An article by Rick Sherman ponders how data marts are finally being supplanted by enterprise data warehouses, at least according to a new TDWI survey. Yet he muddies the waters by saying that it is quicker to produce an EDW with no data marts than one with data marts, and so suggesting that sometimes, maybe, data marts are still the way to go.
Certainly a central enterprise warehouse without data marts or a decent way of producing them is inevitably going to do nothing to reduce the plethora of departmental data marts, since people will certainly want to get data that is relevant to them, and they’ll do it one way or another if the IT department has too big a backlog. But surely this misses the point. Isolated data marts are a major problem – they can never allow the “single view” which many executives need to take across divisional or departmental boundaries, as without something central there are just too many combinations to ever allow consistency. However it should not be an either/or situation. A modern enterprise data warehouse should be perfectly capable of producing useful data marts on an as-needed basis. The data warehouse cannot afford to sit in glorious isolation or it will fall into disuse, and you will end up full circle with numerous disconnected data marts arising to get around its failings.
An important point not made in the article is that in really large organizations you may not be able to get away with just one warehouse. A company with many country operations may want to have a summary global warehouse as well as one per country. Each of these will have dependent data marts. The “local” data warehouses can feed up summary data to the global one. Indeed for truly huge companies this may be the only practical solution. Examples of such an architecture can be found at Shell, BP, Unilever and others. A major advantage of such a federated approach is that political control of the local warehouse remains within the subsidiaries, which avoids “them v us” issues where the local operating units resist something being imposed on them by central office. In this scenario the local subsidiary get a warehouse that suits their needs, and the central office gets its summary information as a side-effect. Such an approach is technically more complicate, but if you use a technology that is capable of being federated (which these do) then the overhead is not great, but you have the key advantage of retaining global v local flexibility.