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

[Cameron Simpson]

On 26Mar2014 20:16, Beartooth <beartooth at comcast.net> wrote:
> On Tue, 25 Mar 2014 16:31:20 -0400, Matthew Miller wrote:
> > Reposted from
> > http://fedoramagazine.org/five-things-in-fedora-this-week-2014-03-25/
> 	[....]
> > Fedora Atomic -------------
> > 
> > Since this week has been a little slow, I’m going to cheat a bit and
> > 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"??

"atomic" is a very old term, even in computing.

It mean indivisible, and is why atoms are called atoms.

In computing, it means any change which does now expose any itermediate
steps: before the change you have state 1, and after the change you
have state 2.

As an example, for a single file update a common practice is to
write the new file to a temporary name, and then rename the completed
temporary file to the original. POSIX guarentees that that is atomic:
before the rename opening the original name gets you the old file,
and after the rename opening the original name gets your the new
file, and there is no gap in the middle where the original name
doesn't work.

When you run RPM, a lot of files get replaced or modified. in place
in your live running OS.

The atomic project is a system for preparing all those changes off to the
side and then cutting over in one go. Likewise for rolling back.

Cheers,
-- 
Cameron Simpson <cs at zip.com.au>

Sorry, baby, I can't take you to the pizza joint tonight, I've got to go
back to the lab and split the atom.     - Ayn Rand, "What is Romanticism?"