See inline comments below:
On Sunday, October 20, 2013 12:34:32 PM UTC-5, Magnus Therning wrote:
> On Sun, Oct 20, 2013 at 12:59:21AM -0700, Blake McBride wrote:
> > Not sure what you mean about designed well, but in order to switch
> > branches without having to do a full rebuild would involve:
> > 1. switching branches would have to auto-delete compiled modules
> > (object files) for source files that aren't contained in the new
> > branch in order to avoid link time collisions. Or, your build
> > process would have to detect left over object files from a
> > branch switch and delete them at build time.
> Why? Isn't a switch from one branch to another basically the same as
> making changes in several files in between builds?
> Why do you need to remove the now unused object files? Surely you
> don't link by using wildcards so the unused object files will simply
> not get linked in to the final result. Of course incremental linking
> won't be possible but triggering a full re-link on switching branches
> would offer a rather cheap solution to that.
I have 15K classes. Yes, 15 thousand. It is a Java web app, and yea, it
does use wildcards.
If I switched branches, the .class files are left over from before the
switch. Sometimes having unnecessary .class files doesn't matter,
sometimes it does!
> > 2. from one branch to another there may be an include file (when
> > using C/C++) that has a difference possibly necessitating a full
> > rebuild.
> Indeed, and again that should be handled exactly the same way as a
> manual change in that header file in between builds on a branch.
You are missing the point. I can't wait for a two hour build every time I
switch projects. If there was a real change to a header file necessitating
a full rebuild I would do it one and be done. I can't do it again and
again every time I switch branches.
> > 3. git would have to restore files using the current date and time
> > (as opposed to their original date/time) in order for the build
> > system to force a recompile on those modules (I checked - git
> > does do this!)
> > I am sure there are many other possible system-specific issues as
> > well, i.e. many situations where switching branches would subtly
> > necessitate a full rebuild. They would present themselves as very
> > hard-to-find bugs that would disappear when a full rebuild occurred.
> Strictly speaking any such thing would be a bug in your build system,
> e.g. incomplete dependency information.
I think the problems I describe are non-issues in small to mid-sized
projects. The problems I describe only appear in very large projects. You
seem to have some idealistic view of how a very large and complex
application can be cleanly structured so that the issues I describe would
> > I can't imagine how any SCMS could solve problems like these.
> > (Although, I ask the question in case there is a solution that
> > eludes me.) Without a solution to problems like these, given a very
> > large system, many of the cool features of a SCMS are not of much
> > use.
> I think you imagine correctly. These problems don't fall within the
> purview of a VCS (git is a VCS, not an SCMS).
Agreed. I suppose reading some of the hype about git, I wanted to
investigate it. Git surely has a nice design. I'll likely make
increasingly greater use of it - even if it doesn't provide me with
scrambled eggs in the morning!
> > I bring all this up not to be difficult. I read about many cool
> > SCMS features, but I can't see how they could be useful in a very
> > large environment that I use all the time. I am wondering if there
> > is a solution I am unaware of. Thinking about it, I suppose there
> > are some design decisions that could be employed that are driven by
> > nothing more than an attempt to resolve SCM branch issues, but there
> > is no way I am aware of to totally fix the fundamental problems.
> Well, many of the issues you mention can be solved by using a build
> system that can take advantage of a shared cache of build objects.
> Connect a CI system that builds each branch regularly and you can
> largely get around those large local re-builds.
> Magnus Therning OpenPGP: 0xAB4DFBA4
> twitter: magthe http://therning.org/magnus
> As far as the laws of mathematics refer to reality, they are not
> certain, and as far as they are certain, they do not refer to reality.
> -- Albert Einstein
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