Hi,

Batch Apex:- Batch Apex allows you to handle more number of records and manipulate them by using a specific syntax.

Need Of Batch Apex:- As you all might know about the salesforce governer limits on its data. When you want to fetch thousands of records or fire DML on thousands of rows on objects it is very complex in salesforce and it does not allow you to operate on more than certain number of records which satisfies the Governer limits.
But for medium to large enterprises, it is essential to manage thousands of records every day. Adding/editing/deleting them when needed.

Salesforce has come up with a powerful concept called Batch Apex. Batch Apex allows you to handle more number of records and manipulate them by using a specific syntax.

How to Work with Apex Batch:- We have to create an global apex class which extends Database.Batchable Interface because of which the salesforce compiler will know, this class incorporates batch jobs. Below is a sample class which is designed to delete all the records of Account object (Lets say your organization contains more than 50 thousand records and you want to mass delete all of them).

global class deleteAccounts implements Database.Batchable
{
global final String Query;
global deleteAccounts(String q)
{
Query=q;
}

global Database.QueryLocator start(Database.BatchableContext BC)
{
return Database.getQueryLocator(query);
}

global void execute(Database.BatchableContext BC,List scope)
{
List <Account> lstAccount = new list<Account>();
for(Sobject s : scope)
{
Account a = (Account)s;
lstAccount.add(a);
}
Delete lstAccount;
}

global void finish(Database.BatchableContext BC)
{
//Send an email to the User after your batch completes
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
String[] toAddresses = new String[] {‘sforce2009@gmail.com’};
mail.setToAddresses(toAddresses);
mail.setSubject('Apex Batch Job is done‘);
mail.setPlainTextBody('The batch Apex job processed ');
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
}
}

///////////////////This is how the batch class is called
id batchinstanceid = database.executeBatch(new deleteAccounts(‘select Id from Account’));

When you instantiate the global class and called it using database.executeBatch, the process gets started. As you have observed the deleteAccounts class accepts your query as a parameter in the constructor and sets it to the string variable called Query.
The start method of deleteAccount class which extends database.batchable interface sets the current query to execute using the Database.getQueryLocator method.
Then the result of the query can be captured in execute method. The scope list of SObjects returned as the result of your query.
When you are executing your batch with a query which returns thousands of records, the batch will be executed with 200 records each time that is salesforce divides the total number of records in to batches. Each batch contains 200 records by default (though this is configurable less than 200 by just introducing the number when you are calling the batch class like database.executebatch(new deleteAccounts(‘your query’), number here);).
After processing each batch, the governor limits are reset.
Once the whole batch of thousands records are done the Finish method gets called and an email will be sent to the specified person

Did this answer your question? If not, let me know what didn't work, or if so, please mark it solved.