I read that
Promises also ensure order-independence. Separating the producer and consumer, so that they can execute independently, is the crux of asynchronous programming. With a promise, you can give a promise to a consumer before giving a resolver to a producer. They can then produce or consume on their own schedule and the promise synchronizes.
Trying to understand the Order Independence described above, I am writing up some code to hopefully demonstrate this. Basically, I wanted to create two asynchronous promises (based on downloading two images at two different times in partA()
and partB()
), and then resolve both using all
.
var axios = require('axios');
function loadImage(imgsrc) {
return axios.get(imgsrc);
}
function partA() {
var promise1 = loadImage("http://cn.bing.com/s/a/hp_zh_cn.png");
var promise2 = setTimeout(partB,20000);
Promise.all([promise1, promise2]).then(function() {
console.log("The images are loaded");
});
};
function partB () {
return loadImage("http://cn.bing.com/s/a/hpc20.png");
};
partA();
Here are my issues and questions are:
When executing partA()
in the last line, I expected that I had to wait 20 seconds to see the success message because of the var promise2 = setTimeout(partB,20000);
line (I had hoped that the two downloads happens 20 seconds apart from each other, for illustration purposes). Maybe I didn't use setTimeout
right. But in any case, I got the success message almost immediately after I call partA() in the babel-node
REPL. How do I properly get the delay?
In this example (if correct), how can I interpret the Order Independence in terms of produce or consume on their own schedule
? Where are the sites of production and consumption?
(This is with babel-node 6.24.1 with --presets es2015
under Ubuntu 16.04)
The problem is, that setTimeout does not return a Promise. You have to insert a promise which executes a timeout to Promise.all.
function partA() {
var promise1 = loadImage("http://cn.bing.com/s/a/hp_zh_cn.png");
var promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
return resolve(partB);
}, 20000);
});
Promise.all([promise1, promise2]).then(function() {
console.log("The images are loaded");
});
};
function partB() {
return loadImage("http://cn.bing.com/s/a/hpc20.png");
};
partA();
In this case, promise2 is actually a promise, which resolves with partB result, after 20 seconds.
setTimeout
does not return a promise.
To see order independence at work consider this example:
var promise1, promise2;
promise1 = loadImage("http://cn.bing.com/s/a/hp_zh_cn.png");
promise2 = loadImage("http://cn.bing.com/s/a/hpc20.png");
promise1.then(function () {
console.log("promise1 finished");
});
promise2.then(function () {
console.log("promise2 finished");
});
Promise.all([promise1, promise2]).then(function() {
console.log("both finished");
});
The output of this script can be either:
promise1 finished
promise2 finished
both finished
or
promise2 finished
promise1 finished
both finished
It just depends on which request completes earlier. If you want a more controlled example consider this:
var promise1, promise2;
promise1 = create_timeout_promise(20);
promise2 = create_timeout_promise(10);
function create_timeout_promise(timeout) {
var promise;
promise = new Promise(function (resolve) {
setTimeout(resolve, timeout);
});
return promise;
}
Now the exepcted output is:
promise2 finished
promise1 finished
both finished
Because promise 2 is going to resolve first.
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