Salesforce - Developer - Queueable

salesforce-developer

https://trailhead.salesforce.com/force_com_dev_intermediate/asynchronous_apex/async_apex_queueable
https://developer.salesforce.com/docs/atlas.en-us.204.0.apexcode.meta/apexcode/apex_enhanced_future_overview.htm

// Salesforce - Developer - Queueable Apex:

public class c1 implements Queueable {
  public void execute(QueueableContext con) {
    ...
  }
}

ID jobID = System.enqueueJob(new c1());

The Queueable interface enable you to add jobs to the queue and monitor them,
which is an enhanced way of running your asynchronous Apex code compared to
using future methods.  Queueable jobs are similar to future methods in that
they are both queued for execution, but queueable Apex provides additional
benefits:

1. Getting an ID for your job: When we submit a job by invoking 
   System.enqueueJob method, the method returns the ID of the new job. Using
   this ID, we can identify your job and monitor its progress.

2. Using non-primitive types: Queueable class can contain member variables
   of non-primitive data types, such as sObject or custom Apex types.  Those
   objects can be accessed when the job executes.

3. Chaining jobs: We can chain one job to another job by starting a second job
   from a running job.  Chaining jobs is useful if we need to do some processing
   that depends on another process to have run first.

Released in Winter '15, Queueable Apex is essentially a superset of future 
methods with some extra #awesomesauce. We took the simplicity of future methods 
and the power of Batch Apex and mixed them together to form Queueable Apex! 
It gives you a class structure that the platform serializes for you, a 
simplified interface without start and finish methods and even allows you to 
utilize more than just primitive arguments! It is called by a simple 
System.enqueueJob() method, which returns a job ID that you can monitor. 

Queueable Apex allows you to submit jobs for asynchronous processing similar to 
future methods with the following additional benefits:

1. Non-primitive types: Your Queueable class can contain member variables of 
   non-primitive data types, such as sObjects or custom Apex types. Those 
   objects can be accessed when the job executes.

2. Monitoring: When you submit your job by invoking the System.enqueueJob 
   method, the method returns the ID of the AsyncApexJob record. You can use 
   this ID to identify your job and monitor its progress, either through the 
   Salesforce user interface in the Apex Jobs page, or programmatically by 
   querying your record from AsyncApexJob.

3. Chaining jobs: You can chain one job to another job by starting a second job 
   from a running job. Chaining jobs is useful if you need to do some sequential 
   processing.

Because queueable methods are functionally equivalent to future methods, most 
of the time you’ll probably want to use queueable instead of future methods. 
However, this doesn’t necessarily mean you should go back and refactor all your 
future methods right now. If you were exceeding a governor limit in your future 
method, or if you think a future method requires a higher limit, you can 
possibly increase the limits for your future method with the Future Methods with 
Higher Limits pilot.

Another reason to use future methods instead of queueable is when your 
functionality is sometimes executed synchronously, and sometimes asynchronously. 
It’s much easier to refactor a method in this manner than converting to a 
queueable class. This is handy when you discover that part of your existing 
code needs to be moved to async execution. You can simply create a similar 
future method that wraps your synchronous method like so:

@future
static void myFutureMethod(List<String> params) {
    // call synchronous method
    mySyncMethod(params);
}

A common scenario is to take some set of sObject records, execute some 
processing such as making a callout to an external REST endpoint or perform some 
calculations and then update them in the database asynchronously. Since @future 
methods are limited to primitive data types (or arrays or collections of 
primitives), queueable Apex is an ideal choice. The following code takes a 
collection of Account records, sets the parentId for each record, and then 
updates the records in the database.

public class UpdateParentAccount implements Queueable {    
    private List<Account> accounts;
    private ID parent;
    public UpdateParentAccount(List<Account> records, ID id) {
        this.accounts = records;
        this.parent = id;
    }
    public void execute(QueueableContext context) {
        for (Account account : accounts) {
          account.parentId = parent;
          // perform other processing or callout
        }
        update accounts;
    }
}

