batch apex to associate a contact to a case

Question

I am using batch apex to associate a contact to a case. I will need to compare all the contacts in the system to the cases with a 'new' status. first_name__c and last_name__c are fields on the case. I want to go over every contact and see if there is a match between the first name and last name on a contact and a 'new' case's first_name__c and last_name__c. If there is 1 match, associate the contact and case. If there are 0 matches, create a new contact and associate it to the case. If there are multiple matches, do nothing and the case will have a null contact. I am using the query locator to get the contacts, which could be millions.

I need to have all the contacts in one batch, can I increase the batch size to 50,000,000? This seems like really bad practice.

If I pass in a batch size of 200 for example, will batch #1 finish before batch #2 begins? For example, in every execute I query for cases with status = 'new'. In the first batch, the contact on the case will be null. If I find a contact and case match in the first batch, it is assigned to the case and the status is unchanged (still 'new'). Will this assignment be reflected in the second batch?

Answer 1

On getting your algorithm implemented, the limiting factors are the governor limits so you always have to watch out for those.

For your particular situation this brute force solution - with a query inside the loop so the batch size must be 200 or less - might work:

public Database.QueryLocator start(Database.BatchableContext context) {
    return Database.getQueryLocator([
                select first_name__c, last_name__c
                from Case
                where Status__c = 'new'
                ]);  
    }
}
public void execute(Database.BatchableContext context, List<Case> cases) {
    for (Case c : cases) {
        Contact[] contacts = [
                select Id
                from Contact
                where FirstName = :c.first_name__c and LastName = :c.LastName
                ];
        ...
    }
}

Instead of iterating over every Contact it queries for the matching Contacts.

Answer 2

Each execute method within a batch operates in a separate transaction context. To say it another way, any changes to the database (DML) within the execute() method in the first chunk of your batch are fully committed by the time the second chunk in your batch is run.