Create edge_cases.md

I meant to publish this at Node Interactive, but just got around to it now.

Author: coreybutler

edge_cases.md

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+#Use Cases
+
+If you created or are maintaining a version manager, please help identify edge/use cases you've encountered.
+
+## Redeployment 
+(by @coreybutler)
+
+In some environments (government, defense, some corporations), the developer environment is locked down. 
+To facilitate this, many organizations utilize Active Directory as a source of supervised software installations.
+
+- Difficult/impossible to track usage.
+- Silent installers are necessary (must be modifiable by the organization)
+- Alternative feed of version availability is necessary to prevent confusion.
+
+## Offline Access Use Case 
+(by @coreybutler)
+
+A state education agency managed technical infrastructure for all school districts, many of which are in rural/remote areas. 
+Some of them have no internet access, and many have very strained bandwidth, yet they want/need to use newer versions of Node. 
+They need a local mirror of Node executables, as well as npm mirrors (optional, but mostly impractical to not have this).
+
+- Difficult/impossible to track usage.
+- Completely different update method (manually copy resources to a mirror from a USB drive or other physically transportable media)
+- A feed of available versions needs to be accessible in a LAN.
+
+## Roaming Profiles & npm Bloat 
+(by @coreybutler)
+
+A company uses roaming profiles,  expecting their node environments to roam with them. Global `node_modules` are a part of this. 
+Due to the number of dependencies, the `node_modules` directory was enormous, and “almost redundant”, meaning many versions of
+the same module with minor variations. This caused the user a 30 minute bootup time while waiting for ALL node environments to copy
+from the server to a new desktop.
+
+- Horrible user experience
+
+## Architecture Awareness 
+(by @coreybutler)
+
+Some native dependencies require different versions of libraries depending on the architecture (32 vs 64 bit). Users need a
+`node_modules` directory specific to a version of node and it’s architecture.
+
+- 2 global node_modules directories per Node.js version if supporting both 32/64-bit (double the already large footprint).
+
+## Yarn: Missing Information 
+(by @coreybutler)
+
+When yarn was released, it was looking for a specific Windows registry key to determine whether Node was installed on the
+system. Different version managers approach the registry differently. Since there is no spec answering “how to determine
+if Node is installed?”, version managers have provided alternative solutions.
+
+- Lacking an “authoritative” spec stifles integration.
+
+## Confused Community 
+(by @coreybutler)
+
+Many people do not understand the differences between version managers. The yarn team [didn’t realize](https://github.com/yarnpkg/yarn/issues/626#issuecomment-252993114)
+people were using certain version managers at all. At NINA'16, @williamkapke mentioned that he just assumed nvm and n were the same thing. 
+Many people confuse nvm and nvm-windows as being the same functional thing for different operating systems.
+
+- Insufficient and/or unclear public messaging.
+
+_NOTE:_ @marcelklehr put together a concise [list of usage types](https://github.com/nodejs/version-management/issues/4#issuecomment-258567834) that would
+make a good starting point to educate the community about the options (or at least make them aware they have a choice).
+
+## Permission Problems (Shims) 
+(by @coreybutler)
+
+Using a shim changes the execution context of the node process (I troubleshoot this all the time in [node-windows](https://github.com/coreybutler/node-windows)).
+A shim may run under a specific user (root, elevated admin), but it treats node as a child process, which (unless specified) runs 
+under the default user context. In other words, it masks the user context.
+
+- Symlinks execute node “directly”, guaranteeing the user context.
+- Shims can potentially change the value of `__dirname, process.cwd()`, and user.
+- Symlinks on Windows can be hard or soft. This choice effects whether it will work across multiple hard drives on a computer.