"Atomic"?? (was Re: Five Things in Fedora This Week (2014-03-25))

[Matthew Miller]

On Wed, Mar 26, 2014 at 08:16:04PM +0000, Beartooth wrote:
> > pull something big from the backlog. Fedora developer Colin Walters has
> > launched a new project called Fedora Atomic. This system constructs
> > git-like trees from existing official Fedora RPMs, and moves
> > operating-system deployment from managing packages to managing these
> > trees, with (as the name suggests) fully-atomic updates and rollbacks.
> 	Reading that, and following the links, makes it seem that there 
> is a new buzzword, "atomic" in some sense which may be apparent to those 
> who use it, but isn't to me. At first, I thought it might be a typo for 
> "automatic."
> 
> 	Can you define the new Fedora-related sense in terms 
> comprehensible to old mossbacks? How about partially or "fully-atomic"??

Old mossbacks should know this one, especially if they have database or
filesystem experience. An atomic transaction is one which can be safely
considered as a single event, and a) failure puts you back as if it didn't
happen and b) it is impossible for something else to access whatever is
affected and find a partial state -- you either get the original or the
update.

A normal RPM or yum update is not atomic -- if you pull the plug partway
through, or if it breaks for some reason, you are left in a middle state,
and, if you access files belonging to packages being updated while the
transaction is in progress, you may get surprises.

"Partially atomic" usually refers to systems where safe rollbacks happen on
failure, but where it might be possible to accidentally make an access while
the thing is being changed with unpredictable results.

-- 
Matthew Miller    --   Fedora Project    --    <mattdm at fedoraproject.org>