Understanding the Batchable Interface
To make your Apex code run as a batch you must sign a contract with the platform.This
contract takes the form of an interface called Batchable that must be implemented by
your code. It requires that you structure your processing logic into the following three
methods:
. start: The start method is concerned with the scope of work, the raw set of
records to be processed in the batch.When a batch is submitted to Salesforce for
processing, the first thing it does is invoke your start method.Your job here is to
return a QueryLocator or an Iterable that describes the scope of the batch job.
.execute: After calling the start method, Force.com has the means to access all
the records you’ve requested that it operate on. It then splits these records into sets
of up to 200 records and invokes your execute method repeatedly, once for each
set of records. At this point, your code can perform the substance of the batch operation,
typically inserting, updating, or deleting records. Each invocation of execute
is a separate transaction. If an uncaught exception is in a transaction, no further
transactions are processed and the entire batch job is stopped.
Caution
Transactions that complete successfully are never rolled back. So, an error in a transaction
stops the batch, but transactions executed up to that point remain in the database. Thinking
of an overall Batch Apex job as transactional is dangerous, because this is not its
default behavior. Additionally, you cannot use savepoints to achieve a single pseudo-transaction
across the entire batch job. If you must achieve job-wide rollback, this can be implemented
in the form of a compensating batch job that reverses the actions of the failed job.
n finish: The finish method is invoked once at the end of a batch job.The job
ends when all transactions in the scope have been processed successfully, or if processing
has failed. Regardless of success or failure, finish is called.There is no
requirement to do anything special in the method.You can leave the method body
empty if no post-processing is needed. It simply provides an opportunity for you to
receive a notification that processing is complete.You could use this information to
clean up any working state or notify the user via e-mail that his batch job is complete
and its outcome.

With this initial walkthrough of the Batchable interface, you can begin to apply it to
your own trigger or Visualforce controller code. If you find a process that is a candidate to
run as a batch, think about how it can be restructured to conform to this interface and
thus take advantage of Batch Apex.