Use Future Methods

Learning Objectives

After completing this unit, you’ll know:

When to use future methods.
The limitations of using future methods.
How to use future methods for callouts.
Future method best practices.

Follow Along with Trail Together

Want to follow along with an instructor as you work through this step? Take a look at this video, part of the Trail Together series on Trailhead Live.

(This clip starts at the 9:39 minute mark, in case you want to rewind and watch the beginning of the step again.)

Future Apex

Future Apex is used to run processes in a separate thread, at a later time when system resources become available.

Note: Technically, you use the @future annotation to identify methods that run asynchronously. However, because "methods identified with the @future annotation" is laborious, they are commonly referred to as "future methods" and that’s how we’ll reference them for the remainder of this module.

When using synchronous processing, all method calls are made from the same thread that is executing the Apex code, and no additional processing can occur until the process is complete. You can use future methods for any operation you’d like to run asynchronously in its own thread. This provides the benefits of not blocking the user from performing other operations and providing higher governor and execution limits for the process. Everyone’s a winner with asynchronous processing.

Future methods are typically used for:

Callouts to external Web services. If you are making callouts from a trigger or after performing a DML operation, you must use a future or queueable method. A callout in a trigger would hold the database connection open for the lifetime of the callout and that is a "no-no" in a multitenant environment.
Operations you want to run in their own thread, when time permits such as some sort of resource-intensive calculation or processing of records.
Isolating DML operations on different sObject types to prevent the mixed DML error. This is somewhat of an edge-case but you may occasionally run across this issue. See sObjects That Cannot Be Used Together in DML Operations for more details.

Future Method Syntax

Future methods must be static methods, and can only return a void type. The specified parameters must be primitive data types or collections of primitive data types. Notably, future methods can’t take standard or custom objects as arguments. A common pattern is to pass the method a List of record IDs that you want to process asynchronously.

public class SomeClass {
  @future
  public static void someFutureMethod(List<Id> recordIds) {
    List<Account> accounts = [Select Id, Name from Account Where Id IN :recordIds];
    // process account records to do awesome stuff
  }
}

The reason why objects can’t be passed as arguments to future methods is because the object can change between the time you call the method and the time that it actually executes. Remember, future methods are executed when system resources become available. In this case, the future method may have an old object value when it actually executes, which can cause all sorts of bad things to happen.

It’s important to note that future methods are not guaranteed to execute in the same order as they are called. We'll say it again, because it's crucial for you to remember: future methods are not guaranteed to execute in the same order as they are called. When using future methods, it’s also possible that two future methods could run concurrently, which could result in record locking and a nasty runtime error if the two methods were updating the same record.

Sample Callout Code

To make a Web service callout to an external service or API, you create an Apex class with a future method that is marked with (callout=true). The class below has methods for making the callout both synchronously and asynchronously where callouts are not permitted. We insert a record into a custom log object to track the status of the callout simply because logging is always fun to do!

The SmsMessage class and SMS_Log__c sObject in the example are hypothetical and shown only to demonstrate the callout pattern.

public class SMSUtils {
    // Call async from triggers, etc, where callouts are not permitted.
    @future(callout=true)
    public static void sendSMSAsync(String fromNbr, String toNbr, String m) {
        String results = sendSMS(fromNbr, toNbr, m);
        System.debug(results);
    }
    // Call from controllers, etc, for immediate processing
    public static String sendSMS(String fromNbr, String toNbr, String m) {
        // Calling 'send' will result in a callout
        String results = SmsMessage.send(fromNbr, toNbr, m);
        insert new SMS_Log__c(to__c=toNbr, from__c=fromNbr, msg__c=results);
        return results;
    }
}

Test Classes

Testing future methods is a little different than typical Apex testing. To test future methods, enclose your test code between the startTest() and stopTest() test methods. The system collects all asynchronous calls made after the startTest(). When stopTest() is executed, all these collected asynchronous processes are then run synchronously. You can then assert that the asynchronous call operated properly.

Test code cannot actually send callouts to external systems, so you’ll have to ‘mock’ the callout for test coverage. Check out the Apex Integration Services module for complete details on mocking callouts for testing.

Here’s our mock callout class used for testing. The Apex testing framework utilizes this ‘mock’ response instead of making the actual callout to the REST API endpoint.

@isTest
public class SMSCalloutMock implements HttpCalloutMock {
    public HttpResponse respond(HttpRequest req) {
        // Create a fake response
        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        res.setBody('{"status":"success"}');
        res.setStatusCode(200);
        return res;
    }
}

The test class contains a single test method (testSendSms() in this example), which tests both the asynchronous and synchronous methods as the former calls the latter.

@IsTest
private class Test_SMSUtils {
  @IsTest
  private static void testSendSms() {
    Test.setMock(HttpCalloutMock.class, new SMSCalloutMock());
    Test.startTest();
      SMSUtils.sendSMSAsync('111', '222', 'Greetings!');
    Test.stopTest();
    // runs callout and check results
    List<SMS_Log__c> logs = [select msg__c from SMS_Log__c];
    System.assertEquals(1, logs.size());
    System.assertEquals('success', logs[0].msg__c);
  }
}

Best Practices

Because every future method invocation adds one request to the asynchronous queue, avoid design patterns that add large numbers of future requests over a short period of time. If your design has the potential to add 2000 or more requests at a time, requests could get delayed due to flow control. Here are some best practices you want to keep in mind:

Ensure that future methods execute as fast as possible.
If using Web service callouts, try to bundle all callouts together from the same future method, rather than using a separate future method for each callout.
Conduct thorough testing at scale. Test that a trigger enqueuing the @future calls is able to handle a trigger collection of 200 records. This helps determine if delays may occur given the design at current and future volumes.
Consider using Batch Apex instead of future methods to process large number of records asynchronously. This is more efficient than creating a future request for each record.

Things to Remember

Future methods are a great tool, but with great power comes great responsibility. Here are some things to keep in mind when using them:

Methods with the @future annotation must be static methods, and can only return a void type.
The specified parameters must be primitive data types or collections of primitive data types; future methods can’t take objects as arguments.
Future methods won’t necessarily execute in the same order they are called. In addition, it’s possible that two future methods could run concurrently, which could result in record locking if the two methods were updating the same record.
Future methods can’t be used in Visualforce controllers in getMethodName(), setMethodName(), nor in the constructor.
You can’t call a future method from a future method. Nor can you invoke a trigger that calls a future method while running a future method. See the link in the Resources for preventing recursive future method calls.
The getContent() and getContentAsPDF() methods can’t be used in methods with the @future annotation.
You’re limited to 50 future calls per Apex invocation, and there’s an additional limit on the number of calls in a 24-hour period. For more information on limits, see the link below.

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