A DB2 federated system is a distributed computing system that consists of:
To client applications, the data sources appear as a single collective database. However, the applications actually interface with a database, called the federated database, that is within the federated server. To obtain data from data sources, they submit queries in DB2 SQL to the federated database. DB2 then distributes the queries to the appropriate data sources, collects the requested data, and returns this data to the applications.
Applications can use DB2 SQL to request values of any data types that DB2 can recognize, except for LOB data types. To write to a data source--for example, to update a data source table--an application must use the data source's own SQL in a special mode called pass-through.
The federated database's system catalog contains information not only about the objects in the database, but also about the data sources and certain tables, views, and functions in them. The catalog, then, contains information about the entire federated system; accordingly, it is called a global catalog.
For a high-level overview of DB2 federated systems, see the Administration Guide: Planning. For an extended overview, see the SQL Reference. For examples of DB2 SQL queries that an application can submit, see Using Distributed Requests to Query Data Sources. For information about Pass-Through, see Using Pass-Through to Query Data Sources Directly.