The content interface is a way for two or more snaps to share something. Sharing happens at the filesystem level so anything that can be expressed as a file can be shared. This includes executables, libraries, data files but also sockets. Let’s look at how this works in a few simple examples. At a very basic level, one directory, file or socket can be made to appear in a place where another snap can access it.
Examples
All of the examples below involve two snaps. One snap is offering some content (using a content slot) and the other snap is consuming that content (using a content plug). Let’s call those snaps producer and consumer.
In all of the cases we see a small set of attributes defined on the particular interface. The producer declares which path can be read (using the read attribute) or written (using write). The consumer uses the target attribute to define where the content should show up at runtime. Both interfaces use the content attribute to describe the content. The content attribute must match on both sides for the connection to happen. Lastly you may notice that both read and write attributes use a list of paths. As of snapd 2.17 only one element may be listed there but this is a place for future expansion.
Sharing read-only snap content
Sharing an executable
producer/snapcraft.yaml
slots:
content:
content: executables
read:
- $SNAP/bin
consumer/snapcraft.yaml
plugs:
content:
content: executables
target: $SNAP/extra-bin
When the two interfaces are connected the consumer snap can invoke executables from $SNAP/extra-bin. The directory can be added to PATH in the wrapper script if desired. The directory can be inspected by any applications that wish to check if the extra executables are available (they can then fail gracefully).
Sharing a C-level library
producer/snapcraft.yaml
slots:
lib0-1604:
content: lib0-1604
read:
- $SNAP/lib
consumer/snapcraft.yaml
plugs:
lib0-1604:
content: lib0-1604
target: $SNAP/extra-libs
When the two interfaces are connected the consumer snap can link to libraries from $SNAP/extra-libs. The directory can be added to LD_LIBRARY_PATH in the wrapper script if desired. While the name of the interface and the value for the content attribute can be anything, it is good practice (and required when sharing content between snap publishers) to follow the form nameAPI-BUILDENV as a reminder to consumers of the API level and build tools used. In the above, the API ‘0’ is used and ‘1604’ is used to denote it was built using the Ubuntu 16.04 LTS toolchain and libraries. API and BUILDENV can be anything that is meaningful to the provider and consumers. For example, the gnome content snap uses gnome-3-26-1604 to denote the full GNOME 3.26 platform libraries and supporting files built on Ubuntu 16.04 LTS.
Sharing writable data
Sharing writable data can be used to share data files or … UNIX sockets for a simple form of IPC among a group of snaps. In subsequent releases of snapd it may become easier to share a subdirectory that is automatically created but this is not supported yet. It is recommended to share all of $SNAP_DATA or $SNAP_COMMON for now.
Sharing writable files
Note that this only works since snapd 2.19.1
producer/snapcraft.yaml
slots:
content:
content: writable-data
write:
- $SNAP_DATA
consumer/snapcraft.yaml
plugs:
content:
content: writable-data
target: $SNAP_DATA
Sharing UNIX sockets
Note that this only works since snapd 2.19.1
TBD: Links to example snaps using this.
producer/snapcraft.yaml
slots:
content:
content: socket-directory
write:
- $SNAP_DATA
consumer/snapcraft.yaml
plugs:
content:
content: socket-directory
target: $SNAP_DATA
When the two interfaces are connected the consumer snap can see the socket in $SNAP_DATA.
Technical background
The content interface is implemented with an interplay of two systems: apparmor and bind mounts. The apparmor sandbox by default allows writes to $SNAP_DATA and reads from $SNAP ([[overview of environment variables|Environment Variables]]). We can take advantage of this by putting content from other location and make it appear to show up in $SNAP or $SNAP_DATA. We can now take data from one snap’s $SNAP (e.g. from /snap/my-snap/1234/content and bind mount it to an empty directory in /snap/my-other-snap/4321/incoming-content). The same can be done for particular files if desired but it requires a pair of interfaces for each file and is more cumbersome.
