1.13Why `upsert` matters so much

You should expect this to come up.

Suppose the integration runs twice.

Using Salesforce create:

run #1 → Account created

run #2 → Another Account created

Oops.

Instead you have:

External_Customer_ID__c

configured as an external ID.

Then:

upsert
using External_Customer_ID__c

means:

does C92834 exist?

YES → update
NO  → insert

MuleSoft's current Salesforce Connector documentation specifically recommends upsert over create in many scenarios to avoid unwanted duplicate records.

This leads into a major integration-development concept we'll cover later:

idempotency

Same request repeated → no unintended side effects.

Your QA instincts should immediately recognize why that's valuable.