Merge pull request #672 from cgwalters/doc-package-managers

docs: Add new package-managers section with guidance

docs/src/SUMMARY.md

@@ -46,6 +46,7 @@
 
 # More information
 
+- [Package manager integration](package-managers.md)
 - [Relationship with other projects](relationships.md)
 - [Relationship with OCI artifacs](relationship-oci-artifacts.md)
 - [Relationship with systemd "particles"](relationship-particles.md)

docs/src/package-managers.md

@@ -0,0 +1,109 @@
+# Package manager integration
+
+A toplevel goal of bootc is to encourage a default model
+where Linux systems are built and delivered as (container) images.
+In this model, the default usage of package managers such as `apt` and `dnf`
+will be at container build time.
+
+However, one may end up shipping the package manager tooling onto
+the end system. In some cases this may be desirable even, to allow
+workflows with transient overlays using e.g. `bootc usroverlay`.
+
+## Detecting image-based systems
+
+bootc is not the only image based system; there are many. A common
+emphasis is on having the operating system content in `/usr`,
+and for that filesystem to be mounted read-only at runtime.
+
+A first recommendation here is that package managers should
+detect if `/usr` is read-only, and provide a useful error
+message referring users to documentation guidance.
+
+An example of a non-bootc case is "Live CD" environments,
+where the *physical media* is readonly. Some Live operating system environments end 
+up mounting a transient writable overlay (whether via e.g. devicemapper or overlayfs)
+that make the system appear writable, but it's arguably clearer not to do so by
+default. Detecting `/usr` as read-only here and providing the same information
+would make sense.
+
+### Running a read-only system via podman/docker
+
+The historical default for docker (inherited into podman) is that
+the `/` is a writable (but transient) overlayfs. However, e.g. `podman`
+supports a `--read-only` flag, and [Kubernetes pods](https://kubernetes.io/docs/reference/kubernetes-api/workload-resources/pod-v1/) offer a
+`securityContext.readOnlyRootFilesystem` flag.
+
+Running containers in production in this way is a good idea,
+for exactly the same reasons that bootc defaults to mounting
+the system read-only.
+
+Ensure that your package manager offers a useful error message
+in this mode. Today for example:
+
+```
+$ podman run --read-only --rm -ti debian apt update
+Reading package lists... Done
+E: List directory /var/lib/apt/lists/partial is missing. - Acquire (30: Read-only file system)
+$ podman run --read-only --rm -ti quay.io/fedora/fedora:40 dnf -y install strace
+Config error: [Errno 30] Read-only file system: '/var/log/dnf.log': '/var/log/dnf.log'
+```
+
+However note that both of these fail on `/var` being read-only; in a default bootc
+model, it won't be. A more accurate check is thus closer to:
+
+```
+$ podman run --read-only --rm -ti --tmpfs /var quay.io/fedora/fedora:40 dnf -y install strace
+...
+Error: Transaction test error:
+  installing package strace-6.9-1.fc40.x86_64 needs 2MB more space on the / filesystem
+```
+
+```
+$ podman run --read-only --rm --tmpfs /var -ti debian /bin/sh -c 'apt update && apt -y install strace'
+...
+dpkg: error processing archive /var/cache/apt/archives/libunwind8_1.6.2-3_amd64.deb (--unpack):
+ unable to clean up mess surrounding './usr/lib/x86_64-linux-gnu/libunwind-coredump.so.0.0.0' before installing another version: Read-only file system
+```
+
+These errors message are misleading and confusing for the user. A more useful error may look like e.g.:
+
+```
+$ podman run --read-only --rm --tmpfs /var -ti debian /bin/sh -c 'apt update && apt -y install strace'
+error: read-only /usr detected, refusing to operate. See `man apt-image-based` for more information.
+```
+
+### Detecting bootc specifically
+
+You may also reasonably want to detect that the operating system is specifically
+using `bootc`. This can be done via e.g.:
+
+`bootc status --format=json  | jq -r .spec.image`
+
+If the output of that field is non-`null`, then the system is a bootc system
+tracking the specified image.
+
+## Transient overlays
+
+Today there is a simple `bootc usroverlay` command that adds a transient writable overlayfs for `/usr`.
+This makes many package manager operations work; conceptually it is similar
+to the writable overlay that many "Live CDs" use. However, one cannot change the kernel
+this way for example.
+
+An optional integration that package managers can do is to detect this transient overlay
+situation and inform the user that the changes will be ephemeral.
+
+## Persistent changes
+
+A bootc system by default *does* have a writable, persistent data store that holds
+multiple container image versions (more in [filesystem](filesystem.md)).
+
+Systems such as [rpm-ostree](https://github.com/coreos/rpm-ostree/) implement
+a "hybrid" mechanism where packages can be persistently layered and re-applied;
+the system effectively does a "local build", unioning the intermediate filesystems.
+
+One aspect of how rpm-ostree implements this is by caching individual unpacked RPMs as ostree commits
+in the ostree repo.
+
+This section will be expanded later; you may also be able to find more information in [booting local builds](booting-local-builds.md).
+
+