6.1Inter-system integration

When integrating multiple systems, simply "connecting them via an API" often fails to meet the requirements. A systems architect must judge which integration approach (REST API, ETL, EAI) to use, and whether to make it synchronous or asynchronous, based on constraints such as data volume, how real-time the processing must be, and differences in availability between the caller and the callee. Getting this judgment wrong leads to failures such as cascading response delays or unhandled temporary data inconsistency.

6.1.1REST APIs, ETL, and EAI

• REST APIs represent resources via HTTP methods (GET/POST/PUT/DELETE, etc.) and URLs, integrating systems loosely coupled. The caller and provider can be developed and deployed independently, and the general-purpose reachability from web/mobile clients is an advantage—but calling one record at a time tends to be inefficient for bulk data integration.
• ETL (Extract/Transform/Load) processes data from multiple systems in the order extract, then transform, then load, bulk-transferring it into a data warehouse or another system. It suits use cases that periodically consolidate large volumes of data, such as nightly batches, but offers low real-time responsiveness.
• EAI (Enterprise Application Integration) mediates heterogeneous in-house systems (accounting, inventory, HR, etc.) through a hub-like platform, so each individual system only needs to connect to the hub. This keeps the number of connections down compared to wiring each system to every other system point-to-point (a "spaghetti" of links), and changes to integration rules can be concentrated in the hub.