To add this class as a job on the queue, execute the following code:

List<Account> accounts = [select id from account where billingstate = ‘NY’];
// find a specific parent account for all records
Id parentId = [select id from account where name = 'ACME Corp'][0].Id;
// instantiate a new instance of the Queueable class
UpdateParentAccount updateJob = new UpdateParentAccount(accounts, parentId);
// enqueue the job for processing
ID jobID = System.enqueueJob(updateJob);

After you submit your queueable class for execution, the job is added to the 
queue and will be processed when system resources become available.  You can use 
the new job ID to monitor progress, either through the Apex Jobs page or 
programmatically by querying AsyncApexJob:

SELECT Id, Status, NumberOfErrors FROM AsyncApexJob WHERE Id = :jobID

// Salesforce - Developer - Queueable Apex - Testing :

The following code sample shows how to test the execution of a queueable job in 
a test method. It looks very similar to Batch Apex testing. To ensure that the 
queueable process runs within the test method, the job is submitted to the queue 
between the Test.startTest and Test.stopTest block. The system executes all 
asynchronous processes started in a test method synchronously after the 
Test.stopTest statement. Next, the test method verifies the results of the 
queueable job by querying the account records that the job updated.

@isTest
public class UpdateParentAccountTest {

    @testSetup 
    static void setup() {
        List<Account> accounts = new List<Account>();
        // add a parent account
        accounts.add(new Account(name='Parent'));
        // add 100 child accounts
        for (Integer i = 0; i < 100; i++) {
            accounts.add(new Account(
                name='Test Account'+i
            ));
        }
        insert accounts;
    }

    static testmethod void testQueueable() {
        // query for test data to pass to queueable class
        Id parentId = [select id from account where name = 'Parent'][0].Id;
        List<Account> accounts = [select id, name from account where name like 'Test Account%'];
        // Create our Queueable instance
        UpdateParentAccount updater = new UpdateParentAccount(accounts, parentId);
        // startTest/stopTest block to force async processes to run
        Test.startTest();        
        System.enqueueJob(updater);
        Test.stopTest();        
        // Validate the job ran. Check if record have correct parentId now
        System.assertEquals(100, [select count() from account where parentId = :parentId]);
    }

}

// Salesforce - Developer - Queueable Apex - Chaining Jobs:

One of the best features of Queueable Apex is job chaining. If you ever need to 
run jobs sequentially, Queueable Apex could make your life much easier. To 
chain a job to another job, submit the second job from the execute() method of 
your queueable class. You can add only one job from an executing job, which 
means that only one child job can exist for each parent job. For example, if 
you have a second class called SecondJob that implements the Queueable interface, 
you can add this class to the queue in the execute() method as follows:

public class FirstJob implements Queueable { 
    public void execute(QueueableContext context) { 
        // Awesome processing logic here    
        // Chain this job to next job by submitting the next job
        System.enqueueJob(new SecondJob());
    }
}

Once again, testing has a slightly different pattern. You can’t chain queueable 
jobs in an Apex test, doing so results in an error. To avoid nasty errors, you 
can check if Apex is running in test context by calling Test.isRunningTest() 
before chaining jobs.

// Salesforce - Developer - Queueable Apex - Things to Remember:

Queueable Apex is a great new tool but there are a few things to watch out for:

1. The execution of a queued job counts once against the shared limit for 
   asynchronous Apex method executions.

2. You can add up to 50 jobs to the queue with System.enqueueJob in a single 
   transaction.

3. When chaining jobs, you can add only one job from an executing job with 
   System.enqueueJob, which means that only one child job can exist for each 
   parent queueable job. Starting multiple child jobs from the same queueable 
   job is a no-no.

4. No limit is enforced on the depth of chained jobs, which means that you 
   can chain one job to another job and repeat this process with each new child 
   job to link it to a new child job. However, for Developer Edition and Trial 
   orgs, the maximum stack depth for chained jobs is 5, which means that you 
   can chain jobs four times and the maximum number of jobs in the chain is 5, 
   including the initial parent queueable job.