@Future vs Batch Apex in salesforce

Future Annotation: 

1. Future Annotation(@Future) is used to separate methods that are to be executed asynchronously.
2. If Future Annotation is not used in a web service callout, the thread will wait until the response comes and other processes will not be executed.
3. Mixed DML Exception can be avoided with the use of @future annotation.
4. @future annotation - Must be static and return void - Specify (callout=true) to allow callouts.
5. Parameters passed to this can only be primitive.
6. Tracking future is difficult as you don't have any jobId to track it.
7. Future can be used to resolve mixed DML exception.

Example: You cannot insert a setup object and a non-setup object in same context. You'll encounter mixed DML exception. @Future comes for rescue in this situation.

public class MixedDMLFuture {

    public static void useFutureMethod() { 

        //First DML operation- Insertion of non-setup object(Account)

        Account a = new Account(Name='Vydehi Hospital'); 

        insert a; 

        

        // This next operation (insert a user with a role)  can't be mixed with the previous insert unless it is within a future method. 

        /*// Other DML Operation -- commented the code with a reason, if you use the commented code you will get mixed DML exception error

        Profile p = [SELECT Id FROM Profile WHERE Name='Standard User']; 

        UserRole r = [SELECT Id FROM UserRole WHERE Name='COO']; 

        // Create new user with a non-null user role ID 

        User u = new User(alias = 'ranvk', email='dummyMail@gmail',                   

                          emailencodingkey='UTF-8', lastname='niru',languagelocalekey='en_US',     

                          localesidkey='en_US', profileid = p.Id, userroleid = r.Id,  

                          timezonesidkey='America/Los_Angeles',  

                          username='dM@gmail.com'); 

        insert u; */

        //Call future method to insert a user(setup object) with a role. 

        UtilClass.insertUserWithRole( 'dummyMail@gmail.com', 'ranvk', 'dM@gmail.com', 'niru'); 

    } 

}

public class UtilClass { 

      @future 

       public static void insertUserWithRole(String uname, String al, String em, String lname) { 

               Profile p = [SELECT Id FROM Profile WHERE Name='Standard User']; 

               UserRole r = [SELECT Id FROM UserRole WHERE Name='COO']; 

               // Create new user with a non-null user role ID 

               User u = new User(alias = al, email=em, emailencodingkey='UTF-8',   

                              lastname=lname, languagelocalekey='en_US', localesidkey='en_US', 

                              profileid = p.Id, userroleid = r.Id, timezonesidkey='America/Los_Angeles',  

                              username=uname); 

               insert u; 

       } 

}

Batch Apex:

1. Batch Apex is used to separate tasks that are going to handle more records(complex long running jobs) in background process. 
2. To avoid hitting governor limits we use Batch Apex for handling bulk records.
3. It can process up to 50 million records. It can also be scheduled to run at a particular time.
4. Only 5 concurrent batch can execute simultaneously. It is difficult to track the progress of the execution.
5. We cannot use @future annotation. As asynchronous call is not allowed from batch apex.

Example: Delete all the account having Rating 'Cold'
global class deleteAccounts implements Database.Batchable{
global Database.QueryLocator start(Database.BatchableContext BC){
string query = 'select id,name,Rating,Owner from Account';
return Database.getQueryLocator(query);
}

global void execute(Database.BatchableContext BC,List scope){
List <Account> lstAccount = new list<Account>();
for(Sobject s : scope){
Account a = new Account();
if(s.Rating=='Cold'){
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[] {'dummyemail@gmail.com'};
mail.setToAddresses(toAddresses);
mail.setSubject('Apex Batch Job is completed');
mail.setPlainTextBody('The batch Apex job processed ');
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
}
}