When is a batch class instantiated when you schedule it?

Question

QUESTION: when you instantiate a class in a scheduled apex job, is the class instantiated at the time of the schedule, or at the scheduled time of execution?

Here's my situation... I have a custom object called Blast, which schedules a bulk SMS send. That object record includes a DateTime field that designates when the blast will happen; for this example, let's say the blast happens next Tuesday at 1:00 pm.

The blast has some unknown number of recipients -- very possibly 20-50k, which are stored as CampaignMembers. So, at the designated time, I have a batch apex class that retrieves all of the recipient info from the CampaignMember records and makes the api call.

When a user creates/updates the Blast record today and sets status to "queued", I'm creating a scheduled apex job, instantiating my batch apex class and providing the blast record, from which it pulls necessary data.

I want to build into the batch apex class a check that verifies the blast record still exists before executing the blast. But I'm not clear on whether that class is instantiated today (when the job is scheduled), or next Tuesday at 1:00pm when it has been scheduled to run.

Answer 1

It's instantiated when new SomeBatchJobName() is called, then serialized when System.scheduleBatch(...) is called. So the data stored in the object will be from the moment of instantiation (now), not when it is eventually pulled from the queue and executed. This is also true for scheduled and queueable jobs, too.

Answer 2

For the benefit of clarity, I'll share my revised code... the object is actually called Wave, not Blast.

global class QueueWave implements Database.Batchable<SObject>, Database.Stateful {
global String waveId;
global String body;
global String messageServiceLabel;
global Integer recipientCount;
global String emailAddress;
global String waveName;
global String userId;

global QueueWave(String waveId) {
    this.waveId = waveId;
}

The constructor does nothing except store the recordId at the time this batch class is scheduled. I'm using try-catch, catching QueryException which aborts the job.

global Database.QueryLocator start(Database.BatchableContext BC){
    String query;
    if(Wave__c.SObjectType.getDescribe().isAccessible()){
        try{
            Wave__c wave = [
                SELECT [fields I need, which may have been updated since batch was scheduled]
                FROM Wave__c
                WHERE Id = :this.waveId
                LIMIT 1];

            //set global vars with data from the wave record
                . . .

            query = // build my query string here

        }catch(QueryException qe){
            System.debug('QueryException on ' + BC.getJobId() + ' ' + qe);
            System.abortJob(BC.getJobId());
        } 
    }  

    return Database.getQueryLocator(query);          
}