Quick answers to key questions

• QueryLocator and Iterable in batch Apex. Database.getQueryLocator returns a SOQL query and can process up to 50 million records, because it skips the 50,000 row limit. An Iterable lets you process a custom collection or complex logic, but the normal row limits apply.
• Order of records in execute. The order of batches is not guaranteed, and each execute call is a separate transaction. You can use ORDER BY in the start query, but do not depend on batch order.
• Batch best practices. Keep logic out of loops that run SOQL or DML, use a smaller scope when you make callouts, avoid heavy use of Database.Stateful, handle partial failures, write tests for a full batch, and keep the query selective.
• Can we call a future method from a batch? No. A future method cannot be called from a batch or another future method. Use queueable Apex instead. Future methods are best for simple, short background work, while batch is for large volumes of records.
• What is queueable Apex? A class that implements Queueable and runs when you call System.enqueueJob(). It can accept complex types as parameters, returns a job Id that you can monitor, and can chain another job.
• Many developers using one sandbox. Give each developer their own developer sandbox or scratch org, keep the code in Git with branches and pull requests, and merge into a shared integration environment. A CI/CD pipeline then validates and deploys it.
• Change sets and Copado. A change set is a bundle of components that you send from one org to another that has a deployment connection. Copado is a DevOps tool that manages user stories, commits changes to Git, promotes them through environments and validates and deploys them with a pipeline.
• CPU time limit exception. It happens when too much work is done in one transaction, such as nested loops, recursion or several triggers and flows running on the same record. Fix it with maps, bulkified code, fewer loops and moving heavy work to async Apex.
• Get all keys from a map. Use myMap.keySet(), which returns a Set.

Batch with failure handling

Use Database.update(records, false) so one failed record does not roll back the whole chunk, keep the failures in a stateful list, and email them in finish.

global class UpdateAccountsBatch implements Database.Batchable<SObject>, Database.Stateful {
    global List<String> errors = new List<String>();

    global Database.QueryLocator start(Database.BatchableContext bc) {
        return Database.getQueryLocator([SELECT Id, Name FROM Account]);
    }
    global void execute(Database.BatchableContext bc, List<Account> scope) {
        Database.SaveResult[] results = Database.update(scope, false);
        for (Integer i = 0; i < results.size(); i++) {
            if (!results[i].isSuccess()) {
                errors.add(scope[i].Id + ': ' + results[i].getErrors()[0].getMessage());
            }
        }
    }
    global void finish(Database.BatchableContext bc) {
        if (!errors.isEmpty()) {
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            mail.setToAddresses(new String[] { 'admin@example.com' });
            mail.setSubject('Batch finished with errors');
            mail.setPlainTextBody(String.join(errors, '\n'));
            Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
        }
    }
}

Update Contact mobile when the Account phone changes

trigger AccountPhoneTrigger on Account (after update) {
    Map<Id, String> newPhones = new Map<Id, String>();
    for (Account a : Trigger.new) {
        if (a.Phone != Trigger.oldMap.get(a.Id).Phone) {
            newPhones.put(a.Id, a.Phone);
        }
    }
    if (newPhones.isEmpty()) return;

    List<Contact> toUpdate = new List<Contact>();
    for (Contact c : [SELECT Id, AccountId, MobilePhone FROM Contact WHERE AccountId IN :newPhones.keySet()]) {
        c.MobilePhone = newPhones.get(c.AccountId);
        toUpdate.add(c);
    }
    update toUpdate;
}

Find duplicates in a List of Strings

Set<String> seen = new Set<String>();
Set<String> duplicates = new Set<String>();
for (String s : items) {
    if (!seen.add(s)) {      // add() returns false if the value is already in the set
        duplicates.add(s);
    }
}
System.debug(duplicates);