I'm having trouble referring you to documentation, which means I either haven't had my coffee yet (true) or we're missing some documentation (update: some documentation here)
When you declare apps in your
snapcraft.yaml, it results in a binary wrapper being generated upon install and placed into
/snap/bin/, named after your package and app name (note that if the app is a service, this wrapper is instead a systemd .service file).
That wrapper contains most of the environment under which the application will run. The two environment variables that are most relevant to this question are
SNAP_DATA is a system-wide writable area (in
/var/snap/). This might be used to host logs for services, for instance.
SNAP_USER_DATA is a user-specific writable area in the home directory of the user running the application (specifically
/home/<user>/snap/). This might be used for user-specific configuration files, etc.
Both of these directories are very important to the upgrade/rollback functionality, since both of them are versioned. That is, each version of a given snap has its own copy of these directories. Let me explain with an example.
Say you install version 1 of the "foo" snap. That will create two directories:
Now say "foo" uses both of these. Maybe it has a service that hosts a database in
SNAP_DATA, and a binary that uses config files in
Now version 2 of "foo" is released, and it's automatically updated. The first thing that happens is that
/var/snap/foo/1 is copied into
/home/<user>/snap/foo/1 is copied into
/home/<user>/snap/foo/2. Then the new version is fired up. It should notice that it's running on old data, and maybe it has some database migrations to run to the database in
SNAP_DATA. It does that, and away it goes.
Now say those migrations fail for whatever reason, and this application needs to be rolled back. It begins using the old version of the /snap/foo application, where
SNAP_DATA was pointing to
SNAP_USER_DATA was pointing to
/home/<user>/snap/foo/1. This picks things up on the old version at the point before the migrations were run, since those operations were run on a copy of the data.
Long story short: don't use the
home interface to store data you can be storing in
SNAP_USER_DATA, since they're an integral part of the upgrade/rollback strategy. Take advantage of them!
UPDATE for v2.0.10:
Two new data directories were also introduced:
SNAP_COMMON sits alongside
SNAP_DATA, but is specifically unversioned. Every revision of the specific snap has access to this directory, so it's not copied upon upgrade/rollback etc. This might be used for particularly large, unversioned files (e.g. raw data that isn't really version-specific).
SNAP_USER_COMMON sits alongside
SNAP_USER_DATA, but is again specifically unversioned. It might be used for storing non-version-specific data per user.
UPDATE for v2.15:
The files placed within
/snap/bin are no longer wrappers that define the environment, but symlinks to
/usr/bin/snap. So the way to determine the environment under which an application runs would be to use
snap run --shell <snap>.<app>, for example:
$ sudo snap install hello-world
$ snap run --shell hello-world
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.
$ env | grep SNAP