Watch an API for Updates - node.js

Question

I am trying to make an API watch to https://api.exchangeratesapi.io/latest
and detect if any changes so I can update my clients just like a web hook notification. But what am doing is
while(true){
fetch(https://api.exchangeratesapi.io/latest)
.then(res => ......
}
I am caching the result and always check if there is any changes, If there is, I will send a request to the client.
I am looking for a better way to pull the data instead of making a while loops

Answer 1

If you use a while loop, you would potentially send many requests before one returns. And even when one returns, it's not going to necessarily be in order. Here is a quick example of what might happen if there is a network spike for example:
const sleep = ms => new Promise(res => setTimeout(res, ms));
async function mockNetworkCall(num) {
const timings = [50, 150, 50]
console.log(`sending request ${num}`);
await sleep(timings[num]);
console.log(`request ${num} finished`)
}
for(let i = 0; i < 3; i++) {
mockNetworkCall(i);
}
You could avoid the while loop if you instead take the approach to only do a new request when the last one finishes. In that case, you will only have a single request active at any one time and you know that you're getting the result in order.
You can wrap the logic for that in a simple function to watch a URL and only re-initiate a request when the previous one is finished. This is the skeleton for such a function - it might need tweaking according to your needs:
function watch({ url, retryCount, lastResponse = null}) {
fetch(url)
.then(res => {
/* do something */
return res;
})
.then(res => watch({url, retryCount, lastResponse: res})) //launch again
.catch(err => {
/* handle error */
console.error("error getting URL", err);
console.warn("retries left: ", retryCount);
if (retryCount-- > 0) {
watch({url, retryCount, lastResponse});
}
})
}
watch({url: "google.com", retryCount: 3});
Or the same using async/await
async function watch({ url, retryCount, lastResponse = null}) {
try {
const res = await fetch(url);
/* do something */
watch({url, retryCount, lastResponse: res}); //launch again
} catch (err) {
/* handle error */
console.error("error getting URL", err);
console.warn("retries left: ", retryCount);
if (retryCount-- > 0) {
watch({url, retryCount, lastResponse});
}
}
}
watch({url: "google.com", retryCount: 3});