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How do I validate the migration?

Short answer: Replay real production traffic through both platforms, compare each against ground truth, and report the distribution of the error — not the mean.
Do not compare the providers against each other. Two wrong answers can agree.

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Why error rates won’t tell you

The failure modes are subtle and delayed. A truck route that omits a constraint returns 200 and looks correct in a diff. A silently transposed matrix produces plausible travel times and wrong assignments. A geocode that falls back to a city centroid renders as a pin on a map. Everything succeeds. You will not notice the regression by watching error rates.
The most dangerous migration outcome is not an outage. It is a system that works, is cheaper, and is quietly wrong in ways that surface as operational incidents six weeks later, with no clear cause.

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What to compare against

SurfaceGround truth
Route durationActual driven duration, from telematics
Route distanceActual driven distance
Route geometryActual driven path, map-matched
GeocodeA held-out set with verified rooftop coordinates
MatrixSampled cells, validated against single-route calls
Truck routesTrap geometry — pass/fail

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Report distributions, not means

SELECT provider,
  percentile_cont(0.50) WITHIN GROUP (ORDER BY error) AS p50,
  percentile_cont(0.90) WITHIN GROUP (ORDER BY error) AS p90,
  percentile_cont(0.99) WITHIN GROUP (ORDER BY error) AS p99
FROM comparison GROUP BY provider;
A platform whose durations are unbiased on average but have twice the variance produces twice as many late deliveries.The mean will tell you nothing about that. p99 is where failed deliveries live.

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Measure confidence calibration

The metric almost nobody measures, and the one that determines data quality. Among results the platform scored above 0.9, what fraction were actually correct? A geocoder with a lower match rate and well-calibrated confidence is more useful than one with a higher match rate and confidence that doesn’t predict correctness. The first lets you route uncertain records to an exception queue. The second silently corrupts your database.
Do not port your confidence threshold across platforms. queryScore: 0.9 does not mean what partial_match: false means. Recalibrate against your own ground truth.

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Truck routing is a gate, not a metric

Route a 410 cm vehicle through the 11foot8 bridge (Durham NC), Storrow Drive (Boston), and the Southern State Parkway (Long Island). Any path returned is a failure. Run the same three in car mode as a control. All three must route — proving the test exercises the constraint rather than failing for an unrelated reason. Keep these assertions in CI permanently, not just during migration. A refactor that drops vehicle[height] is invisible in code review.

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Test the rollback

A migration you cannot reverse is not a migration. It is a bet.Write the rollback procedure. Then execute it once in production, deliberately, during a low-traffic window. Flip the flag. Verify traffic moves. Flip it back.An untested rollback is a hypothesis.
If rollback takes twenty minutes, nobody will pull the trigger during an incident. They will debug forward, badly, at 2am.

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Common misconceptions

“Cost validation is enough.” A cheaper wrong route is worse than an expensive right one. “We’ll compare the two providers.” Against ground truth. Two wrong answers agree. “The mean error looks fine.” Report p90 and p99. “We’ll delete the trap tests after cutover.” They’re the thing that catches the refactor two years from now.

Google Migration Architecture

Shadow comparison, per-endpoint flags, tested rollback.

How do I test before production?

Entitlement checks and trap geometry.

HERE Geocoding vs Google

How to benchmark on your own address dataset.

Truck Routing

The trap geometry, in detail.
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