What are the limitations/benefits on using MSM instead of MSI?

Question

I'm currently building a product distributed through MSI Windows Installer. That product is being integrated by our customers using different forms such us inside their own MSI, using bootstrapper/chainner like WiX Burn or authoring tools like InstallShield.

Having this scenario in mind, I always wanted to know what are the limitations and/or benefits to use Merge Modules (MSM) instead of keeping an MSI, and also what are the nowadays recommended approach about choosing one against the other.

Answer 1

On paper merge modules are fine, but in the real world I find them clunky to update and hence error prone since they may be merged into many setups before being discovered to be defective. As a result I do not recommend merge modules at all. I prefer a single MSI that can be run as a batch process via a bootstrapper or batch file and that can also be updated easily. This avoids all kinds of problems that are not generally intuitive.

I want to add that merge modules work well for truly shared files installed in locations in the file system that are meant for shared files and that change infrequently. These are generally OS-runtimes. These merge modules are generally heavily tested and work ok. However, often I see people use merge modules for files that end up changing frequently and that they then end up installing in different locations in different flavors in an ad-hoc fashion. This kind of use is a total mess and a hugely wasted effort.

Having said all that - I have indeed used merge modules successfully when I have needed advanced release management with repetitive, non-changing inclusion of a set of files via a merge module into several setups. Even then I ran into a version issue after a while with a couple of files needing update, and subsequent, minor errors with the wrong merge module being used when I left the project to someone else. I also experienced having to rebuild all setups due to a minor merge module bug fix. All setups then had to go through QA again. Very frustrating with such tight coupling.

If your requirements are simple and you are not taking on a huge multi-product release project sharing a bunch of files, use MSI instead of MSM. Easier to comprehend, generally less work to deal with, more atomic updates and less risk of introducing the same error in many setups due to merge module update or design problems.

Answer 2

There's nothing that wrong with merge modules. Their primary use (which hasn't been mentioned) is sharing. If you want the same set of shared files in multiple MSI files, they need the same set of component guids to preserve the sharing rules. Or if you are giving files to clients for them to use (like Microsoft) in their MSI builds then give them merge modules. That's one of the reasons MS and other vendors redistribute merge modules so that everyone can build their MSIs and install them on the same system without file sharing disasters. I've also seen merge modules used as a common UI for MSI files. But mainly they are essential to make sure that shared files are used correctly. I'll tell you from experience that the disaster resulting from incorrect use of shared files is much worse that any perceived difficulty with using merge modules. Note also that they are universal and can be included in all tools that build MSI files.

I've never found merge modules difficult to patch, version, or fix. Major upgrades aren't a problem. The only potential issue I've seen is with build processes that rebuild all the binaries in a merge module during creation of a patch (.msp) build. If only one binary needs a fix but you compile them all, their versions and internals may change enough so that the patch process (the delta between two MSI files and their content) will tell you that they need to be included in the patch because they've changed, but this issue can be avoided if it is in fact an issue.