Making Callouts for Long-Running requests using Continuation object in Salesforce ?

Sometimes we may encounter webservice callouts in business scenarios that will take longer to respond. This is because the other end need to do significant amount of work prior to responding back. if the other end process huge number of transactions during peak loads, the response will be even slower.

Salesforce has a governor limit of maximum of 10 synchronous requests that last longer than five seconds. Asynchronous callouts that are made from a Visualforce page don’t count this limit (e.g. @future method).

To resolve above Salesforce limitation, we can use Continuation Class which makes asynchronous callout using REST and SOAP services. Using this class, we can make a long running request from a Visualforce page to external systems, and its response is returned through a callback method.

Execution Flow of an Asynchronous Callout - Image from Salesforce documentation

In the above example, Warranty information is accessed from an external service and the service is called by an action button on a Visualforce page. This page is accessed by hundreds of employees in an organization. So, when all of employees click the same button to get the account information from the external systems, the actions exceed the limit of maximum of 10 synchronous requests that last longer than five seconds. This limit can be overcome by using the asynchronous callout using Continuation Class .

Callout request using Continuation object

 
string requestLabel;
//Create continuation with a timeout
Continuation con = new Continuation(40);

//Set callback method
con.continuationMethod='processWarranty';

//Create callout request 
HttpRequest req1 = new HttpRequest();

req1.setMethod('GET');

req1.setEndpoint(LONG_RUNNING_SERVICE_URL);

this.requestLabel = con.addHttpRequest(req1);

 // Callback method 
 public Object processWarranty() {   
      // Get the response by using the unique label
      HttpResponse response = Continuation.getResponse(this.requestLabel);
      // Set the result variable that is displayed on the Visualforce page
      this.result = response.getBody();
      
      // Return null to re-render the original Visualforce page
      return null;
 }