSEurl);

  // Handle default event
  this.eventSource.onmessage = (event: MessageEvent) => {
    this.zone.run(() => this.processSseEvent(event));
  };

  // Implement custom event handlers...
  /*
    enum SSE_EVENTS [
      NewDocument: 'NewDocument',
      //...
    ]
  */
  Object.keys(SSE_EVENTS).forEach(key => {
    this.eventSource.addEventListener(SSE_EVENTS[key], event => {
      this.zone.run(() => this.processSseEvent(event));
    });
  });

  // Handle connection opened event
  this.eventSource.onopen = () => {
    this.reconnectFrequencySec = 1;
  };

  // Handle errors
  this.eventSource.onerror = (error: any) => {
    this.reconnectOnError();
  };
}

Refer to my previous answer: Handling SSE reconnect on error, which provides a comprehensive SSE service implementation with all necessary functionalities.

Exploring the integration of Server-Sent Events in Angular

Answer №1

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