RNEventSource in Expo Causes Infinite Loop with Repeated 'attempt: 1' SSE Events

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I'm using RNEventSource in a React Native Expo managed project to handle Server-Sent Events (SSE). While my backend streams data correctly (verified via Postman), the frontend gets stuck in an infinite loop, receiving the same event repeatedly. The logs only show the following response:

BASH
LOG  Open SSE connection.
LOG  Stream Data: {"attempt": 1, "run_id": "1efb7463-0ff8-60c3-bd10-17ccc210899f"}
WARN  Unexpected Data: {"run_id":"1efb7463-0ff8-60c3-bd10-17ccc210899f","attempt":1}
LOG  Stream Data: {"attempt": 1, "run_id": "1efb7463-0ff8-60c3-bd10-17ccc210899f"}
WARN  Unexpected Data: {"run_id":"1efb7463-0ff8-60c3-bd10-17ccc210899f","attempt":1}
...

The backend sends exactly the same data that the frontend is trying to handle, and after testing with Postman the stream works as expected.

The frontend code looks like this:

JAVASCRIPT
const askDogy = async (request) => {
  const url = `${assitantBaseUrl}/threads/${threadId}/runs/stream`
  const requestBody = JSON.stringify({
    assistant_id: assistantID,
    input: {
      messages: [{ role: 'user', content: request }],
    },
    stream_mode: ['messages'],
  })

  const options = {
    body: requestBody,
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      Accept: 'text/event-stream',
    },
  }

  eventSourceRef.current = new RNEventSource(url, options)

  eventSourceRef.current.addEventListener('open', () => {
    console.log('Open SSE connection.')
  })

  eventSourceRef.current.addEventListener('message', (event) => {
    console.log('Raw Event:', event)
    try {
      const data = JSON.parse(event.data)
      console.log('Parsed Data:', data)

      if (data.content) {
        console.log('AI Reply Content:', data.content)
        setChatResponse('Bot', data.content, true)
      } else {
        console.warn('Unexpected Data:', data)
      }
    } catch (error) {
      console.error('Error parsing event data:', error)
    }
  })

  eventSourceRef.current.addEventListener('error', (error) => {
    console.error('Stream error:', error)

    if (error.status === 409) {
      console.log('Conflict detected, creating a new thread...')
      // Create a new thread and retry
      tempThreadId = await createThread()
      if (!tempThreadId)
        throw new Error('Failed to create a new thread after conflict')
      setThreadId(tempThreadId)

      // Retry the stream with the new thread ID
      console.log('Retrying with new thread ID:', tempThreadId)
      await askDogy(request) // Recursive call with the new thread
    } else {
      handleStopStream()
    }
  })
}

Debugging Observations:

The log repeatedly shows the same attempt: 1 data without progressing.
The backend should provide incremental updates, but they are never reflected on the frontend.

Questions:

Is there a known issue with RNEventSource or its compatibility with React Native (Expo managed projects)?
Are there specific handling techniques in RNEventSource for parsing incremental updates?
What alternative npm packages or libraries can manage server-sent events in React Native/Expo?

Things I've Tried:

Verified that the backend complies with the SSE specification.
Ensured the RNEventSource initialization uses the proper headers and POST method.
Implemented maximum retries and timeout logic (now removed) to handle backend failures.
Tested the backend stream with Postman, confirming correct behavior.

Comments 0

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Answers 1

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1 Answer

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is some insight and recommendations regarding the issues you’re seeing with RNEventSource:

1. Known Compatibility/Behavior Issues

RNEventSource on Expo/React Native:
There have been reports that RNEventSource may behave unexpectedly under certain React Native configurations (and Expo managed projects are no exception). In some cases, it may not process incremental chunks as expected, causing the same event to be parsed repeatedly. The library relies on correctly formatted and flushed SSE streams, and if the native environment buffers data differently than a browser, you may observe issues.

2. Handling Incremental Updates

SSE Formatting:
Ensure that your backend strictly follows the SSE spec. This means:
- Each event should be prefixed with data: , and events should end with two newline characters (\n\n).
- Verify that the stream is flushed immediately after sending each chunk. If the flush is delayed or if the event isn’t terminated properly, RNEventSource might re-read the same data over and over.
Library-Specific Behavior:
RNEventSource might buffer data or interpret line-breaks differently in a React Native (or Expo) environment. If the event chunks arrive too slowly or with unexpected formatting, this can lead to events being reprocessed and logged as identical.

3. Alternative Libraries/Approaches

Alternatives to RNEventSource:
If you continue to see issues, you have a few options:
- Fetch Streams:
  Use the native fetch API (or libraries built on top of it) to handle streaming responses. You can read the response using response.body.getReader() (when available) and manually parse chunks. Note that support for streaming may vary on React Native versions.
- Other SSE Libraries:
  Consider trying alternative libraries that implement SSE clients. Some developers have used packages like fetch-event-source (which works on Node and modern browsers) by polyfilling or adapting it for React Native. However, adaptions might be required given that not every library is tailored for the React Native runtime.
Custom Implementation:
If the SSE protocol is your only need, implementing a lightweight custom parser using the streaming fetch API might give you tighter control over how events are parsed and ensure they’re processed incrementally as intended.

Debugging Checklist

Backend Verification:
Use a tool like Postman (which you’ve done) to confirm the endpoint streams data as expected. Double-check that each event ends with \n\n to signal the end of an event.
Headers and Options:
Make sure the headers (especially Accept: text/event-stream) and the POST method are in line with what your backend expects for SSE connections.
Recursive Calls:
In your error handler, the recursive call to askDogy(request) on a conflict (HTTP 409) might inadvertently cause repeated connections in some cases. Although this may not be the root cause here, it’s worth reviewing in a production scenario.

Summary

RNEventSource Issues: There isn’t a widely reported “bug” but community feedback suggests it can be finicky with incremental updates and buffering in Expo/React Native.
Incremental Updates: Ensure the streaming endpoint flushes and terminates events properly (using double newlines).
Alternatives: Consider using a streaming fetch implementation or alternative SSE libraries (like fetch-event-source) that can be adapted to React Native.

By reviewing the stream format and possibly exploring an alternative method for handling SSE, you might overcome the reoccurrence of identical events and get the incremental updates you expect.

If you have more questions or need further assistance troubleshooting the stream parsing, feel free to ask!

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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