Don't Panic — System Integration

System integration is the business of making separately funded systems deliver one outcome together — an order captured here, a payment authorized there, a shipment scheduled somewhere else — without any of them needing to know the others' internals. That is the whole job. Everything else is choosing how the exchange happens.

Before this had a name, the default was point-to-point: every system wired to every other, each connection hardcoded, each change rippling through the estate until nobody dared touch anything. The cure is to draw the business flow first — which system owns which fact, which arrow carries which exchange — and only then pick a technology. Skip the map and you get the knot back, just with newer tools.

Four interaction styles cover almost every real boundary. A synchronous API when the caller needs an answer now. A command on a queue when the work can be deferred, buffered, retried. An event when independent consumers should react to a fact. A batch when the exchange is large or scheduled and real time is not an option. They are not gradations of sophistication — a nightly batch can be exactly right; an event stream can be overkill.

The distinction that breaks things if you miss it: a command asks for a specific action and is high-value (deliver it at least once, usually process it exactly once), while an event announces a fact and expects no particular reaction. Sending "do this" as a broadcast event, or treating a past-tense notification as an instruction, is how duplicate orders and double billing get born.

The boring list is the one that saves you. Every boundary needs a contract, identity, authorization, versioning, a timeout or delivery behavior, idempotency, observability, ownership, and a recovery path. Leave any implicit and it becomes the next incident.

Read the Intro for the four styles and the boundary concerns. The Cheatsheet keeps the command/event split, the broker properties, and the reliability patterns at hand. The Reference Links take you to the Azure Architecture Center guides that settle the details.