Batch Apex

To use batch Apex, write an Apex class that implements the Salesforce-provided interface Database.Batchable
Batch Apex is asynchronous execution of Apex code, specially designed for processing a large number of records and has greater flexibility in governor limits than the synchronous code.

global class UpdateAccountFields implements Database.Batchable<sObject>,
Database.Stateful{

global final String Query;
global final String Entity;
global final String Field;
global final String Value;

global UpdateAccountFields(String q, String e, String f, String v){
Query=q; Entity=e; Field=f;Value=v;
}

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

global void execute(Database.BatchableContext BC, List<sObject> scope){
for(Sobject s : scope){s.put(Field,Value); 
update scope;
}

global void finish(Database.BatchableContext BC){
AsyncApexJob a = [SELECT Id, Status, NumberOfErrors, JobItemsProcessed,
TotalJobItems, CreatedBy.Email FROM AsyncApexJob WHERE Id =:BC.getJobId()];

// Send an email to the Apex job's submitter notifying of job completion.
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
String[] toAddresses = new String[] {a.CreatedBy.Email};
mail.setToAddresses(toAddresses);
mail.setSubject('Apex Sharing Recalculation ' + a.Status);
mail.setPlainTextBody
('The batch Apex job processed ' + a.TotalJobItems +
' batches with '+ a.NumberOfErrors + ' failures.');
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
}
}

UpdateAccountFields be = new UpdateAccountFields();
database.executeBatch(be,200); //maximun 200

Advantages:

• Every transaction starts with a new set of governor limits, making it easier to ensure that your code stays within the governor execution limits.
• If one batch fails to process successfully, all other successful batch transactions aren’t rolled back.
• If your code accesses external objects and is used in batch Apex, use Iterable<sObject> instead of Database.QueryLocator.

Batch Apex Governor Limits

Keep in mind the following governor limits for batch Apex.

• Up to 5 batch jobs can be queued or active concurrently.
• Up to 100 Holding batch jobs can be held in the Apex flex queue.
• In a running test, you can submit a maximum of 5 batch jobs.
• The maximum number of batch Apex method executions per 24-hour period is 250,000, or the number of user licenses in your org multiplied by 200—whichever is greater.
• A maximum of 50 million records can be returned in the Database.QueryLocator object. If more than 50 million records are returned, the batch job is immediately terminated and marked as Failed.
• If the start method of the batch class returns a QueryLocator, the optional scope parameter of Database.executeBatch can have a maximum value of 2,000. If set to a higher value, Salesforce chunks the records returned by the QueryLocator into smaller batches of up to 2,000 records. If the start method of the batch class returns an iterable, the scope parameter value has no upper limit. However, if you use a high number, you can run into other limits.
• The start, execute, and finish methods can implement up to 100 callouts each.