Spring Boot Interview Question - @Async Broke Our Audit Logs

A payment service stores request metadata in MDC so every log line contains the request information.

The controller logs look correct:

But every log produced inside sendReceipt() shows:

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Why did the audit context disappear?

Most logging frameworks store MDC using ThreadLocal. The values belong only to the current thread.

The executor picks another thread from its pool. That thread has a completely different ThreadLocal map.

Hence MDC.get("requestId") returns null

Unlike method arguments, ThreadLocal state is never copied automatically. Even if the thread comes from the same executor every time, the context is not transferred.

How would you fix it without changing every @Async method in the application?