A connection is not an agreement.
Two systems exchanging data does not mean they agree about it. Customer statuses, identifiers and timestamps can carry different meanings on either side of an interface.
Begin with the shared concepts. Decide which system owns each record, how updates are represented and what to do when values conflict.
Plan for partial failure.
Networks fail and dependencies become unavailable. An integration needs an explicit approach to timeouts, retries and duplicate events. The right behaviour depends on the operation: repeating a read is different from repeating a payment instruction.
Give operators enough information to identify a failed exchange without exposing sensitive data in logs. Decide which failures can recover automatically and which require a person.
Maintain the contract.
Document the expected structure and meaning of messages. Agree how changes will be introduced and how both sides can validate compatibility.
Ownership should remain clear after launch. Someone needs to respond when the contract changes, the data quality degrades or the business process evolves. Reliable integration is an ongoing relationship between systems and teams.