Timed promise queue / throttle

问题:

I have a request-promise function that makes a request to an API. I'm rate-limited by this API and I keep getting the error message:

Exceeded 2 calls per second for api client. Reduce request rates to resume uninterrupted service.

I'm running a couple of Promise.each loops in parallel which is causing the issue, if I run just one instance of Promise.each everything runs fine. Within these Promise.each calls they lead to the same function a with a request-promise call. I want to wrap this function with another queue function and set the interval to 500 milliseconds so that a request isn't made after one another, or parallel, but set to that time, on queue. The thing is I still need these promises to get their contents even if it takes a rather long time to get a response.

Is there anything that will do this for me? Something I can wrap a function in and it will respond at a set interval and not in parallel or fire functions one after another?

Update: Perhaps it does need to be promise specific, I tried to use underscore's throttle function

var debug = require("debug")("throttle")
var _ = require("underscore")
var request = require("request-promise")

function requestSite(){
  debug("request started")
  function throttleRequest(){
    return request({
      "url": "https://www.google.com"
    }).then(function(response){
      debug("request finished")
    })
  }
  return _.throttle(throttleRequest, 100)
}

requestSite()
requestSite()
requestSite()

And all I got back was this:

$ DEBUG=* node throttle.js 
throttle request started +0ms
throttle request started +2ms
throttle request started +0ms

回答1:

Update

The last answer was wrong, this works but I still think I can do better:

// call fn at most count times per delay.
const debounce = function (fn, delay, count) {
    let working = 0, queue = [];
    function work() {
        if ((queue.length === 0) || (working === count)) return;
        working++;
        Promise.delay(delay).tap(() => working--).then(work);
        let {context, args, resolve} = queue.shift();
        resolve(fn.apply(context, args));
    }
    return function debounced() {
        return new Promise(resolve => {
            queue.push({context: this, args: arguments, resolve});
            if (working < count) work();
        });
    };
};

function mockRequest() {
    console.log("making request");
    return Promise.delay(Math.random() * 100);
}

var bounced = debounce(mockRequest, 800, 5);
for (var i = 0; i < 5; i++) bounced();
setTimeout(function(){
    for (var i = 0; i < 20; i++) bounced();
},2000);

---

So you need to make the requests throttle function-wide - that's fine. Promises have queueing pretty much built in.

var p = Promise.resolve(); // our queue

function makeRequest(){
    p = p.then(function(){ // queue the promise, wait for the queue
        return request("http://www.google.com");
    });
    var p2 = p; // get a local reference to the promise
    // add 1000 ms delay to queue so the next caller has to wait 
    p = p.delay(1000); 
    return p2;
};

Now makeRequest calls will be at least 1000ms apart.

jfriend has pointed out that you need two requests per second and not a single one - this is just as easily solvable with a second queue:

var p = Promise.resolve(1); // our first queue
var p2 = Promise.resolve(2); // our second queue

function makeRequest(){

    var turn = Promise.any([p, p2]).then(function(val){ 

        // add 1000 ms delay to queue so the next caller has to wait 
        // here we wait for the request too although that's not really needed, 
        // check both options out and decide which works better in your case
        if(val === 1){
            p = p.return(turn).delay(1, 1000);
        } else {
            p2 = p2.return(turn).delay(1, 1000); 
        }
        return request("http://www.google.com");
    });

    return turn; // return the actual promise
};

This can be generalized to n promises using an array similarly