Getting Blank response when i return Array on continuation in Lightning component's Server side method

Question

In my requirement, I have to make multiple callouts with the Lightning component's server-side page load method. So here I have created multiple HTTP request(i.e: 20) objects and bind each 3 in different continuation instances(20/3 => 7). So in under this solution, I have to return a List from Apex to the client-side server as a response JSON. But when I do so, I received a blank object, instead of a JSON object.

Code Block here:

// Action method
@AuraEnabled(continuation=true cacheable=true)
public static List<continuation> startRequest() {
    Map<String,String> headers=new Map<String,String>();
    String access_token = 'L90NMyHS2BcLSdtQ2iyJyr1MD5WdGjEt';
    headers.put('Content-Type','application/x-www-form-urlencoded');
    headers.put('Accept','application/json');
    headers.put('Authorization','Bearer ' + access_token);
    List<String> folderList = new List<String>{'108947290168', '108950868379'};
    List<continuation> lstCon = new List<continuation>();
    // Create continuation. Argument is timeout in seconds.
    for(String objFol : folderList){
        Continuation con = new Continuation(40);
        LONG_RUNNING_SERVICE_URL = LONG_RUNNING_SERVICE_URL+'objFol';
        // Set callback method
        con.continuationMethod='processResponse';
        // Set state

        // Create callout request
        HttpRequest req = new HttpRequest();
        req.setMethod('GET');
        req.setEndpoint(LONG_RUNNING_SERVICE_URL);
        system.debug('req--'+req);
        for(String ss : headers.keyset()){
            req.setHeader(ss,headers.get(ss));   
        }

        // Add callout request to continuation
        String strteemp = con.addHttpRequest(req);
        con.state = strteemp;
        lstCon.add(con);
    }
    // Return the continuations
    return lstCon;
}

// Callback method
@AuraEnabled(cacheable=true)
public static Object processResponse(List<String> labels, Object state) {
    // Get the response by using the unique label
    List<String> lstRes = new List<String>();
    for(String objStr : labels){
        HttpResponse response = Continuation.getResponse(objStr);
        // Set the result variable
        String result = response.getBody();
        lstRes.add(result);
    }
    return lstRes;
}

Note: For POC purpose when I did the same flow for a single request(without iterating the for loop) with a single continuation object I got the response correctly from the @AuraEnabled apex method to the client-side(lightning controller.js).

Answer 1

Late to the party, I would say ditch continuation. Its no longer needed. The only reason we used continuation is when there we had long-running callouts from UI. But from Winter 20...

Callouts Are Excluded from Long-Running Request Limit

Every org has a limit on the number of concurrent long-running Apex requests. This limit counts all requests that run for more than 5 seconds (total execution time). However, HTTP callout processing time is no longer included when calculating the 5-second limit. We pause the timer for the callout and resume it when the callout completes.

https://releasenotes.docs.salesforce.com/en-us/winter20/release-notes/rn_apex_request_limit.htm