How do you implement and test an Apex HTTP callout?

Implementation: Use HttpRequest and HttpResponse classes with an Http instance:
Http h = new Http(); HttpRequest req = new HttpRequest(); req.setEndpoint('callout:MyNC/api/v1/resource'); req.setMethod('POST'); req.setHeader('Content-Type', 'application/json'); req.setBody(JSON.serialize(payload)); req.setTimeout(20000); HttpResponse res = h.send(req); if(res.getStatusCode() == 200) { ... }

Testing: Salesforce does not make real callouts in tests. Implement the HttpCalloutMock interface and register it with Test.setMock(HttpCalloutMock.class, new MyMock()). The mock returns a predefined HttpResponse. Use MultiStaticResourceCalloutMock for multi-callout tests.

Timeout: req.setTimeout() accepts milliseconds (1000–120000). Always set a timeout to avoid transactions hanging indefinitely.

Always test both success and error response codes, and verify idempotent retry logic.