@sentry/nextjs docs: Why do you need to flush when deploying to Vercel?

[benj-dobs]

In the @sentry/nextjs docs, Creating a custom _error page, the getInitialProps sample code contains the following snippet:

  if (err) {
    Sentry.captureException(err);

    // Flushing before returning is necessary if deploying to Vercel, see
    // https://vercel.com/docs/platform/limits#streaming-responses
    await Sentry.flush(2000);

    return errorInitialProps;
  }

The contents of the given URL are, in its entirety:

Streaming Responses

Vercel supports streaming responses using Edge Functions.

See this example on how to create a stream that sends a response asynchronously.

This does not explain why Sentry needs to flush if we're deploying to Vercel.

So why does it?

[lforst]

Hi, vercel might have updated the docs behind the link in the code so it's not relevant anymore.

In essence, we're flushing here because we want all outgoing requests to sentry to be complete before continuing. Why? Vercel uses AWS lambdas behind scenes and those lambdas freeze after sending a response. We don't want the sending of error events to be interrupted by the freeze so we're just blocking the freeze until they're sent - otherwise errors might not get set to sentry.

[benj-dobs]

Thank you @lforst, that's a helpful explanation :)

[vladanpaunovic]

There is an open issue now. I suggest we follow the conversation in #5131

[benj-dobs]

I think that's a different issue