Question 1

How do you efficiently query the nearest available vehicle to a pickup location?

Accepted Answer

Store active vehicle locations in Redis using geospatial commands. On each GPS update: GEOADD fleet_vehicles {lng} {lat} vehicle:{id}. When a trip is requested: GEOSEARCH fleet_vehicles FROMLONLAT {pickup_lng} {pickup_lat} BYRADIUS 20 km ASC COUNT 20 returns the 20 nearest vehicles sorted by distance in O(N+log M) where M is total vehicles. Filter by vehicle status (AVAILABLE) and vehicle type in the application layer. For scaling: partition by city or region into separate Redis keys (fleet_vehicles:nyc, fleet_vehicles:sf) to keep each key manageable. Update vehicle location in Redis on every GPS event (every 5-30 seconds). Expire vehicle entries if no update received for 5 minutes -- vehicle is offline.

Question 2

How do you handle GPS data loss and vehicle tracking gaps?

Accepted Answer

GPS signals can be lost in tunnels, parking garages, or dead zones. Strategies: (1) Dead reckoning: use the last known speed and heading to estimate position during gaps. Store speed and heading in the Redis vehicle hash. (2) Stale location detection: if the last GPS event is older than 2 minutes, mark the vehicle's displayed location as approximate. The UI shows a grayed-out icon. (3) On reconnect: when GPS resumes, the device sends a burst of buffered events (if it has local storage). The stream processor ingests them in order (use event timestamp, not arrival timestamp). (4) Trip distance correction: if GPS was lost for part of a trip, use odometer delta (from OBD-II) to estimate the distance driven during the gap.

Question 3

How do you implement geofencing alerts for a fleet?

Accepted Answer

A geofence is a polygon or circle defining an allowed or restricted area. Store geofences in PostGIS: table with (geofence_id, fleet_id, name, type ALLOWED/RESTRICTED, geometry POLYGON). On each GPS event, check if the vehicle is inside its assigned geofences: SELECT geofence_id FROM geofences WHERE ST_Contains(geometry, ST_Point({lng}, {lat})). If a vehicle exits an ALLOWED zone or enters a RESTRICTED zone: publish a GeofenceViolation event, notify the fleet manager via push/email, log the violation. Performance: spatial index (GIST) on geometry makes the ST_Contains query fast even with thousands of geofences. For higher throughput: cache geofence geometries for each vehicle in memory and run the containment check in the application layer using a fast geometry library (Shapely, S2).

Question 4

How does the driver assignment algorithm handle multiple simultaneous trip requests?

Accepted Answer

Naive approach: process requests serially -- first request gets the best available driver, second gets the next best. Problem: two requests arriving within milliseconds both see the same driver as available and both assign them. Solution: use optimistic locking. When assigning driver D to trip T: UPDATE drivers SET status=ON_TRIP, current_trip_id=T WHERE driver_id=D AND status=AVAILABLE. If rows_affected == 0: driver was assigned concurrently, retry with the next candidate. This is a single atomic compare-and-swap using the database. For high-concurrency dispatch (100s of requests/sec): use Redis SETNX as a distributed lock per driver: SET driver:{id}:lock trip:{id} NX EX 5. Only the first request succeeds; others try the next available driver. Release the lock after the database assignment commits.

Question 5

How do you calculate and store trip routes efficiently?

Accepted Answer

On trip creation: call a routing engine (OSRM self-hosted, Google Directions API, Valhalla) with origin and destination. Store the result: route_polyline (encoded Google polyline, ~1KB for a typical urban route), estimated_distance_km, estimated_duration_min, turn_by_turn_directions (JSON array). Push the route to the driver's device via WebSocket. Re-routing triggers: if the driver deviates significantly from the route (> 200m off route for > 30 seconds), request a new route from the current GPS position. For efficiency: run all routing requests through a queue, not inline in the trip creation API. Routing takes 100-500ms; the trip can be created and confirmed before the route is ready. Cache routes for common origin-destination pairs (e.g., airport to hotel district).

Low-Level Design: Fleet Management System — Vehicle Tracking, Driver Assignment, and Route Optimization

Core Entities

Real-Time Vehicle Tracking

Driver and Vehicle Assignment

Route Optimization

Maintenance Scheduling

Analytics and Reporting