Dart Web Server: prevent crash

Issue

Id’like to develop a web services + web sockets server using dart but the problem is I can’t ensure the server’s high availability because of uncatched exceptions in isolates.

Of course, I have try-catched my main function, but this is not enough.

If an exception occurs in the then() part of a future, the server will crash.

Which means that ONE flawd request can put the server down.

I realize that this is an open issue but is there any way to acknoledge any crash WITHOUT crashing the VM so that the server can continue serving other requests ?

Thank you.

Solution

What I’ve done in the past is use the main isolate to launch a child isolate which hosts the actual web server. When you launch an isolate, you can pass in an “uncaught exception” handler to the child isolate (I also think you should be able to register one at the top-level as well, to prevent this particular issue, as referenced by the issue in the original question).

Example:

import 'dart:isolate';

void main() {
  // Spawn a child isolate
  spawnFunction(isolateMain, uncaughtExceptionHandler);
}

void isolateMain() {
  // this is the "real" entry point of your app
  // setup http servers and listen etc...
}

bool uncaughtExceptionHandler(ex) {
  // TODO: add logging!
  // respawn a new child isolate.
  spawnFunction(isolateMain, uncaughtException);
  return true; // we've handled the uncaught exception
}

Answered By – Chris Buckett

Answer Checked By – David Goodson (FlutterFixes Volunteer)