Most new business apps are not built in isolation. They have to read from an old ERP, push data to an accounting package, or sync with a third-party platform. The integration layer is where many projects slip, because it is planned last.

Here is a practical checklist to work through when a new web app, SaaS product or mobile app has to talk to existing systems.

1. Map every system before you design endpoints

List each system the new product must exchange data with: legacy apps, department tools, third-party platforms

For each one, note the direction of data (in, out or both) and how often it moves

Decide which system is the source of truth for each data type

2. Make the API layer secure by default

Authenticate every call, including internal service-to-service traffic

Give each integration the least access it needs

Keep secrets out of source code and rotate them on a schedule

Keep an inventory of every API you expose, and retire the ones nobody calls

3.…