Application integration

Application integration is the design of interactions between independently operated applications. It defines how systems exchange information, request actions, and coordinate business processes. For example, an online store might connect its checkout application to payment processing, inventory management, fulfillment, and customer notifications. These systems need agreed interfaces and business semantics, but they do not need to share a programming language, database, or deployment schedule.

Synchronous integration usually uses request-response APIs when a caller needs an immediate answer. A checkout service might call an inventory API before confirming availability. This approach is straightforward, but the caller depends on the downstream system's response time and availability. Timeouts, bounded retries, circuit breakers, and concurrency limits help prevent a slow dependency from exhausting the caller's resources. A timeout does not prove that an operation failed: a payment might succeed even if its response is lost.

Asynchronous integration uses queues, publish-subscribe messaging, or event routing to separate when work is submitted from when it is processed. A queue can buffer shipping requests during a traffic spike while workers process them at a sustainable rate. Publish-subscribe allows an order-created event to reach billing, analytics, and notification consumers independently. These patterns reduce direct timing dependencies, but introduce processing delays, backlog management, and the need to communicate pending states to users.

Message delivery guarantees must be understood at the application boundary. Many messaging systems can redeliver messages, so consumers should make repeated processing safe, often by recording a business operation identifier. Ordering may be limited to a partition or message group rather than an entire system. A broker's exactly-once feature does not automatically make external database updates or payment calls occur exactly once. Failed messages need bounded retries, a dead-letter destination where appropriate, and an operational process for investigation and replay.

Contracts are as important as transport. An event such as OrderAccepted should have a documented meaning, schema, identifier, timestamp, and compatibility policy. Producers should avoid changes that unexpectedly break existing consumers. Integration layers may translate formats or protocols between a legacy application and a cloud service, but excessive transformation can conceal business rules and make troubleshooting difficult.

Multi-step processes can use orchestration, where a workflow explicitly coordinates activities, or choreography, where applications react to events without a central coordinator. A travel reservation workflow might reserve a flight and hotel, then run compensating actions if a later step fails. Compensation is a business action, such as canceling a reservation; it is not necessarily a perfect reversal or an atomic database rollback. Local transactions and a transactional outbox can help prevent a database update from succeeding while its associated event is never published.

Secure integration requires authenticated identities, authorization for each operation or destination, encrypted transport, and careful handling of credentials and sensitive payloads. Correlation identifiers, distributed traces, queue-age metrics, and workflow histories make failures traceable across applications. Managed integration services reduce infrastructure maintenance, but do not remove responsibility for contracts, duplicate handling, quotas, costs, or recovery. Application integration is also not simply data replication: its primary concern is enabling applications to collaborate, although moving and transforming data is often part of that collaboration.