Parallel installs enable you to run multiple instances of the same snap on the same system. Each instance is completely isolated from all other instances, including its name, configuration, interface connections, data locations, services, applications and aliases.
This feature is currently considered experimental. In particular, snap install will fail if the same snap is already installed. See below for a workaround and see Current limitations for more details. As a result, to experiment with parallel installs, an experimental feature-flag must first be enabled:
$ sudo snap set system experimental.parallel-instances=true
The process of installing multiple instances of a snap is identical to installing a single instance except you must provide a unique identifier, called an instance key, for every additional snap instance you install:
$ sudo snap install hello-world hello-world_foo hello-world 6.3 from Canonical✓ installed hello-world_foo 6.3 from Canonical✓ installed
When installing from snap file, the instance key is set by passing
$ sudo snap install --name hello-world_foo hello-world_27.snap
The instance key needs to be manually appended to the snap name, and takes the following format:
For example, the following are valid instance names, for example:
Only lowercase letters or digits are valid, and the instance name can be up to 10 characters long. After being set during the installation of a snap, an instance name cannot be changed.
When working with instances, the vast majority of snap commands function just as they would with a single snap. To remove an instance, for example, use remove:
$ snap remove hello-world_foo hello-world_foo removed
Interfaces work across multiple snap instances just as they do from any one snap to another. For example, xkcd-webserver includes a network plug, as will all of its instances, any of which can be connected to the system’s :network slot:
$ snap connections xkcd-webserver_foo Interface Plug Slot Notes network xkcd-webserver_foo:network :network - network-bind xkcd-webserver_foo:network-bind :network-bind - $ sudo snap disconnect xkcd-webserver_foo:network $ snap connections xkcd-webserver_foo Interface Plug Slot Notes network xkcd-webserver_foo:network - - network-bind xkcd-webserver_foo:network-bind :network-bind - $ sudo snap connect xkcd-webserver_foo:network $ snap connections xkcd-webserver_foo Interface Plug Slot Notes network xkcd-webserver_foo:network :network manual network-bind xkcd-webserver_foo:network-bind :network-bind -
As with Interfaces, Services function the same with multiple instances of a snap as they do from any one snap to another.
However, with multiple instances, you’re more likely to run into port allocation issues, such as two web servers needing access to port 80. In such cases, only the service from one instance will be active.
$ snap services Service Startup Current xkcd-webserver.xkcd-webserver enabled active xkcd-webserver_foo.xkcd-webserver enabled inactive
You can see why xkcd-webserver_foo.xkcd-webserver is inactive by looking at its logs:
$ sudo snap logs xkcd-webserver_foo.xkcd-webserver 2018-10-03T12:31:59Z xkcd-webserver_foo.xkcd-webserver: OSError: [Errno 98] Address already in use (...)
As with single snap service collisions, the solution is to stop the service on one instance and start the service on the other:
$ sudo snap stop xkcd-webserver.xkcd-webserver Stopped. $ sudo snap start xkcd-webserver_foo.xkcd-webserver Started. $ snap services Service Startup Current xkcd-webserver.xkcd-webserver enabled inactive xkcd-webserver_foo.xkcd-webserver enabled active
Snap application names for multiple instances are adjusted according to the following pattern:
When an application name matches the snap name, a short alias is created to match the
For example, with hello-world and hello-world_foo installed, hello-world.env has an alias of hello-world_foo.env in the _foo instance:
$ hello-world.env |grep SNAP_INSTANCE_NAME SNAP_INSTANCE_NAME=hello-world $ hello-world_foo.env |grep SNAP_INSTANCE_NAME SNAP_INSTANCE_NAME=hello-world_foo
As with regular snaps, aliases can be added separately:
$ sudo snap alias hello-world_bar bar_env Added: - hello-world_bar as bar_env $ sudo snap alias hello-world_foo foo_env Added: - hello-world_foo as foo_env $ snap aliases Command Alias Notes hello-world_bar bar_env manual hello-world_foo foo_env manual
Aliases from instances generate conflict errors, just as they would with distinct snaps:
$ sudo snap alias hello-world_foo bar_env error: cannot perform the following tasks: - Setup manual alias "bar_env" => "hello-world" for snap "hello-world_foo" (cannot enable alias "bar_env" for "hello-world_foo", already enabled for "hello-world_bar")
When a snap application is run, its environment is populated with a number of
SNAP* environment variables.
The following environmental variables are potentially affected when installing multiple instances of a snap:
For example, if hello-world and its instance, hello-world_foo, are installed, the following name variables would be set:
The data and mount points of parallel installed snaps are kept separate on the host filesystem. Assuming the snap mount directory is
/snap, the following locations are used:
Within the mount namespace of a snap, adjustments are made to map instance-specific locations to the snap locations. Taking hello-world as an example, system data locations are set to the following:
However, due to security concerns, user data locations are not similarly adjusted:
There are a few limitations with parallel installs that will be addressed in subsequent
Installing multiple instances of a snap using classic confinement is not currently supported, although this is a planned feature for the future.
As described in Snap environment, data and namespace, the user data locations are kept separate across multiple instances. Applications that hard code paths to either user data directories or the XDG runtime directory may not function correctly as the AppArmor profile has been updated to allow accessing instance specific paths only.
Applications need to use
SNAP_USER_COMMON or fallback to using
Applications built using most popular frameworks, such as Gtk/glib or Qt, and that are already capable of observing both
XDG_RUNTIME_DIR, should continue to work.
Snaps may provide APIs consumed by other snaps or the host system at some well known locations or addresses, such as DBus service names, /dev/shm objects or semaphores, abstract socket addresses. While the default AppArmor template has been updated to allow only instance specific access, connecting interfaces may allow a wider access capabilities that can lead to conflicts between instances of the same snap.
When snapd installs a snap with services, the services are automatically started. Services trying to bind to the same port may fail or function incorrectly. Such snaps should deliver means for configuration of the service via configure hooks.
A demo snap https://github.com/bboozzoo/parallel-installs-demo provides an example of reconfiguration via configure hooks.
Services exporting the API on DBus under a well known name may conflict with other instances of the same snap. Those may require fixes from application or snap developers.
The default AppArmor profile allows instance specific access only. Applications hardcoding paths can break. Note that some interfaces such as
mir allow a wider access to
/dev/shm. Such snap would need to be updated to made instance aware.
Snaps using socket activation will collide with each other when parallel installed. The socket unit file may need manual adjustment.
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