Add the option to not use a cache

Author: dguo

text/0000-no-cache-option.md

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+- Start Date: 2017-03-14
+- RFC PR:
+- Yarn Issue: [yarnpkg/yarn#986](https://github.com/yarnpkg/yarn/issues/986)
+
+# Summary
+
+Allow users to bypass the global cache.
+
+# Motivation
+
+For certain situations, it would be preferrable to not use a global cache for
+packages because the cache takes space and the step of copying from the cache
+to `node_modules` can take a significant amount of time. Some examples are:
+
+1. Building a Docker image. Images are isolated, so there's no point in having
+a cache within the image. If anything, it has the negative effect of increasing
+the image size.
+
+2. Running Yarn in a CI context. This is another example of a potentially
+isolated environment where we would just want to put the installed packages
+directly into `node_modules`. While sophisticated CI setups might have
+multiple projects share the same cache, there are certainly setups that don't
+do that.
+
+Overall, the desired effects of bypassing the cache are to increase the speed
+and decrease the disk usage of certain Yarn commands (i.e., install, add,
+upgrade).
+
+# Detailed design
+
+There should be a CLI switch `--no-cache` that tells Yarn to skip doing any work
+with the cache.
+
+Furthermore, there could be an equivalent `.yarnrc` setting of `no-cache` that
+does the same thing when set to `true`. This way, the developer doesn't need to
+remember to use the CLI switch every time.
+
+# How We Teach This
+
+We would update the [documentation](https://yarnpkg.com/en/docs/cli/cache) for
+the `yarn cache` command.
+
+# Drawbacks
+
+Increases the complexity of the implementation of Yarn.
+
+# Alternatives
+
+In the linked issue, there is a suggestion to set the cache folder to some
+in-memory or temporary location, but this solution is hacky and doesn't really
+help with the speed concern because Yarn still has to do the work of copying
+the packages to `node_modules`.
+
+The same reasoning applies to the option of immediately running
+`yarn cache clean`.
+
+# Unresolved questions