Batch Apex conflicts and design? - salesforce

Question

I need help with a design issue and what happens in Batch Apex.
This is the scenario we have:
We have a territory object, then when you update a single field needs to update a field on UPTO hundreds of thoursands of contacts. To do this, I am using Batch Apex. And invoking it on the territory record before it’s updated.
Question:
Say the user updates the territory from A to B, and clicks save. This causes a big batch of contacts to get updated and take a while Then, he changes B to C. Are we guaranteed that the final update on all impacted records will be C? How come?
Or, is there a way to schedule your batch jobs? I’m looking into asyncApexJob and using that as a framework…
Is there a better design?

Answer 1

Batch Apex doesn't work the same way a Trigger works. The only way the situation described in your Question 1 would occur is if you were to call/execute a batch from a Trigger, and I would highly recommend avoiding that, if it's even possible.
(and 3.) Batches are typically scheduled to run over-night, or during off hours, using the Apex Scheduler. This is the recommended solution.

Answer 2

First, you will want to put the logic in the AFTER UPDATE trigger on the Territory object, not the BEFORE UPDATE section. As a general rule, if you need to update a field or value on the record/object that the trigger action is for (i.e. the Territory object in your case) then you use the BEFORE UPDATE or BEFORE INSERT section(s), and if you want to create/update/delete other records/objects (i.e. Contacts in your case) you use the AFTER UPDATE or AFTER INSERT section(s).
Second, I do not think there is anything wrong with initiating the batch apex process from your trigger.
For example, let us say you have a batch class called "BatchUpdateContactsBasedOnTerritory." And this class has three (3) key features:
it implements "Database.Stateful" in addition to "Database.Batchable"
it has a constructor method that takes a list of territories as an argument/parameter
it has a member variable to hold the list of territories that are passed in
Part of your batch class:
global list<Territory> TerritoryList;
global BatchUpdateContactsBasedOnTerritory(list<Territory> updatedTerritories){
TerritoryList = updatedTerritories;
}
Your trigger:
trigger TerritoryTrigger on Territory (after delete, after insert, after undelete, after update, before delete, before insert, before update)
{
if(trigger.isInsert)
{
if(Trigger.isBefore){
// before insert event not implemented
}
else if(Trigger.isAfter){
// after insert event not implemented
}
}else if(trigger.isUpdate){
if(Trigger.isBefore){
// before update event not implemented
}
else if(Trigger.isAfter){
// after update event - call batch class to process 1000 records at a time
Database.ExecuteBatch(new BatchUpdateContactsBasedOnTerritory(trigger.new),1000);
}
}else if(trigger.isDelete){
if(Trigger.isBefore){
// before delete event not implemented
}
else if(Trigger.isAfter){
// after delete event not implemented
}
}
else if(Trigger.isUnDelete){
// undelete event not implemented
}
}