How GeoBlood Uses MongoDB Geospatial Queries to Match Blood Donors in Emergencies

GeoBlood is a blood donor registry platform that uses MongoDB's geospatial indexing to match nearby compatible donors during medical emergencies. A naive approach of fetching all donors and filtering in application memory fails at scale, scanning entire collections and wasting CPU and memory resources. The system also accounts for blood type compatibility rules, such as O-negative donors being eligible for B-negative recipients, which a simple query would miss. Additional factors like recent donation history and donor responsiveness must be modeled to avoid routing requests to ineligible or unreliable donors. The project also highlights a critical GeoJSON pitfall where longitude and latitude must be stored in reverse of the common human convention, with incorrect ordering silently returning wrong results rather than throwing errors.