1.
.
WWoorrkkiinngg wwiitthhmmuullttiippllee ggiitt rreeppoossiittoorriieess
Julien Pivotto
T-Dose, Eindhoven
October 26, 2014

2.
.
wwhhooaammii
JJuulliieenn PPiivvoottttoo
• System administrator at inuits.eu
• Git user for 5+ years
• DevOps believer
• Open-source defender since 2004
• roidelapluie on twitter/github

3.
.
OOppss <<33 DDeevv
• Infrastructure as Code
▶ SCM all the things
▶ Monitoring
▶ Configuration
▶ Application deployment
• Taking part of software development
▶ Understanding
▶ Monitoring
▶ High Availability
▶ …

4.
.
MMee aanndd ggiitt
• I first used subversion (10 years ago)
• Private forges and sourceforge
• 5 years later a lot of projects moved to git
• The Puppet community also uses a lot git
• Git/Hg are the de-facto standards in OS
projects

5.
.
Git is a free and open source distributed version control system
designed to handle everything from small to very large projects
with speed and efficiency.
Source: http://git-scm.com/

6.
.
ggiitt lloogg
• Initiated in 2005 by Linus Torvalds
• Replacement of closed-source BitKeeper
• Created for the Linux Kernel development
• Now used by thousands of projects

7.
.
ggiitt iiss ddiissttrriibbuutteedd
• Everything can be done in local (exept
pull&push)
• Work with several remotes
• Share code with anyone
• No unique central repository
• A lot of workflows possible

8.
.
BBrraanncchhiinngg aannddmmeerrggiinngg iiss eeaassyy
• You can branch locally
• Merging is easy and simple
• You can squash commits

9.
.
HHooww ttoo aacccceessss aa rreeppoo
• Locally (Local filesystems)
• SSH
• HTTP/HTTPs
• Git protocol

10.
.
ggiitt iiss ooppeenn--ssoouurrccee
• Published under GPL-2
• Written in C
• A lot of frontends/backends
• A lot of libraries
• A lot of hosting services
• And a lot of users

11.
.
MMuullttiippllee ggiitt rreeppooss:: wwhhyy??
• Code isolation: plugins, libraries
• Getting faster feedback from testing
• Contributing back to upstream projects
• Separate apps that are released together
• Share code between projects

12.
.
MMuullttiippllee ggiitt rreeppooss uusseeccaasseess
• Infrastructure as Code
• Projects released together
• Projects that embed external libraries
• …

13.
.
git submodules

14.
.
ggiitt ssuubbmmoodduulleess
• Built-in with git
• Embeds external repositories in a
subdirectory or your repo
• Points these external repositories in a
particular commit

15.
.
ggiitt ssuubbmmoodduulleess
• Submodules act like traditional git
repositories
• Super repo contains a file with the
submodules urls
• Super repo contains objects that can be
updated with commit hashes

16.
.
IInniittiiaattee ssuuppeerr--rreeppoo
.
$ git init .
Initialized empty Git repository in /home/roidelapluie
/super-repo/.git/
$ > README
$ git add README
$ git commit -m "Initial commit"
[master (root-commit) b526be8] Initial commit
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 README
.

17.
.
IInniittiiaattee ssuubbrreeppoo
.
$ git init .
Initialized empty Git repository in /home/roidelapluie
/subrepo/.git/
$ > README
$ git add README
$ git commit -m "Initial commit"
[master (root-commit) 41e64b6] Initial commit
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 README
.

18.
.
AAdddd ffiirrssttmmoodduullee iinn ppaarreenntt rreeppoo
.
$ git submodule add ../subrepo/ subrepo
Cloning into 'subrepo'...
done.
$ git status
On branch master
Changes to be committed:
(use "git reset HEAD <file>..." to unstage)
.
new file: .gitmodules
new file: subrepo

19.
.
WWhhaatt jjuusstt hhaappppeenneedd??
• git created a .gitmodules file
• It cloned the subrepo in a directory
• It created a git object that points to the repo
• That object contains the git revision

20.
.
..ggiittmmoodduulleess ffiillee
.
$ cat .gitmodules
[submodule "subrepo"]
.
path = subrepo
url = ../subrepo/
• Unique file for all the submodules
• Only contains the path and the url
• In this case, we use relative URL

21.
.
CCoommmmiittiinngg oouurr cchhaannggee
.
$ git commit -m "Add subrepo"
[master 3251a63] Add subrepo
2 files changed, 4 insertions(+)
create mode 100644 .gitmodules
create mode 160000 subrepo
.

22.
.
CClloonniinngg tthhaatt ssuuppeerr rreeppoo
.
$ git clone /home/roidelapluie/super-repo/ .
Cloning into '.'...
done.
$ ls subrepo/
$ git submodule init
Submodule 'subrepo' (/home/roidelapluie/subrepo)
registered for path 'subrepo'
$ git submodule update
Cloning into 'subrepo'...
done.
Submodule path 'subrepo': checked out '41e64b6bc0c97052
1c296b9c5e6a0b9144778962'
.

