Hello Community,
I am currently working on an application displaying live data for railway departure boards. When developing or building locally - independent of OS or build environment - everything works fine, all trains are displayed correctly. But once deployed to vercel certain trains have duplicate list entries. I have tracked down the Issue to a feature called irisOverride in my code:
`const processedItems = new Map<string, Stop>();
function processItems(items: WebAPIStop[], isArrival: boolean) {
// This functtion takes in a departing train and populates the map with the data. The code is fully available at the github page linked below
}
processItems(arrivals.items, true);
processItems(departures.items, false);
// Process IRIS data
irisData.stops.forEach((irisItem) => {
const key = irisItem.line.name + "-" + irisItem.line.fahrtNr;
const existing = processedItems.get(key); // <- This is the critical point: On vercel the get fails and therefore a new (duplicate) entry is created.
if (
!existing &&
cutoffTimestamp < moment(irisItem.when.arrival).valueOf()
) {
// If no existing data, create new entry with IRIS data
processedItems.set(key, {})//<- Excluded Object structture`
I think somehow Vercel is “loosing” the data in the processed items map. Or maybe one of you has an idea where the issue lies?
To reproduce this Issue just visit https://db-live-monitor.vercel.app/board/KD. Even though most information is in German I think that the general issue should be visible when paying attention to departure times and line names.
I see that you see the item twice on the screen. Now the function you are referring to won’t lose the processedItems Map because it is scoped within the function call. So, each function invocation will lead to a new map being created independent of others.
Coming to the file itself, I see two caches declared up top:
// Add caching for IRIS data with a short TTL (e.g., 30 seconds)
const IRIS_CACHE = new Map();
const IRIS_CACHE_TTL = 30000; // 30 seconds
// Add caching for all API responses, not just IRIS
const API_CACHE = new Map();
const API_CACHE_TTL = 30000; // 30 seconds
Now, these Maps will be reused or not depending on if the serverless function is hot or reusing the same instance, which might be the root cause. I would say, try to debug your code assuming these Maps will or will not have the value cached. Maybe that changes the answer for you.
Generally, it is recommended to make your serverless functions stateless by using external data stores instead of relying on a global variable.
Hey,
Firstly: Thank you for your feedback, but I have found the solution on my own. Even though your answer is not the cause of the problems, I think it might be a good idea to later switch to something like Redis for example!
In this case the problem was simple (and a bit stupid I guess). It turns out that running services that heavily depend on dates and schedules (like railways do in most cases) on a provider with a different timezone is not a good idea. After looking at the data I realized, due to Germany being GMT+1 my API was fetching data for the past hour and this caused the problems.
Sorry for bothering you as this is clearly not Vercel’s fault.
