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Motivation

Lazy initialization for RecordHeader was introduced in KAFKA-10438, improving performance but also creating unexpected side effects.
Since the Consumer is not thread-safe, the same assumption naturally extends to ConsumerRecord. However, users often assume that read-only access across threads is safe.
With lazy initialization, this assumption no longer holds, which can lead to unexpected behavior.
So far, three concurrency-related issues (KAFKA-12999, KAFKA-17725, KAFKA-18470) have been reported in connection with RecordHeader data access.
Making RecordHeader thread-safe would help avoid user confusion and prevent similar issues in the future.

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Testing will be carried out using the JMH benchmark described above.
If the benchmark completes successfully without any NullPointerException, it demonstrates that RecordHeader is thread-safe.
Furthermore, it confirms that double-checked locking performs significantly better than full-method synchronization.

Rejected Alternatives

Full-Method Synchronization

Although it seems simple, it introduces significant overhead to key()andvalue() on every method invocation.