23.
.
SShhoorrccuutt ##11
.
$ git submodule init
$ git submodule update
.
.
$ git submodule update --init
Submodule 'subrepo' (/home/roidelapluie/subrepo)
registered for path 'subrepo'
Cloning into 'subrepo'...
done.
Submodule path 'subrepo': checked out '41e64b6bc0c97052
1c296b9c5e6a0b9144778962'
.

24.
.
SShhoorrccuutt ##22
.
$ git clone /home/roidelapluie/super-repo/ .
$ git submodule init
$ git submodule update
.
.
$ git clone --recurse-submodules /home/roidelapluie/
super-repo/ .
Cloning into '.'...
done.
Submodule 'subrepo' (/home/roidelapluie/subrepo)
registered for path 'subrepo'
Cloning into 'subrepo'...
done.
Submodule path 'subrepo': checked out '41e64b6bc0c97052
1c296b9c5e6a0b9144778962'
.

25.
.
WWoorrkkiinngg iinn aa ssuubbmmoodduullee
.
$ cd subrepo
$ > newfile
$ git add newfile
$ git commit -m "add newfile"
[detached HEAD 6ee0e71] add newfile
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 newfile
$ git push
fatal: You are not currently on a branch.
To push the history leading to the current (detached HEAD)
state now, use
.
git push origin HEAD:<name-of-remote-branch>

26.
.
CChheecckkiinngg oouutt aa bbrraanncchh
.
$ git checkout master
Warning: you are leaving 1 commit behind, not connected
to any of your branches:
6ee0e71 add newfile
If you want to keep them by creating a new branch, this
may be a good time to do so with:
git branch new_branch_name 6ee0e71
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.
.

27.
.
MMeerrggiinngg oouurr ccoommmmiitt
.
$ git merge 6ee0e71
Updating 41e64b6..6ee0e71
Fast-forward
newfile | 0
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 newfile
.

28.
.
PPuusshhiinngg oouurr ccoommmmiitt
.
$ git push
Counting objects: 2, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (2/2), 247 bytes | 0 bytes/s, done.
Total 2 (delta 0), reused 0 (delta 0)
To /home/roidelapluie/subrepo
.
41e64b6..6ee0e71 master -> master

29.
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PPuusshhiinngg oouurr ccoommmmiitt
.
$ date > newfile
$ git commit -am "update newfile"
[master 8a56806] update newfile
1 file changed, 1 insertion(+)
$ git push
Counting objects: 3, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 310 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To /home/roidelapluie/subrepo
.
6ee0e71..8a56806 master -> master

30.
.
LLeett''ss rreewwiinndd....
• A submodule not attached to a branch
▶ Because submodules are referenced by their commit hash
only
▶ It is called the DETACHED HEAD
• How to push then?
▶ The easy/safe option is to checkout a branch
• You only need to do that once

31.
.
ggiitt ssuubbmmoodduullee ffoorreeaacchh
• git submodule foreach allow you to run a
command on each repo
▶ git submodule foreach git checkout master
▶ git submodule foreach ls
▶ git submodule foreach git commit -am "foo"

32.
.
MMeeaannwwhhiillee,, oonn tthhee ssuuppeerr--pprroojjeecctt……
.
$ git status
On branch master
Changes not staged for commit:
(use "git add <file>..." to update what will be
committed)
(use "git checkout -- <file>..." to discard changes in
working directory)
.
modified: subrepo (new commits)
no changes added to commit (use "git add" and/or "git
commit -a")

33.
.
UUppddaattee aa ssuubbrreeppoo
.
$ git commit -am "update subrepo"
[master 731626b] update subrepo
1 file changed, 1 insertion(+), 1 deletion(-)
.

34.
.
LLooggss
.
$ git log -p --submodule=log subrepo/
commit 731626bc53d8c248647443b4c0523d96d316cd24
Author: Julien Pivotto <roidelapluie@inuits.eu>
Date: Sat Oct 25 22:26:56 2014 +0200
.
update subrepo
Submodule subrepo 41e64b6..01c1b9c:
> work on newfile
> update newfile
> add newfile

35.
.
CChhaannggee aa ssuubbmmoodduulleess UURRLL
• Edit .gitmodules and commit the file
• Run git submodule sync
• Now the submodule will use the new URL

36.
.
ggiitt ssuubbmmoodduullee ----rreeccuurrssiivvee
• Run commands recursively
• You can get multiple levels of submodules
• But you add complexity
▶ Each level is 1 more commit you need to write
▶ Playing with remote urls becomes tricky of you work with
remotes

37.
.
SSuubbmmoodduulleess iinntteeggrraattiioonn
• Integrated in git, standard
• Integrated in CI tools (Jenkins)
• Nothing specific needed for pulling
• Need to checkout the submodules after
pulling

38.
.
SSuubbmmoodduulleess iinn GGeerrrriitt
• Gerrit integrates submodules
• It can create commits in "super-repo" if a
code change is merged
• Not a perfect way of doing if changes in two
repo's are depending of each other

39.
.
SSuubbmmoodduulleess iinn GGeerrrriitt
• Gerrit is integrated in CI tools
• Gerrit allows fine tuning ACL
• Gerrit code reviews with submodules
changes don't display changes in the
submodules

40.
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SSuubbmmoodduulleess pprrooss aanndd ccoonnss