Snappy Changes

As announced last Saturday, Snappy Ubuntu Core 2.0 has just been tagged and made its way into the archives of Ubuntu 16.04, which is due for the final release in the next days. So this is a nice time to start covering interesting aspects of what is being made available in this release.

A good choice for the first post in this series is talking about how snappy performs changes in the system, as that knowledge will be useful in observing and understanding what is going on in your snappy platform. Let’s start with the first operation you will likely do when first interacting with the snappy platform — install:

% sudo snap install ubuntu-calculator-app
120.01 MB / 120.01 MB [===============================] 100.00 % 1.45 MB/s

This operation is traditionally done on analogous systems in an ephemeral way. That is, the software has either a local or a remote database of options to install, and once the change is requested the platform of choice will start acting on it with all state for the modification kept in memory. If something doesn’t go so well, such as a reboot or even a crash, the modification is lost.. in the best case. Besides being completely lost, it might also be partially applied to the system, with some files spread through the filesystem, and perhaps some of the involved hooks run. After the restart, the partial state remains until some manual action is taken.

Snappy instead has an engine that tracks and controls such changes in a persistent manner. All the recent changes, pending or not, may be observed via the API and the command line:

% snap changes
ID   Status  ...  Summary
1    Done    ...  Install "ubuntu-calculator-app" snap

(the spawn and ready date/time columns have been hidden for space)

The output gives an overview of what happened recently in the system, whether pending or not. If one of these changes is unintendedly interrupted for whatever reason, the daemon will attempt to continue the requested change at the next opportunity.

Continuing is not always possible, though, because there are external factors that such a change will generally depend upon (the snap being available, the system state remaining similar, etc). In those cases, the change will fail, and any relevant modifications performed on the system while attempting to accomplish the defined goal will be undone.

Because such partial states are possible and need to be handled properly by the system, changes are in fact broken down into finer grained tasks which are also tracked and observable while in progress or after completion. Using the change ID obtained in the former command, we can get a better picture of what that changed involved:

% snap changes 1
Status ...  Summary
Done   ...  Download snap "ubuntu-core" from channel "stable"
Done   ...  Mount snap "ubuntu-core"
Done   ...  Copy snap "ubuntu-core" data
Done   ...  Setup snap "ubuntu-core" security profiles
Done   ...  Make snap "ubuntu-core" available
Done   ...  Download snap "ubuntu-calculator-app"
Done   ...  Mount snap "ubuntu-calculator-app"
Done   ...  Copy snap "ubuntu-calculator-app" data
Done   ...  Setup snap "ubuntu-calculator-app" security profiles
Done   ...  Make snap "ubuntu-calculator-app" available

(the spawn and ready date/time columns have been hidden for space)

Here we can observe an interesting implementation detail of the snappy integration into Ubuntu: the ubuntu-core snap is at the moment ~80MB, and contains the software bundled with the snappy platform itself. Instead of having it pre-installed, it’s only pulled in when the first snap is installed.

Another interesting implementation detail that surfaces here is the fact snaps are in fact mounted rather than copied into the system as traditional packaging systems do, and they’re mounted read-only. That means the operation of having the content of a snap in the filesystem is instantaneous and atomic, and so is removing it. There are no partial states for that specific aspect, and the content cannot be modified.

Coming back into the task list, we can see above that all the tasks that the change involved are ready and did succeed, as expected from the earlier output we had seen for the change itself. Being more of an introspection view, though, this tasks view will often also show logs and error messages for the individual tasks, whether in progress or not.

The following view presents a similar change but with an error due to an intentionally corrupted system state that snappy could not recover from (path got a busy mountpoint hacked in):

% sudo snap install xkcd-webserver
[\] Make snap "xkcd-webserver" available to the system
error: cannot perform the following tasks:
- Make snap "xkcd-webserver" available to the system
  (symlink 13 /snap/xkcd-webserver/current: file exists)

% sudo snap changes 2
Status  ...  Summary
Undone  ...  Download snap "xkcd-webserver" from channel "stable"
Undone  ...  Mount snap "xkcd-webserver"
Undone  ...  Copy snap "xkcd-webserver" data
Undone  ...  Setup snap "xkcd-webserver" security profiles
Error   ...  Make snap "xkcd-webserver" available to the system

Make snap "xkcd-webserver" available to the system

2016-04-20T14:14:30-03:00 ERROR symlink 13
    /snap/xkcd-webserver/current: file exists

Note how reassuring that report looks. It says exactly what went wrong, at which stage of the process, and it also points out that all the prior tasks that previously succeeded had their modifications undone. The security profiles were removed, the mount point was taken down, and so on.

This sort of behavior is to be expected of modern operating systems, and is fundamental when considering systems that should work unattended. Whether in a single execution or across restarts and reboots, changes either succeed or they don’t, and the system remains consistent, reliable, observable, and secure.

In the next blog post we’ll see details about the interfaces feature in snappy, which controls aspects of confinement and integration between snaps.


This entry was posted in Architecture, Article, Cloud, Design, Go, Project, Security. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *