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Do We Really Need Semantic Layers?

It used to be that a semantic layer was the sine qua non of a sophisticated BI deployment and program. Today, I’m not so sure.

A semantic layer is a set of predefined business objects that represent corporate data in a form that is accessible to business users. These business objects, such as metrics, dimensions, and attributes, shield users from the data complexity of schema, tables, and columns in one or more back-end databases. But a semantic layer takes time to build and slows down deployment of an initial BI solution. Business Objects (now part of SAP) took its name from this notion of a semantic layer, which was the company’s chief differentiator at its inception in the early 1990s.

A semantic layer is critical for supporting ad hoc queries by non-IT professionals. As such, it’s a vital part of supporting self-service BI, which is all the rage today. So what’s my beef? Well, 80% of most BI users don’t need to create ad hoc queries. The self-service requirements of “casual users” are easily fulfilled using parameterized reports or interactive dashboards which do not require semantic layers to build or deploy.

Accordingly, most pureplay dashboard vendors don’t incorporate a semantic layer. Corda, iDashboards, Dundas, and others are fairly quick to install and deploy precisely because they have a lightweight architecture (i.e., no semantic layer). Granted, most are best used for departmental rather than enterprise deployments, but nonetheless, these low-cost, agile solutions often support sophisticated BI solutions.

Besides casual users, there are “power users” who constitute about 20% of total users. Most power users are business analysts who typically query a range of databases, including external sources. From my experience, most bonafide analysts feel constrained by a semantic layer, preferring to use SQL to examine and extract source data directly.

So is there a role for a semantic layer today? Yes, but not in the traditional sense of providing “BI to the masses” via ad hoc query and reporting tools. Since the “masses” don’t need such tools, the question becomes who does?

Super Users. The most important reason to build a semantic layer is to support a network of “super users.” Super users are technically savvy business people in each department who gravitate to BI tools and wind up building ad hoc reports on behalf of colleagues. Since super users aren’t IT professionals with formal SQL training, they need more assistance and guiderails than a typical application developer. A semantic layer ensures super users conform to standard data definitions and create accurate reports that align with enterprise standards.

Federation. Another reason a semantic layer might be warranted is when you have a federated BI architecture where power users regularly query the same sets of data from multiple sources to support a specific application. For example, a product analyst may query historical data from a warehouse, current data from sales and inventory applications, and market data from a syndicated data feed. If this usage is consistent, then the value of building a semantic layer outweighs its costs.

Distributed Development. Mature BI teams often get to the point where they become the bottleneck for development. To alleviate the backlog of projects, they distribute development tasks back out to IT professionals in each department who are capable of building data marts and complex reports and dashboards. To make distributed development work, the corporate BI team needs to establish standards for data and metric definitions, operational procedures, software development, project management, and technology. A semantic layer ensures that all developers use the same definitions for enterprise metrics, dimensions, and other business objects.

Semi-legitimate Power Users. You have inexperienced power users who don’t know how to form proper SQL and aren’t very familiar with the source systems they want to access. This type of power user is probably more akin to a super user than a business analyst and would be a good candidate for a semantic layer. However, before outfitting these users with ad hoc query tools, first determine whether a parameterized report, an interactive dashboard, or a visual analysis tool (e.g., Tableau) can meet their needs.

So there you have it. Semantic layers facilitate ad hoc query and reporting. But the only people who need ad hoc query and reporting tools these days are super users and distributed IT developers. However, if you are trying to deliver BI to the masses of casual users, then a semantic layer might not be worth the effort. Do you agree?

Posted by Wayne Eckerson on July 28, 2010


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