Correct pattern for flushing a (Sentry) event queue from middleware

[rpinna]

[Question]

Thank you for the great work on this package, I just integrated it and it went very smoothly.

We use Sentry for crash reporting. A natural use case for a middleware is to log all exceptions and rejected promises to Sentry.

I'm not quite sure how to handle the requirement to flush the Sentry event queue. You can refer to this issue, but the TL;DR is that it's possible to drop Sentry events from a lambda function if you don't make sure that its background event queue is drained. This isn't an edge case, it happens regularly if you don't protect against it.

This is what I have for the middleware right now. Could you take a look at the two calls to sentry.flush and advise if this is the correct pattern?

If this is of interest, please let me know if you would like me to submit a PR.

import middy from '@middy/core';

interface Breadcrumb {
  message?: string;
  data?: {
    [key: string]: any;
  };
}

interface SentryLike {
  addBreadcrumb(breadcrumb: Breadcrumb): void;
  captureException(exception: any): string;
  flush(timeout?: number): Promise<boolean>;
}

interface Config {
  sentry: SentryLike;
  injectInContext?: boolean;
}
export function sentryMiddleware<TEvent = any, TResponse = any>(
  config: Config,
): middy.MiddlewareObject<TEvent, TResponse> {
  const { injectInContext, sentry } = config;
  return {
    before: (handler: any, next: any) => {
      if (injectInContext !== false) {
        handler.context.sentry = sentry;
      }
      return next();
    },
    after: async () => {
      await sentry.flush();
    },
    onError: async (handler: any) => {
      const { error, event } = handler;
      sentry.addBreadcrumb({
        message: 'lambda event',
        data: event,
      });
      sentry.captureException(error);
      await sentry.flush();
      return error;
    },
  };
}

[willfarrell]

That pattern looks right to me. Because this depends on a 3rd party service we likely won't add into core at this time. I'd be happy to approve a PR to add it to the community middleware list.