I've been experimenting with a code that involves adding promises to a queue for processing in a non-blocking manner. One code snippet behaves as anticipated while the other doesn't, leaving me puzzled.
Problematic Code:
const axios = require('axios');
const promiseQueue = require('promise-queue');
let currentQueueSize = 0;
const maxQueueSize = 5;
// eslint-disable-next-line new-cap
const queue = new promiseQueue(maxQueueSize, maxQueueSize);
const requestData = () => {
currentQueueSize++;
queue
.add(() => axios.get('https://catfact.ninja/fact'))
.then(result => {
console.log(result.data);
})
.catch(e => {
console.log(e);
})
.finally(() => {
currentQueueSize--;
});
};
const run = async () => {
while (true) {
if (currentQueueSize <= maxQueueSize) {
console.log('current queue size ', currentQueueSize);
requestData();
}
}
};
run();
I was under the impression that this code would add items to the queue, decrease the queue size upon resolution, and allow for more additions. However, the then block within the requestData function is never executed.
In contrast, when I modify the run code as follows:
const run = async () => {
while (true) {
if (currentQueueSize <= maxQueueSize) {
console.log('current queue size ', currentQueueSize);
ingestData();
} else {
await new Promise((resolve, _reject) => {
const waiter = () =>
setTimeout(() => {
if (currentQueueSize <= maxQueueSize) {
console.log(`Queue size ${currentQueueSize} now under max queue size of ${maxQueueSize}, resuming...`);
resolve(true);
} else {
console.log(`Waiting for queue size to reduce, current queue size ${currentQueueSize}`);
waiter();
}
}, 1000);
waiter();
});
}
}
};
The execution of then, catch, and finally is as expected, allowing continuous additions to the queue as promises are resolved. What distinguishes the dysfunctional first code from the functional second one?