Await for proper connection to rabbit in nodejs

Question

I try to write my simple eventemitter wrapper for amqplib/callback_api. I have trouble with handling sitaution when rabbit is not available or disconnected. I have method getConnect which returns Promise, which resolves when connection established. But if connection is refused Promise obviously rejects. How to force this method do reconnection while connection will not wstablished

/**
     * Async method getConnect for connection
     * @returns {Promise<*>}
     */
    getConnect = async () => {
        return new Promise((resolve, reject) => {
            amqp.connect(this.config.url, async function(err, conn) {
                    if (err) {
                        reject(err);
                    }
                    resolve(conn);
            })
        })
    };

Whole code is here https://github.com/kimonniez/rabbitEE

Maybe, I'm already very sleepy, but I'm completely confused :) Thanks in advance!

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

Wrap your `Promise` inside an `Observable`

Promise is not built to handle "retry" logic. If you want to do that, you should look into Observables using the rxjs library. This will allow you to retry using an arbitrary time interval while catching errors.

const { from, interval, of } = rxjs;
const { catchError, mergeMap, tap, skipWhile, take } = rxjs.operators;

const THRESHOLD = 3;
const RETRY_INTERVAL = 1000;

// Equivalent to 'amqp.connect'
const functionThatThrows = number =>
  number < THRESHOLD
    ? Promise.reject(new Error("ERROR"))
    : Promise.resolve("OK");

// Equivalent to `getConnect`
const getConnect = () =>
  interval(RETRY_INTERVAL)
    .pipe(
      mergeMap(x => from(functionThatThrows(x)).pipe(catchError(e => of(e)))),
      skipWhile(x => {
        const isError = x instanceof Error;
        if (isError) console.log('Found error. Retrying...');
        return isError;
      }),
      take(1)
    ).toPromise();

// Resolve only if the inner Promise is resolved
getConnect().then(console.log);

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/6.4.0/rxjs.umd.min.js"></script>

Explanation

Create a source with an interval of 1000. Meaning that it will retry each second
Call your amqp.connect which is equivalent to functionThatThrows in my example
Catch the error using the catchError operator and return it
Skip while the returned object is an error. This will allow your to resolve only if your Promise has been resolved and not rejected
Take the first resolved result using take(1)
Convert your observable into a promise using the toPromise utility function
Call your function and attach then like you do with a standard Promise

Thanks a lot! Actually I need to modificate my code for this new logic but anyway now it seems greate! — kimonniez, Feb 28 '19 at 08:21

score 0 · Answer 2 · answered Feb 27 '19 at 22:15

If you just want to keep tryng connecting until a connection is made, you can wrap the getConnect method into a new keepConnect method:

keepConnect = async () => {
   while (true) {
      try {
         let conn = await getConnect()
         return conn
      } catch (e) {}
  }
}

But I think it would be better to implement something like a "try to connect for n times", by changing the while condition. In general, a "while true" solution is not clean and could perform bad, with the risk to slow down the event loop (imagine if the connect method will always return an error in few milliseconds).

You could also implement a system of progressive delays between connection attempts, using the keepConnect wrapper as idea.

If you instead want to reconnect when the connection is lost, then this is related to Rabbit (that I don't know) and his events.

Await for proper connection to rabbit in nodejs

2 Answers2

Wrap your Promise inside an Observable

Explanation

Wrap your `Promise` inside an `Observable`