Keep your ovens clean

Let’s assume for a moment that you are a baker, producing different types of pastries in your small bakery. The production process is always the same: prepare the dough, put it in the oven, wait some time and retrieve the most delicious buns or bread. If we can abstract the real baking process to these steps, it’s the same as with software: prepare the sourcecode, put it in the compiler, wait some time and retrieve the most delicious binary or executable. There is only one difference: The oven of the baker is a self-contained, closed system, while our compilers require a distinct system setup around them in order to produce anything edible. The oven is independent from the kitchen around it, the compiler is depedent on the environment. To finish the analogy, what would a baker say if he can’t bake bread in his oven unless he nurtures a certain type of yeast in his kitchen?

A most unpleasant case

While developing a platform dependent application recently, we met a most unpleasant case of build dependency on a third-party library. It was an old dynamic link library (DLL) that requires registering in the windows registry. There was no other way than to register the DLL using the regsrv32 utility. If you didn’t do this, the build process would abort with an error stating unmet dependencies. If you ran the resulting program on a machine without registered DLL, it would crash with a runtime error complaining about the missing registry entry. And by the way, there are two totally independent regsrv32 utilities on a 64-bit windows system, one for 32-bit and one for 64-bit registrations. No, the name of the latter one isn’t regsvr64, that would be way too easy.

We accepted the fact that you need to prepare your system if you want to run the program, but we quarreled a lot with the nuisance that you need to alter your system just to build the software. This process of alteration is called snowflaking in the DevOp mentality and it’s not a desired activity. We would need to alter every build machine in our continuous integration cluster that comes into contact with the project. And we would need to de-snowflake them again afterwards, because this kind of tinkering adds up to inscrutable side-effects very fast.

A practicable workaround

We found a way around the abovementioned snowflaking for our build servers. It’s not a solution, it’s only a workaround, as it solves the immediate problem but produces some lesser problems on the way. Let’s look at what we did.

At first, our situation could be described with this module diagram:

dependency1We couldn’t modify the problematic DLL itself, it was a given binary. But we could wrap it in our own DLL. Wrapping less pleasant things into something you can control is a proven technique even in baking, by the way. We now had a system layout that looks like this:

dependency2Nothing gained so far, just that we now have a layer outside our system that can provide the functionality of the DLL and is actually under our control. The wrapper really does nothing on its own but to forward each call to the DLL. To profit from this indirection, we need to introduce another module, like this:

dependency3The second module provides the same interface as the first, but does nothing, not even forwarding anywhere. It’s a complete stub, just there to be uncomplicated during the build process. The goal is to build the system using this “empty” DLL and then replace it with the “problematic” DLL afterwards. The only question is: how do we build the problematic DLL? Here’s the workaround part of the solution: We actually had to compile the problematic DLL on a snowflaked system and add it to the project repository. Good thing our target system’s specification is known, so we only need to do this for one platform. Because we are reasonably sure that the DLL interface will not change over time (it had every opportunity in the last ten years and didn’t use it), we can assume that the interface of our two wrapper DLLs also won’t change. So it’s not too problematic to check in a precompiled binary that needs to satisfy an interface that’s reproduced with every build cycle. Still, we need to keep an eye on the method signatures of our two wrapper DLLs. If one of them changes, the modification needs to be replicated on the other wrapper, too. It’s a classic duplication.

When we balanced the duplication in the interfaces of the two wrapper DLLs against the snowflaking of every CI and developer machine, we found our aversion against snow outweighing the other negative aspects. Your mileage may vary.

Conclusion

We kept our build ovens clean by introducing a wrapping layer around the problematic depedency and then using the benefits of indirection by switching to a non-problematic stub during the build cycle. The technique is very old, but still use- and powerful.

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