Why is my caching inconsistent? - google-apps-script

Question

I am trying setup caching for a spreadsheet custom funciton but the results seem to be inconsistent/unexpected. Sometimes I get the cached results, sometimes it refreshes the data. I've set the timeout to 10 seconds, and when I refresh within 10 seconds, sometimes it grabs new data, sometimes it caches. Even after waiting more than 10 seconds since last call, sometimes I get the cached results. Why is there so much inconsistency in the spreadsheet function? (or am I just doing something wrong?). When I call the function directly within the actual script, it seems to be much more consistent but sometimes I get inconsistenties/unexpected results.
function getStackOverflow(){
var cache = CacheService.getPublicCache();
var cached = cache.get("stackoverflow");
if(cached != null) {
Logger.log('this is cached');
return 'this is cached version';
}
// Fetch the data and create an object.
var result = UrlFetchApp.fetch('http://api.stackoverflow.com/1.1/tags/google-apps-script/top-answerers/all-time');
var json = Utilities.jsonParse(result.getContentText()).top_users;
var rows = [],data;
for (i = 0; i < json.length; i++) {
data = json[i].user;
rows.push(data.display_name);
}
Logger.log("This is a refresh");
cache.put("stackoverflow",JSON.stringify(rows),10);
return rows;
}

Answer 1

You cant use custom functions like that. Its documented.
Custom functions must be deterministic, they have always the same output given fhe same input (in your case none since you are passing no parameters.
the spreadsheet will remember the values for each input set, basically like a second layer of cache that yiu have no control.