128-bit trace IDs exist because coordination-free random generation requires a collision space so vast that the birthday paradox never becomes a practical threat.
Key Takeaways
Trace IDs are generated independently at each service entry point and propagated via headers like traceparent; no central counter is feasible.
Collision risk follows the birthday paradox formula P ≈ 1 - e^(-k²/2N); with 64-bit IDs, a billion traces already hits ~2.7% collision risk.
128-bit space (N = 2¹²⁸ ≈ 3.4×10³⁸) keeps k²/2N near zero even at quadrillion-scale trace volumes.
256 bits would double storage and bandwidth on every span, header, log line, and index with no realistic safety gain over 128.
128 bits matches UUID size, meaning every database, language, and protocol already handles it natively.
