Write an Apex REST callout that posts data from a Salesforce trigger to an external system

In-depth guide

Steps

1. Create a named credential in Setup to store the external endpoint URL and authentication, avoiding hardcoded URLs or credentials in Apex code
2. Write a future method (or Queueable Apex class) annotated with @future(callout=true) that accepts primitive arguments — Apex triggers cannot make synchronous callouts
3. Inside the future/Queueable, construct an HttpRequest using the named credential as the endpoint prefix (callout:MyNamedCredential/path) and set appropriate headers and JSON body
4. Call HttpClient.send() and inspect HttpResponse.getStatusCode(); throw a custom exception for non-2xx responses to trigger retry logic or alerting
5. In the trigger, collect the relevant record IDs and field values, then invoke the future/Queueable asynchronously — do not pass SObject instances directly
6. Add a try/catch around the enqueue call in the trigger to handle the AsyncException that fires when the async queue is full (governor limit: 50 future calls per transaction)