Introduction to Semantic Layers

Introduction to Semantic Layers
The semantic layer is a metadata layer that abstracts the complexities of the data source. The end user sees only a logical grouping of available, well-named fields for use and does not have to concern them with the details of database design or need any SQL knowledge.
In Business Objects XI, two semantic layers are supported business views and universes. With the merging of Crystal Decisions and Business Objects, a decision was made to keep business views from Crystal Decisions and universes from Business Objects.
Why Use a Semantic Layer
Semantic layers offer wonderful advantages over traditional report design processes by removing the most difficult data-intensive tasks, as described in the following sections, from report design. Furthermore, it allows for reuse of components and promotes the concept of changing a component once and having the change applied to multiple reports.
Abstract the Complexities of the Database
With large complex data warehousing projects using many tables and complex joins, the report author might not have the requisite knowledge. If the report author is required to join the tables for the reports, he might not use the most efficient join, resulting in poor performance. Semantic layers allow for a division in labor, whereby the more technical database administrators and developers can create the joins between tables and data sources and the business users can concentrate on designing reports to satisfy their requirements.
Putting Report Design in the Hands of Business People
As a result of this division of labor and the reuse of the Business Objects repository, business people do not have to be as technically savvy, effectively resulting in less technical skill being required to develop reports.
Support for a Wide Range of Data Sources
Both metadata layers provide a wide range of database support, including the majority of relational database vendors. Business views also include the ability to connect to some no relational data sources; for example, a user could abstract the complexity of an XML file by encapsulating the XML with a business view.
Reuse of Metadata Components
One major drawback to specifying the data joins in each report is that this creates a large amount of redundant work. Secondly, should the database change, there is no way to effect this change other than changing each of the individual reports. A semantic layer allows the administrator to create this join once and should this join change, the administrator only needs to change it once.
Security
The semantic layer allows the administrator to set up security so that data is filtered based on who the user is. This can be based off of security within an entitlements database, or the Business Objects user model can be used to create the security model. This way rows and columns can be secured by users or groups.
The Business Objects repository also secures the semantic layers and only those users with the required permissions can access them. For example, it is generally accepted auditing practice that accounts receivable personnel should not be able to see accounts payable information and vice versa. By using the user group functionality in Business Objects Enterprise, the accounts receivable group would be given access to only the accounts receivable semantic layer.
Transportability
The semantic layers provide the ability to export a business view to an XML file or a universe to an .unv file. These files can then be easily imported into another Business Objects Enterprise repository. This simple form of transportability makes semantic layer swapping a simple exercise. It also simplifies the storage of source code, should the organization have a source code storage requirement.