System Integration Planning: APIs, Data and Ownership

Follow one exchange end to end.

Choose a representative exchange, such as an order moving from a website into an internal application. List the sending system, receiving system, fields and trigger. Decide which system owns each value. If two systems can update the same record, document how conflicting updates will be handled before building the interface.

Design for delayed and repeated messages.

An integration can receive the same request twice or lose a response after the receiver has completed its work. Discuss identifiers, retries and reconciliation with the implementation team. Define how users see a pending or failed exchange, and who can safely correct it without creating duplicate business records.

Make the interface reviewable.

Record examples of accepted and rejected inputs, expected responses and access requirements. Include a change process so that one system cannot silently break the other. Review the contract with both owners and test representative failures before considering the integration ready for operational use.

Walk through the life of an exchanged record

Choose one record, such as an order, and follow its creation, update, cancellation and correction. At every boundary, identify the sending system, receiving system and source of truth. Confirm the actual interface available rather than assuming that an application exposes the data you need.

Review authentication, field meanings, identifiers and expected timing with both owners. Test a duplicate message, a delayed update and an unavailable destination. Decide how to distinguish a rejected request from an uncertain outcome. Arrange reconciliation so the operating team can find records that disagree after an interruption. These decisions should be captured before implementation begins.

An interface contract planning sheet

This is a discussion template, not a substitute for the technical contract. Attach the relevant API specification or file format after the owners agree the behaviour.

• Purpose and direction: why the exchange occurs and which system initiates it.
• Record identity: stable identifiers, field definitions and ownership of each value.
• Trigger and timing: event, schedule, expected delay and ordering assumptions.
• Access and limits: authentication, permissions and constraints confirmed by the provider.
• Failure handling: timeouts, duplicates, retries, rejection reasons and human escalation.
• Verification: test examples, reconciliation checks, versioning and named support owners.

Do not equate a response with a completed business action

A successful technical response may acknowledge receipt without confirming that a downstream process finished. Agree what each status means and how completion is checked. Conversely, a timeout may leave an uncertain result, so retrying a consequential action without checking can create duplication.

Avoid relying on field names alone. Two systems may use the same label for different concepts. Keep mappings and interface changes visible to both teams, including who approves a breaking change.

Should an integration be real time?

Choose timing around the business need and the capabilities of the connected systems. A scheduled exchange may be sufficient for some tasks; others need an event-driven flow. Document the acceptable delay and recovery approach before choosing the implementation pattern.

ILLUSTRATIVE PLANNING EXAMPLE

Example: connecting orders and inventory

An order system sends confirmed purchases to inventory. Decide which system owns stock counts, when a reservation occurs and how cancellations reverse it. Ask what happens when the same message arrives twice or inventory is temporarily unavailable. Assign an owner to each interface and agree representative integration tests.

Create an interface sheet covering fields, triggers, permissions, duplicate handling and recovery. Confirm assumptions with both system owners before estimating the connection.

FURTHER READING

Official resources to explore

Explore the related official guidance below. The templates and examples are practical starting points to adapt to your own project.

• MDN: overview of HTTP