API-first with legacy systems: replacing the spaghetti one route at a time

A typical enterprise in the region runs an ERP, a few line-of-business systems, a customer portal and a mobile app, joined by dozens of point-to-point connections built over the years. Each new project adds another. Eventually nobody can change one system without breaking two others.

Put an interface in front of the core

Instead of connecting every new application directly to the database of an old system, publish a small set of well-designed APIs in front of it: customers, orders, invoices, shipments. New applications use the APIs; the old connections are moved over one at a time. This approach, sometimes called the strangler pattern, lets the core system be replaced later without touching the consumers.

Design the contract first

• write the API specification, usually in OpenAPI, before any code;
• agree it with the teams who will use it;
• version it, and never break an existing version without notice;
• publish it in a developer portal with examples and a test environment.

Make it safe to retry

Networks fail. A payment or an order sent twice must not be processed twice, so write operations should accept an idempotency key. For events that other systems need to know about, such as a shipment leaving a port, publish them through a message broker using an outbox table, so that the database change and the event cannot drift apart.

Security and visibility

An API gateway handles authentication with OAuth 2.0 or mutual TLS, rate limits and logging in one place. Correlation identifiers carried through every call let support teams follow a single request across systems. Without them, an integration fault becomes a meeting between four vendors.

Run it like a product

APIs need owners, a backlog and service levels, not only a project team that leaves after go-live. Measure availability, latency and error rates per API, and review who is using what before you change it.

Our integration team designs API layers, event streams and gateways around existing ERP and line-of-business systems, and can run them as a managed service afterwards.