what is the parameter database.batchablecontext?

Question

I was learning batch apex and it has three method which is start,execute and finish. Each method has a parameter Database.BatchableContext.

What is this parameter and why it is being used in all three methods?

Answer 1

A common use case for this is in the finish() method to see if any of the execute transactions had errors

public void finish(BatchableContext bc) {

  AsyncApexJob[] jobs = [SELECT Id, NumberOfErrors 
    FROM AsyncApexJob 
    WHERE Id = :bc.getJobId()];

  if (jobs[0].NumberOfErrors > 0) {..do something, perhaps log/message}
}

For start(), you might want to abort the job if you detect another batchable already running.

Answer 2

Personally, I've never used that parameter. But it is described in BatchableContext Interface as:

BatchableContext Interface

Represents the parameter type of a batch job method and contains the batch job ID.

Useful I guess if you want to manipulate the job that runs the batchable e.g. to abort it via System.abortJob.