Integration strategies can be described as either loosely coupled or tightly coupled. In the former, the systems pass data between them without regard to the specific methods each uses to manage that data (I'm over-simplifying these descriptions for the sake of clarity). As long as each system knows what the data represents, it can function effectively, and the integrated applications can perform coordinated functions as required. While a system may need data from another system to produce the desired results, the systems still can function independently.
In a tightly coupled environment, systems are linked so that each system requires functionality from the other system to get its job done. It will begin a process, pass data to another system, wait for the other system to process that data and return an answer or an update (or request data from another system to complete its function), and then complete its original process.
Typically, tightly coupled systems depend on remote-procedure calls to support the inter-system relationships. And loosely coupled systems often rely on a bridge system like IBM's Websphere MQ, which passes data in the form of messages (hence the name "message-oriented middle-ware").
Both tightly coupled and loosely coupled systems also can rely on application programming interfaces to pass data. These are pre-programmed doorways, or "sockets," into a system that permits entry of data according to predefined protocols.
The Impact of the Web
As with everything else, the Web has had an impact on systems integration—in fact, it has profoundly changed the concept of what systems integration means. No longer is systems integration seen simply as sharing data among a group of systems within a single enterprise, or among enterprises linked by a proprietary network. The future of systems integration is to link any system to any other system using protocols designed to support business process integration (BPI).