METADATA TOOLS

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At the last count, there are three basic kinds of metadata:

Business semantic metadata. This is the kind of metadata that describes, in precise business terms, what each element of data actually means. Recall the discussion What is a sale? The business semantic metadata will define a sale as being, for instance:

The quantity and total order value of an order for a single product by a customer, before discount

The description should leave no room for ambiguity. It must be possible to define every entity, relationship, and attribute in the system.

Transformational metadata. This describes all the properties of the data element and includes:

Source system from which the element was extracted

Frequency of extraction

Changes that have been made

Level of retrospection that applies

Dependencies on other data elements

Again, this information should be held for every piece of information in the data warehouse.

Navigational metadata. This is the sort of metadata that comes with most products. It is metadata that enables the product to operate properly. For instance, the product might need to store information about the location of its own files and programs, etc.

All products claim to have metadata. This is because it is fashionable to have metadata, and vendors feel that it somehow enhances their products if the architectural diagrams can show a layer of metadata (checkout the EASI data architecture in Figure 7.1!!). One major RDBMS vendor used to have schema tables in which the tables, columns , users, etc., got stored. These schema tables are no longer schema tables; they are metadata and they've got diagrams to prove it. This is a good example of navigational metadata. When we in data warehousing refer to metadata, we generally mean the business semantic and transformational kind of metadata.

Active Versus Passive Metadata

If we are thinking of purchasing a metadata tool, then it is worth considering the difference between active and passive metadata management systems:

Active metadata management. This is where the metadata management is part of the data warehouse operation. This means that, in order for us to process any item of data, we have to register that data with the system. Some ETL products include a set of metadata functions within the product. However, these tend to be a combination of transformational and navigational metadata. Typically these products are not strong on business semantic metadata.

Passive metadata management. These are separate products that sit outside of the system. They do not participate in the actual operation of the system.

Typically, these products are stronger in the area of business semantic metadata than the active metadata management products.

The active metadata management systems have the distinct advantage that they are more likely to be complete and more likely to be kept up to date because any changes that we make to the data or to the processes will be recorded automatically into the active repository. With passive systems, there is a reliance on the users of the metadata management system to, initially, complete it and, subsequently, keep it up to date. Experience of passive documentation systems shows that there is usually an initial enthusiasm that tails off pretty quickly. The result is that these systems are rarely kept up to date.

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