SSAS Compare: an intern’s journey

About a month ago, David mentioned an intern working in the BI Tools Team. That intern happens to be me! In five weeks’ time, I’ll start my second year of Computer Science at the University of Cambridge and be a full-time student again, but for the past eight weeks, I’ve been living a completely different life.

As Jon mentioned before, the teams here at Red Gate are small and everyone (including the interns!) is responsible for the product as a whole. I’ve attended planning sessions, UX tests, daily meetings, and everything else a full-time member of the team would; I had as much say in where we would go next with the product as anyone; I was able to see that what I was doing was an important part of the product from the feedback we got in the UX tests. All these things almost made me forget that this is just an internship and not my full-time job.

First steps at Red Gate

Being based in Cambridge, Red Gate has many Cambridge university graduates working for them. They also hire some Cambridge undergraduates for internships each summer. With its popularity with university graduates and its great working environment, Red Gate has managed to build up a great reputation. When I thought of doing an internship here in Cambridge, Red Gate just seemed to be the obvious choice for my first real work experience.

On my first day at Red Gate, David, the lead developer for SSAS Compare, helped me settle in and explained what I’d be doing. My task was to improve the user experience of displaying differences between MDX scripts by syntax highlighting, script formatting, and improving the difference identification in the first place. David suggested how I should approach the problem, but left all the details and design decisions to me. That was when I realised how much independence and responsibility I’d have.

What I’ve done

If you launch the latest version of SSAS Compare and drill down to an MDX script difference, you can see the changes that have been made. In earlier versions, you could only see the scripts in plain text on both sides – either in black or grey, depending on whether they were the same or not. However, you couldn’t see exactly where the scripts were different, which was especially annoying when the two scripts were large – as they often are. Furthermore, if parts of the two scripts were formatted differently, they seemed to be different but were actually the same, which caused even more confusion and made it difficult to see where the differences were.

All these issues have been fixed now. The two scripts are automatically formatted by the tool so that if two things are syntactically equivalent, they look the same – including case differences in keywords! The actual difference is highlighted in grey, which makes them easy to spot. The difference identification has been improved as well, so two scripts aren’t identified as different if there’s just a difference in meaningless whitespace characters, or when you have “select” on one side and “SELECT” on the other. We also have syntax highlighting, which makes it easier to read the scripts.

How I did it

In order to do the formatting properly, we decided to parse the MDX scripts. After some investigation into parser builders, I decided to go with the GOLD Parser builder and the bsn-goldparser .NET engine.

GOLD Parser builder provides a fairly nice GUI to write, build, and test grammar in. We also liked the idea of separating the grammar building from parsing a text. The bsn-goldparser is one of many .NET engines for GOLD, and although it doesn’t support the newest features of GOLD Parser, it has “the ability to map semantic action classes to terminals or reduction rules, so that a completely functional semantic AST can be created directly without intermediate token AST representation, and without the need for glue code.” That makes it much easier for us to change the implementation in our program when we change the grammar. As bsn-goldparser is open source, and I wanted some more features in it, I contributed two new features which have now been merged to the project.

Unfortunately, there wasn’t an MDX grammar written for GOLD already, so I had to write it myself. I was referencing MSDN to get the formal grammar specification, but the specification was all over the place, so it wasn’t that easy to implement and find. We’re aware that we don’t yet fully support all valid MDX, so sometimes you’ll just see the MDX script difference displayed the old way. In that case, there is some grammar construct we don’t yet recognise. If you come across something SSAS Compare doesn’t recognise, we’d love to hear about it so we can add it to our grammar.

When some MDX script gets parsed, a tree is produced. That tree can then be processed into a list of inlines which deal with the correct formatting and can be outputted to the screen.

Doing all this has led me to many new technologies and projects I haven’t worked with before. This was my first experience with C# and Visual Studio, although I have done things in Java before. I have learnt how to unit test with NUnit, how to do dependency injection with Ninject, how to source-control code with SVN and Mercurial, how to build with TeamCity, how to use GOLD, and many other things.

What’s coming next

Sadly, my internship comes to an end this week, so there will be less development on MDX difference view for a while. But the team is going to work on marking the differences better and making it consistent with difference indication in the top part of comparison window, and will keep adding support for more MDX grammar so you can see the differences easily in every comparison you make.

So long! And maybe I’ll see you next summer!