Writing Batch Jobs/Apex in Salesforce

Batch Apex

When to use Batch Apex, think of Batch Job when data is huge, with a min of 10,000 to 1 million records, like updating records nightly manner, weekly or Monthly batch job really helpful.

To use batch Apex, One must write an Apex class that implements the Salesforce-provided interface Database.Batchable, and then invoke the class programmatically.

Following are main/mandatory methods to write Apex Batch Job,

1. Start grab the record you want to operate upon
   
2. Execute (perform the operation)
   
3. Finish (close, and if required send confirmation email)

global class Batch_Class_Example implements Database.Batchable < sObject > {

    // Constructor (Optional) - you can use this for Initialisation just like in your controllers
    global Batch_Class_Example() {

    }

    // Start Method
    global Database.QueryLocator start(Database.BatchableContext BC) {
        return Database.getQueryLocator('<Query_goes_here>');
        // returns up to 50 million records
    }

	// Execute Method
    global void execute(Database.BatchableContext BC, List < sObject > scope) {
        // Batch Logic goes here
        for(Sobject s : scope){
          // perform for loop logic here
        }      
    }

	// Finish Method
    global void finish(Database.BatchableContext BC) {
        // Use this method to send confirmation emails or execute post-processing operations
    }
}

To invoke a batch class, instantiate it first and then call Database.executeBatch with the instance of your batch class:

Batch_Class_Example bObj = new Batch_Class_Example();
Id batchprocessid = Database.executeBatch(bObj, <batch_size>);

Default size is 200

Monitoring

To monitor or stop the execution of the batch Apex job, navigate as follows. From Setup, click Monitoring | Apex Jobs or Jobs | Apex Jobs.