E-commerce monolith migration

A mid-size European fashion retailer runs its e-commerce on a 10-year-old monolith: Java 8 / Spring, about 2 million lines, one PostgreSQL database of 1.2 TB with 350 tables and heavy use of stored procedures and cross-module joins. It serves 8 countries, 3 currencies and 4 languages, with about 40,000 orders a day and peaks of 12x during sales. The monolith contains: the storefront (server-rendered, plus a separate mobile app hitting the same endpoints), catalogue and search (a Lucene index rebuilt nightly), pricing and promotions (the most complex module, 200,000 lines, with country-specific rules nobody fully understands), cart and checkout with three payment providers, order management, inventory synchronised every 15 minutes with the warehouse system via file exchange, customer accounts and loyalty, returns, and a back-office used by 300 staff. Deployments happen every two weeks as a single artefact with a 30-minute maintenance window; test coverage is 25% and mostly unit tests. Five teams of 8 developers, each organised around a business area but all committing to the same repository.

Plan the migration to independently deployable services over 12 months with no feature freeze, no unplanned downtime and the ability to roll back every step, preserving the peak-season capacity (sales in January and July must not be put at risk).