This is the last part in my series of things I wished I knew about before starting a Microsoft BI project. I’ll be taking my summer vacation now so the blog will be quiet the next month. After the break I will revise a couple of the tips based on feedback so stay tuned.
#26: Decide how to source your data in Analysis Services and stick with it.
Ideally you will source your data from a correctly modeled star schema. Even then you may need to massage the source data before feeding it into SSAS. There are two ways of accomplishing this: Through views in the database or through data source views (dimensional) or queries (tabular). Unless you are unable to create views in your database (running on a prod system etc) I would strongly suggest using them. This will give you a clean separation of logic and abstraction between the SSAS solution and the data source. This means that clients connecting to the data warehouse directly will see the same data model as the SSAS solution. Also migrating between different front-ends (like dimensional and tabular) will become much simpler. In my solutions I never connect to tables directly I always bind to views for everything and never implement any logic in the DSV or via queries.
#27: Have some way of defining “current” time periods in your SSAS solution
Most SSAS solutions have a time dimension with dates, months, years, etc. In many ways its the most important dimension in your solution as it will be included in most reports / analyses as well as form the basis for a lot of calculations (see previous tips). Having a notion of what is the current period in your time dimension will greatly improve the usability of your solution: Reports will automatically be populated with the latest data without any user interaction. It can also simplify ad-hoc analysis by setting the default members to the most current date / month / year so that when users do not put these on one of the axes it will default to the most recent time period. There are a number of ways of implementing this including calculated members and named sets (for dimensional) and calculations for Tabular and the internet is abundant with sample solutions. Some of them are fully automated (using VBA time functions) and some require someone to manually set the current period. I prefer to use the latter if possible to avoid reports showing incorrect data if something went wrong in the ETL.
#28: Create a testable solution
This is a really big topic so I will emphasize what I have found most important. A BI solution has a lot of moving parts. You have your various source systems, your ETL pipeline, logic in the database, logic in your SSAS solution and finally logic in your reporting solution. Errors happen in all of these layers but your integration services solution is probably the most vulnerable part. Not only do technically errors occur, but far more costly are logic errors where your numbers don’t match what is expected. Luckily there are a lot of things you can do to help identify when these errors occur. As mentioned in tips #6 and #7 you should use a framework. You should also design your solution to be unit testable. This boils down to creating lots of small packages that can be run in isolation rather than large complex ones. Most importantly you should create validation queries that compares the data you load in your ETL with data in the source systems. How these queries are crafted varies from system to system but a good starting point would be comparisons of row counts, sums of measures (facts) and number of unique values. The way I do it is that I create the test before building anything. So if I am to load customers that have changed since X, I first create the test query for the source system (row counts, distinct values etc.) then the query for the data warehouse together with a comparison query and finally I start building the actual integration. Ideally you will package this into a SSIS solution that logs the results into a table. This way you can utilize your validation logic both while developing the solution but also once its deployed. If you are running SQL Server 2012 you might want to look into the data tap features of SSIS that lets you inspect data flowing through your pipeline from the outside.
#29: Avoid the source if you are scaling for a large number of users
Building a BI solution to scale is another very large topic. If you have lots of data you need to scale your ETL, Database and SSAS subsystems. But if you have lots of users (thousands) your bottleneck will probably be SSAS. Concurrently handling tens to hundreds of queries with acceptable performance is just not feasible. The most effective thing is to avoid this as much as possible. I usually take a two pronged approach. Firstly I implement as much as possible as standard (“canned”) reports that can be cached. Reporting Services really shines in these scenarios. It allows for flexible caching schemes that in most circumstances eliminates all trips to the data source. This will usually cover around 70-80% of requirements. Secondly I deploy an ad-hoc cube specifically designed and tuned for exploratory reporting and analysis. I talked about this in tip #17. In addition you need to consider your underlying infrastructure. Both SSRS and SSAS can be scaled up and out. For really large systems you will need to do both, even with the best of caching schemes.
#30: Stick with your naming standards
There are a lot objects that need to be named in a solution. From the more technical objects such as database tables and SSIS packages to objects exposed to users such as SSAS dimensions and measures. The most important thing with naming conventions is not what they are, but that they are implemented. As I talked about in tip #24 changing a name can have far reaching consequences. This is not just a matter of things breaking if you change them but consider all of the support functionality in the platform such as logging that utilize object names. Having meaningful, consistent names will make it a heck of a lot easier to get value out of this. So at the start of the project I would advise to have a “naming meeting” where you agree upon how you will name your objects. Should dimension tables be prefixed with Dim or Dim_? Should Dimension names be plural (CustomerS) or singular (Customer), etc.