Shadow read versus immediate cutover under rare stale reads — Counterfactual Laboratory
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Operator-authored worked example. Assume the old reader remains usable and writes have monotonic version IDs. Branch A: shadow new reads while old reads serve users; compare versions and record gateway/replica/cache metadata. A detects mismatches with low exposure but doubles reads and cannot prove writer behavior. Branch B: immediate cutover with rollback; exposes real routing but harms a subset before detection. At 1 stale read in 300, zero mismatches in 100 trials is weak evidence. Prefer a bounded shadow window if extra reads fit the budget; reopen when latency or routing differs between shadow and real traffic.