Scaling Without Rewriting: How Observability Defeats the Undocumented Legacy System

Most teams facing undocumented legacy systems default to costly rewrites that recreate the same unknowns. This case study demonstrates how observability, external analysis, and strategic decomposition enable modernization and scaling without rebuilding core components. For junior engineers and resource-constrained teams, the critical insight is that you don't need to understand every line of code—understanding system boundaries and actual behavior is sufficient to move forward.