A correlation ID (request ID) is a unique identifier added to a request and included in logs. Generate it at the edge (HTTP middleware/filter) or accept it from upstream, then pass it through all layers and background jobs triggered by the request.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "correlation-id-in-a-monolith:-what-is-it-and-whe"
function explain() {
// Start from the core idea:
// A correlation ID (request ID) is a unique identifier added to a request and included in lo
}