When a platform serves 1.2 billion monthly users processing 469 million posts annually, every architectural decision carries enormous weight. Reddit’s journey from a Lisp application rewritten in Python to a modern microservices architecture reveals a masterclass in managing technical evolution at scale. What makes Reddit’s approach particularly instructive isn’t just the technologies they chose, but how they orchestrated transitions without breaking the experience for hundreds of millions of users.

Most companies at scale rewrite their early technology choices. Shopify took the opposite path. During Black Friday 2025, they processed 90 petabytes of data, handled 14.8 trillion database queries, and peaked at 489 million requests per minute—all while running one of the largest Ruby on Rails monoliths in production.