docs: Add apply-live

docs/apply-live.md

@@ -0,0 +1,72 @@
+---
+nav_order: 6
+---
+
+# Architecture of apply-live
+{: .no_toc }
+
+1. TOC
+{:toc}
+
+## Copying into an "underlay"
+
+As noted in the architecture doc, everything in rpm-ostree is oriented
+around creating and managing hardlinked complete bootable filesystem trees.
+
+In this flow then, `rpm-ostree install --apply-live strace` will first
+create a new pending deployment, run sanity tests on it, prepare it to be booted, etc.
+
+However, the first time `apply-live` is invoked, we create an `overlayfs`
+mount over `/usr`.  It's mounted `ro` from the perspective of the rest
+of the system, but rpm-ostree can write to it.
+
+## Package and filesystem diffs
+
+When `apply-live` is invoked, rpm-ostree computes the diff between
+the source and target OSTree commit for `/usr`.  If this is the *first* `apply-live`,
+the source commit is the booted commit.  For subsequent invocations,
+it will be based on the current live commit.
+
+We also compute a package-level diff; this is how `apply-live`
+currently distinguishes between pure package additions versus upgrades.
+
+## Copying data for /usr
+
+Per the core OSTree model, almost everything we care about is in `/usr`.
+So the first step is to apply the diff to the transient writable `overlayfs`.
+
+One downside is that that this diff will take extra memory and disk space
+proportional to its size.
+
+## Updating /etc
+
+The second aspect we need to take care of is `/etc`.  Normally, the libostree
+core handles the `/etc` merge during shutdown as part of `ostree-finalize-staged.service`,
+but we need to do it now in order to ensure that we get new config files
+(or remove ones).
+
+Note that the changes in `/etc` are persistent, live-applied changes there are
+also hence not updated transactionally.  It is hence possible for configuration
+files to "leak" from partially applied live updates.
+
+## Updating /var
+
+Normally, libostree core never touches `/var`.  Today rpm-ostree generates
+`systemd-tmpfiles` snippets for RPM packages which contain directories in
+`/var`.  In a regular update, these will hence be generated at boot
+time by `systemd-tmpfiles-setup.service`.
+
+But here, we need to do this live.  So rpm-ostree directly starts a
+transient systemd unit running `systemd-tmpfiles`.
+
+## Tracking live state
+
+Because the `overlayfs` is transient (goes away on reboot), the `apply-live`
+operation also writes its state into the transient `/run` directory, specifically
+a stamp file is stored at `/run/ostree/deployment-state/$deployid/`.
+
+Currently, there is also a persistent ostree ref `rpmostree/live-apply` for
+the current live commit.  Eventually the goal is that libostree itself would
+gain direct awareness of live apply, and we wouldn't write a persistent ref.
+
+