3. Concepts
• Large, data-intensive processing.
• Scope: set of records Batch Apex works on. One to 50
million records. Scope is usually expressed as a SOQL
statement, which is contained in a Query Locator, a system
object that is exempt from the normal governor limits on
SOQL.
• Batch Job: Asynchronous, run in background. UI to list,
monitor and cancel batch jobs.
• Transactions: Units of work running the same code within a
batch job, subject to governor limits. Up to 200 records.
The scope is split into several transactions, each committed
to the database independently. Stateless. Parallel or serial.
• 5 Active Batch Jobs per Org.
4. Batchable Interface
• start() - return a QueryLocator or an Iterable that
describes the scope of the batch job.
• execute() – splits the records from the previous step
into several transactions, each of which operates on up
to 200 records.
• The records are provided in the scope argument of the
execute method.
• Each invocation of execute is a separate transaction.
• If an uncaught exception is in a transaction, no further
transactions are processed and the entire batch job is
stopped.
5. Transactions in Batch Job
• Transactions that complete successfully are never
rolled back.
• If an error in a transaction stops the batch, the
transactions executed up to that point remain in
the database.
• Cannot use savepoints to achieve a single
pseudo-transaction across the entire batch job.
• If you must achieve job-wide rollback, this can be
implemented in the form of a compensating
batch job that reverses the actions of the failed
job.
6. Batchable (contd…)
• finish() - Invoked once at the end of a batch
job.
• The job ends when all transactions in the
scope have been processed successfully, or if
processing has failed.
• Regardless of success or failure, finish is
always called.
• No code required in the finish method.
7. Batch Apex Class
• Must implement the Database.Batchable
interface. Parameterized interface, so needs a
type name. Use SObject for batches with a
QueryLocator scope, or any database object
type for an Iterable scope.
• The class must be global, hence methods must
be global as well.
8. Batchable Context
• The BatchableContext object argument in all
three methods contains a method called getJobId
to get unique ID of the current batch job.
• jobID can be used to look up additional
information about the batch job in the standard
database object AsyncApexJob.
• You can also pass this identifier to the
System.abortJob method to stop processing of
the batch job.
9. Guidelines
• Single DB Object – source data from a single
DB object, no web services,
• Simple scope of work – single SOQL typically.
• Minimal shared state – each unit of work is
independent.
• Limited transactionality – rollback requires
custom code.
• Not time critical – no guarantee on when it is
executed and how long it will run.
10. Running Batch Jobs
• 4 ways: execute from a VF page, schedule it or kick off
from a trigger or run it from the Execute Anonymous
view in the Force.com IDE.
• Execute from the Anonymous view.
HelloBatchApex batch = new HelloBatchApex();
Id jobId = Database.executeBatch(batch);
System.debug('Started Batch Apex job: ' + jobId);
Pass an extra argument to executeBatch to control the scope.
12. 4 Different Logs in Debug Logs
• Results of evaluating the code in the Execute
Anonymous view.
• Invocation of the start method to prepare the
dataset for the batch.
• Results of running the execute method, where
the batch job performs its work on the subsets of
the data.
• All the transactions have been processed, so the
finish method is called to allow post-processing
to occur.
13. Stateful Batch
• Batch Apex is stateless by default. That means for
each run of the execute method, you receive a
fresh copy of your object. All fields of the class ,
both static and instance are initialized.
• If your batch process needs information that is
shared across transactions, one approach is to
make the Batch Apex class itself stateful by
implementing the Stateful interface.
• This instructs Force.com to preserve the values of
your static and instance variables between
transactions.
14. Compare QueryLocator and Iterable
Batch
Feature QueryLocator Iterable Batch
Number of Records 1 to 50 million Max of 50k records
How it works? Uses exactly 1 SOQL
without governor limits
Custom Apex Code, can
use complex criteria,
subject to governor limits.
15. Iterable Batch
• Global class that must implement 2 interfaces:
the Iterator interface and the Iterable interface.
• Iterator – hasNext() and next() global methods.
• Iterable – single global method called Iterator().
• You could write two separate classes, one to
implement each interface. Or you can implement
both interfaces in the same class.
16. Limitations of Apex Batch
• No future methods allowed.
• Run as system user, so have permission to read and
write all data in the org.
• Max heap size is 6MB.
• Exactly 1 callout to external systems allowed for each
invocation of start, execute and finish. Must implement
Database.AllowsCallouts interface.
• Reduce the default 200 records per transaction if
intensive work needs to be done that may exceed the
governor limits.
• Max number of queued or active jobs in an org is 5.
17. Testing Batch Apex
• Batch Apex can be tested like any Apex code, although
you are limited to a single transaction’s worth of data
(one invocation of the execute method).
• A batch job started within a test runs synchronously,
and does not count against the organization’s limit of 5
batch jobs.
public static testmethod void testBatch() {
Test.startTest();
HelloBatchApex batch = new HelloBatchApex();
ID jobId = Database.executeBatch(batch);
Test.stopTest();
}
18. Schedule Batch Apex
• Enables any Apex code, not just Batch Apex, to be
scheduled to run asynchronously at regular time intervals.
• An Apex class that can be scheduled by Force.com must
implement the Schedulable interface. Marker Interface =>
no methods.
• Code that is executed by the scheduler runs as the system
user, so sharing rules or other access controls are not
enforced.
• At most 10 classes can be scheduled at one time.
• Programmatic approach:
System.schedule('Scheduled Test', '0 0 1 * * ?',
new HelloSchedulable());
19. Create Scheduled Batch Job
global class HelloSchedulable
implements Schedulable {
global void execute(SchedulableContext sc)
{
HelloBatchApex batch = new
HelloBatchApex();
Database.executeBatch(batch);
}
}
