I have an Express.js website that utilizes socket.io. For each user (socket) that connects, a function is called that can take a long time to resolve.
server.js
const app = express();
const server = http.createServer(app);
const io = new Server(server);
io.on('connection', async (socket) => {
console.log(`${socket.id}: socket connected`);
console.log(`${socket.id}: computation started`);
await longRunningTask();
console.log(`${socket.id}: computation completed`);
socket.on('disconnect', () => {
console.log(`${socket.id}: socket disconnected`);
// TODO: Check if `longRunningTask` has resolved. If it has not, cancel it
});
});
This is the current flow of the program given a user disconnects before the longRunningTask resolves:
User visits website (they are assigned socket.id tCzH)
tCzH: socket connected
tCzH: computation started
User disconnects
tCzH: socket disconnected
Some time passes
tCzH: computation completed
My goal is to terminate the longRunningTask immediately given the socket is disconnected. What would be the best approach?
Unfortunately, it is impossible to terminate the execution of a function from outside its' scope in Javascript. Even if you wrap your longRunningTask in a Promise and reject that Promise manually, it will not prevent the rest of its stack from execution.
Implementation of longRunningTask has to be modified so it is dependent on a global variable that is in the outer scope, and once this variable becomes for example false it should return or break itself from within.