Talk I have given at T-Dose about git submodules and git slave and different worklow around them

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. .
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. .
SSuubbmmoodduulleess pprrooss aanndd ccoonnss
Each commit in submodule is not in
super-repo
It is easy to rollback by accident a
submodule
Getting logs is not easy
You can forget to update super-repo
Submodules are not attached to a branch

41. .
SSuubbmmoodduulleess pprrooss aanndd ccoonnss
+ Built-in
+ Can use relative urls
+ You can control the version of the
submodules
+ Supported by many tools
+ Easy rollbacks of submodules
+ git bissect works (on the super repo)

42. .
gitslave a.k.a. gits

43. .
ggiittssllaavvee
• First public release in 2010
• Wrapper around git
• gits can be used in place of git
• Run each git commands in each subrepos

44. .
IInniittiiaattee ggiittss
.
$ git init .
Initialized empty Git repository in /home/roidelapluie/
superrepo/.git/
$ gits prepare
[master (root-commit) 594c492] gits creating .gitslave
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 .gitslave
.

45. .
AAdddd aa ggiitt rreeppoo
.
$ gits attach /home/roidelapluie/subrepo/ subrepo
Cloning into 'subrepo'...
done.
[master 926321f] gits adding /home/roidelapluie/
subrepo/ subrepo
2 files changed, 2 insertions(+)
create mode 100644 .gitignore
$ cat .gitslave
/home/roidelapluie/subrepo/ subrepo
.

46. .
CClloonniinngg aa rreeppoo
.
$ gits clone ../superrepo/ .
Cloning into '.'...
done.
Cloning into 'subrepo'...
done.
.

47. .
WWoorrkkiinngg iinn ssuubbmmoodduulleess
.
$ cd subrepo
$ date newfile
$ git add newfile
$ gits commit -m Work on newfile
On: subrepo:
[master 1230de3] Work on newfile
.
1 file changed, 1 insertion(+), 1 deletion(-)
On: (super-repo):
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working directory clean

48. .
PPuusshhiinngg cchhaannggeess
.
$ gits push
On: subrepo:
To /home/roidelapluie/subrepo/
.
fbcf73c..1230de3 master - master
On: (super-repo):
Everything up-to-date

49. .
PPuulllliinngg cchhaannggeess
.
$ gits pull
On: (super-repo), subrepo:
Already up-to-date.
.

50. .
ggiitt ssllaavvee wwoorrkkffllooww
• Subrepos are always attached to a branch
• You can work within them normally
• Run almost any git command with gits

51. .
PPeerrffoorrmmaannccee
• gits --parallel 8: run 8 parallel git commands
• No extra commit in the super repository
• Close to traditional git workflow

52. .
AA bbrraanncchhiinngg aapppprrooaacchh
• Use gits branch and gits checkout to switch
between branches
• All the repositories will switch branches (e.g
1.0, 1.1)
• Easy merge with gits merge
• Avoid upstream traditional branch names

53. .
ggiittss oouuttppuutt
• gits will simplify the output of git
commands
• It will group the repos by output
• Easier to see the status, the remotes, etc

54. .
PPrrooss aanndd ccoonnss ooff ggiittssllaavvee
Not integrated in tools
Requires an additional binary
Not integrated in git
Hard to rollback on every repositories

55. .
PPrrooss aanndd ccoonnss ooff ggiittssllaavvee
+ Close to git workflow
+ Less error-prone
+ Use traditional git commands
+ Every git push arrives in the repo

56. .
MMiittiiggeeaattee ggiittssllaavveess'' iissssuueess
• Use git hooks to trigger tests on CI
▶ Polling SCM does not scale anyway
▶ Use the Quite period feature
• Use CI to add the same tag in each
repository
▶ Rollback to a tag, not a commit
▶ Add a tag at each commit

57. .
Additional tips

58. .
UUssee SSSSHH ssoocckkeettss
.
$ cat .ssh/config
Host githost
.
controlmaster auto
controlpath /tmp/ssh-%r@%h:%p
ControlPersist 300
• Use only one SSH connection
• Do not disconnect SSH

59. .
FFiinndd tthhee ttaagg ooff aa ccoommmmiitt
.
$ git name-rev --name-only 106cb38dd0bc5ea2d2d0adef0b40
06bd61884e42
remotes/origin/8.5.2~1
.

60. .
ggiitt aauuttooccoorrrreecctt
.
$ git config help.autocorrect 10
$ git pusk
WARNING: You called a Git command named 'pusk', which does not Continuing under the assumption that you meant 'push'
in 1.0 seconds automatically...
.

61. .
UUssee ggiitt hhooookkss
• Multiple repositories = a lot of pull
• Jenkins pulling X repos X times every 5
minutes is a bad idea
• Git hooks to the resue
• Reduce load on Jenkins and the git server
• Reduce the time before Jenkins launches
builds

62. .
GGPPGG--SSiiggnn yyoouurr ccoommmmiittss
• git commit -S
• Sign PR, merge commits
• Do not sign every commit (then it becomes
useless)

63. .
KKeeeeppiinnggmmuullttiippllee rreemmootteess iinn ssyynncc
• git remote set-url --add
• Pull push will ose both urls

64. .
HHoommeewwoorrkk
• Git subtree (also built-in)
• Repo (from Android)
• gr
• …

65. .
CCoonncclluussiioonnss
• Git is an awesome, extandable tool
• There are several ways to manage multiple
git repositories
• Working with multiple repos is easy
• You have to learn tools anyway
• Experiment them and pick one :-)

66. .
TThhaannkk yyoouu
Any question?

67. .
CCoonnttaacctt
Julien Pivotto
julien@inuits.eu
@roidelapluie
INUITS bvba
Belgium
+32 473 441 636
https://inuits.eu