Update caching documentation

doc/ci/caching/index.md

@@ -23,61 +23,55 @@ how it is defined in `.gitlab-ci.yml`.
 
 NOTE: **Note:**
 Be careful if you use cache and artifacts to store the same path in your jobs
-as **caches are restored before artifacts** and the content would be overwritten.
-
-Don't mix the caching with passing artifacts between stages. Caching is not
-designed to pass artifacts between stages. Cache is for runtime dependencies
-needed to compile the project:
-
-- `cache`: **Use for temporary storage for project dependencies.** Not useful
-  for keeping intermediate build results, like `jar` or `apk` files.
-  Cache was designed to be used to speed up invocations of subsequent runs of a
-  given job, by keeping things like dependencies (e.g., npm packages, Go vendor
-  packages, etc.) so they don't have to be re-fetched from the public internet.
-  While the cache can be abused to pass intermediate build results between
-  stages, there may be cases where artifacts are a better fit.
+as **caches are restored before artifacts** and the content could be overwritten.
+
+Don't use caching for passing artifacts between stages, as it is designed to store
+runtime dependencies needed to compile the project:
+
+- `cache`: **For storing project dependencies**
+
+  Caches are used to speed up runs of a given job in **subsequent pipelines**, by
+  storing downloaded dependencies so that they don't have to be fetched from the
+  internet again (like npm packages, Go vendor packages, etc.) While the cache could
+  be configured to pass intermediate build results between stages, this should be
+  done with artifacts instead.
+
 - `artifacts`: **Use for stage results that will be passed between stages.**
-  Artifacts were designed to upload some compiled/generated bits of the build,
-  and they can be fetched by any number of concurrent Runners. They are
-  guaranteed to be available and are there to pass data between jobs. They are
-  also exposed to be downloaded from the UI. **Artifacts can only exist in
-  directories relative to the build directory** and specifying paths which don't
-  comply to this rule trigger an unintuitive and illogical error message (an
-  enhancement is discussed at
-  [https://gitlab.com/gitlab-org/gitlab-foss/issues/15530](https://gitlab.com/gitlab-org/gitlab-foss/issues/15530)
-  ). Artifacts need to be uploaded to the GitLab instance (not only the GitLab
-  runner) before the next stage job(s) can start, so you need to evaluate
-  carefully whether your bandwidth allows you to profit from parallelization
-  with stages and shared artifacts before investing time in changes to the
-  setup.
-
-It's sometimes confusing because the name artifact sounds like something that
-is only useful outside of the job, like for downloading a final image. But
-artifacts are also available in between stages within a pipeline. So if you
-build your application by downloading all the required modules, you might want
-to declare them as artifacts so that each subsequent stage can depend on them
-being there. There are some optimizations like declaring an
-[expiry time](../yaml/README.md#artifactsexpire_in) so you don't keep artifacts
-around too long, and using [dependencies](../yaml/README.md#dependencies) to
-control exactly where artifacts are passed around.
-
-In summary:
-
-- Caches are disabled if not defined globally or per job (using `cache:`).
-- Caches are available for all jobs in your `.gitlab-ci.yml` if enabled globally.
-- Caches can be used by subsequent pipelines of that same job (a script in
-  a stage) in which the cache was created (if not defined globally).
-- Caches are stored where the Runner is installed **and** uploaded to S3 if
-  [distributed cache is enabled](https://docs.gitlab.com/runner/configuration/autoscale.html#distributed-runners-caching).
-- Caches defined per job are only used, either:
-  - For the next pipeline of that job.
-  - If that same cache is also defined in a subsequent job of the same pipeline.
-- Artifacts are disabled if not defined per job (using `artifacts:`).
-- Artifacts can only be enabled per job, not globally.
-- Artifacts are created during a pipeline and can be used by the subsequent
-  jobs of that currently active pipeline.
-- Artifacts are always uploaded to GitLab (known as coordinator).
-- Artifacts can have an expiration value for controlling disk usage (30 days by default).
+
+  Artifacts are files generated by a job which are stored and uploaded, and can then
+  be fetched and used by jobs in later stages of the **same pipeline**. This data
+  will not be available in different pipelines, but is available to be downloaded
+  from the UI.
+
+The name `artifacts` sounds like it's only useful outside of the job, like for downloading
+a final image, but artifacts are also available in later stages within a pipeline.
+So if you build your application by downloading all the required modules, you might
+want to declare them as artifacts so that subsequent stages can use them. There are
+some optimizations like declaring an [expiry time](../yaml/README.md#artifactsexpire_in)
+so you don't keep artifacts around too long, or using [dependencies](../yaml/README.md#dependencies)
+to control which jobs fetch the artifacts.
+
+Caches:
+
+- Are disabled if not defined globally or per job (using `cache:`).
+- Are available for all jobs in your `.gitlab-ci.yml` if enabled globally.
+- Can be used in subsequent pipelines by the same job in which the cache was created (if not defined globally).
+- Are stored where the Runner is installed **and** uploaded to S3 if [distributed cache is enabled](https://docs.gitlab.com/runner/configuration/autoscale.html#distributed-runners-caching).
+- If defined per job, are used:
+  - By the same job in a subsequent pipeline.
+  - By subsequent jobs in the same pipeline, if the they have identical dependencies.
+
+Artifacts:
+
+- Are disabled if not defined per job (using `artifacts:`).
+- Can only be enabled per job, not globally.
+- Are created during a pipeline and can be used by the subsequent jobs of that currently active pipeline.
+- Are always uploaded to GitLab (known as coordinator).
+- Can have an expiration value for controlling disk usage (30 days by default).
+
+NOTE: **Note:**
+Both artifacts and caches define their paths relative to the project directory, and
+can't link to files outside it.
 
 ## Good caching practices
 

doc/ci/yaml/README.md

@@ -1514,8 +1514,10 @@ globally and all jobs will use that definition.
 
 #### `cache:paths`
 
-Use the `paths` directive to choose which files or directories will be cached. You can only specify paths within your `$CI_PROJECT_DIR`.
-Wildcards can be used that follow the [glob](https://en.wikipedia.org/wiki/Glob_(programming)) patterns and [filepath.Match](https://golang.org/pkg/path/filepath/#Match).
+Use the `paths` directive to choose which files or directories will be cached. Paths
+are relative to the project directory (`$CI_PROJECT_DIR`) and cannot directly link outside it.
+Wildcards can be used that follow the [glob](https://en.wikipedia.org/wiki/Glob_(programming))
+patterns and [filepath.Match](https://golang.org/pkg/path/filepath/#Match).
 
 Cache all files in `binaries` that end in `.apk` and the `.config` file:
 
@@ -1744,8 +1746,9 @@ be available for download in the GitLab UI.
 
 #### `artifacts:paths`
 
-You can only use paths that are within the local working copy.
-Wildcards can be used that follow the [glob](https://en.wikipedia.org/wiki/Glob_(programming)) patterns and [filepath.Match](https://golang.org/pkg/path/filepath/#Match).
+Paths are relative to the project directory (`$CI_PROJECT_DIR`) and cannot directly
+link outside it. Wildcards can be used that follow the [glob](https://en.wikipedia.org/wiki/Glob_(programming))
+patterns and [filepath.Match](https://golang.org/pkg/path/filepath/#Match).
 
 To restrict which jobs a specific job will fetch artifacts from, see [dependencies](#dependencies).