Batch Apex
Apex class must implement Database.Batchable interface, which will allow them to write code against start, execute and finish methods.
A batch class can be invoked by calling Database.executeBatch. It needs the instance of the class and batch size.
Using SOQL, status of the batch job can be monitored programmatically by querying AsyncApexJob. The same is also possible from “Apex Jobs” in setup.
It is used to process millions of records asynchronously.
Apex jobs always get executed in a separate transaction.
Every batch apex transactions has their own governor limit(new limit always). Thus if one batch fails, it will not impact other batches.
Start Method:
It is used to collect records or objects which will be passed to the method “execute”.
It returns either a Database.QueryLocator object or an Iterable.
Execute Method:
This method is responsible for doing the actual processing work.
Default batch size is 200.
Parameter it needs – a reference to Database.BatchableContext object and a list of sObjects.
Finish Method:
This method is used to do post-processing operations likes sending email etc.
You can chain a batch job by calling Database.executeBatch or System.scheduleBatch from the finish method of the current batch class. The new batch job will start after the current batch job finishes.
Scheduled Apex
Apex class must implement Schedulable interface. By implementing this interface, an apex class can be scheduled at specific time/day in the Apex Scheduler.
System.Schedule method is used to schedule an instance of the class.
It should implement the execute method(this is the only method). Parameter – SchedulableContext object.
If the class is getting scheduled from Trigger, Governor limit should be checked.
Once a class is scheduled, as CronTrigger object record is created. The getTriggerId method can be used to return the ID of CronTrigger record.
Additional processing should be done in a separate class outside of execute method.
Synchronous Web service callouts are not supported from scheduled apex. To do that, make an asynchronous callout in a method annotated with @future(callout=true) and then call it from scheduled apex. Note – If scheduled apex executes batch job, then callouts are possible as they are supported from batch class.
Maximum 100 scheduled Apex jobs at one time is possible.
