Shadow Traffic
Run the new implementation against real inputs without affecting users. A sensor that produces differential evidence with zero user risk.
Shadow traffic mirrors real requests to the new version, discards the responses, and compares the new version’s behavior to the old. It’s differential testing with real production inputs.
Sensor properties
| Property | Value |
|---|---|
| Oracle strength | High — divergence on real inputs is strong evidence |
| Independence | High — production inputs |
| Scope | System-level |
| Feedback latency | Minutes |
| Actionability | Guiding — shows divergent requests and responses |
| Type | Retrospective |
What it cannot detect
Shadow traffic can only compare what both versions produce. If both have the same bug, it won’t be detected. Also, side effects (database writes, external calls) must be carefully isolated.