Integrations

The contract comes first

Every exchange declares a schema: what comes in is checked before it touches anything, what goes out is documented. The payoff shows up on integration day: instead of discovering fields by trial and error, there is a document stating what arrives and what comes back — and a test that fails when someone changes it.

Duplicates, retries and a safety net

Messages arrive twice, out of order, or never. Every state-changing operation is written so it can be repeated without harm, and every queue has a retry ceiling with growing backoff.

When a vendor's connection proves unreliable you don't beg it: you put a periodic check beside it that realigns state. It costs little and removes a whole class of phone calls.

Systems that don't want to talk

Business systems with interfaces from the 2000s, platforms exposing a slice of what you need, public administration portals that accept a file and nothing else: the work is a layer that normalises all of it into one model, and keeps each vendor's quirks in a single place.

Which system owns each piece of data

Before a line is written we decide, for every piece of information, which system is the source of truth: stock is owned by the business system, orders are born in the online shop, the customer record belongs to whoever saw them first. There is no universally right answer — there is the right one for how you work, and it must be written down.

When that decision isn't taken, the integration takes it for you: last writer wins, and six months later nobody can explain why a price changed.

When the other side goes quiet

A link doesn't fail when the vendor returns an error: it fails when it answers "fine" and does nothing, or when the same order arrives twice because the confirmation was lost on the way.

So every state-changing operation is written to be repeatable without harm, every queue has a retry ceiling with growing backoff, and when the retries run out the thing doesn't die quietly: someone is told.

And when a vendor's link proves unreliable, you don't beg it: you put a periodic check beside it that realigns state. It costs little and removes a whole class of phone calls.

Old systems, government portals, files

Not everything exposes modern interfaces, and that is fine: you work with what exists. Scheduled exports, watched folders, files dropped on a server, sometimes read-only access to the database.

These are less elegant routes and perfectly workable, provided the same discipline applies: a declared format, a check on what comes in, and an alert when last night's file didn't arrive. The whole difference between an integration and a script lies there.