TL;DR — Zero-Downtime Data Migration

You run a user-generated content platform with 500M documents stored in MongoDB (3 sharded clusters). The application uses MongoDB's flexible schema heavily — documents have varying fields, nested arrays, and embedded objects. Read queries average 15K QPS, writes average 5K QPS. The database costs $45K/month due to the large working set and shard count. Key features: All reads and writes continue working throughout migration. Data consistency verified between source and target databases.