1. MERGE MANAGEMENT

2. TRAINING GOALS
Describe problems and
risks emerging during
merge operations in
VCSs
Show how to mitigate
those risks and avoid
problems related to
merging
2

3. TRAINING PLAN
1. What is merging?
2. Why do we need to manage merging?
3. Merging machinery
4. Branch types
5. Merging operation – maturity testing
6. Merging in action
7. Advanced merge management
8. Criteria of successful merge
3

4. INTRODUCTION TO MERGE
MANAGEMENT
4 Basic ideas and principles

5. WHAT IS MERGING?
• In general sense:
combination of two (or more) different objects into
the whole in the form of new object with unified
properties
• Applicably to Version Control Systems:
fundamental operation that reconciles multiple
changes made to a revision-controlled collection
of files
5

6. WHY DO WE NEED TO MANAGE
MERGING?
1. Result of merging often might be unpredictable
2. Nobody can guarantee that result of merging is
the one you really wanted to achieve
3. Few people know the machinery of merging
process
4. If something unpredictable happened during
merging, almost nobody can explain why.
5. This is all because merging is the one of the most
non-trivial operations of version control systems
6

7. LET’S DIVE INTO THE MERGING MACHINERY
7

8. COMMON BUZZWORDS OF
MERGING MACHINERY
diff patch
branch merge
conflict cherrypicking
8

9. COMMON BUZZWORDS OF
MERGING MACHINERY
diff patch
branch merge
There are two diffs:
diff and diff3
conflict cherrypicking
9

10. DIFF (TWO-WAY MERGE)
 Based on the longest common subsequence (LCS)
algorithm
 Not to confuse with longest common substring algorithm
 Complexity of LCS algorithm is O(mn); m, n – lengths of
the strings
 Therefore, diff works using line by line comparison
 Uses four formats: normal, context, unified, ed
 Can compare directories and their content recursively
 But does not track directories move/rename
10

11. DIFF (TWO-WAY MERGE)
11

12. DIFF3 (THREE-WAY MERGE)
Diff3 utility is the basis for:
Manual comparison of three files
Automatic merging
Generation of conflict situations
Textual representation of the conflicts
12

13. DIFF3 (THREE-WAY MERGE)
You can think of diff3 as subtracting older
from theirs and adding the result to mine …
or
… as merging into mine the changes that
would turn older into theirs
13

14. DIFF3 (THREE-WAY MERGE)
14

15. COMMON BUZZWORDS OF
MERGING MACHINERY
diff patch
branch merge
patch is often
complementary to the
diff
conflict cherrypicking
15

16. PATCH
You can think about patch – as a:
 Useful utility for applying specific source code change
set to the working copy
 Reversed diff
 Imitation of distributed version control in centralized
VCSs
16

17. PATCH
Interesting facts about patch:
 Merging functionality in DVCSs is implemented via
series of subsequent patches applied.
 There is no patch format extension for directories
structure modification. Beware about move, rename and
delete operations
 svn merge command is like patch, but with additional
special abilities such as: tree structure changes tracking
and mergeinfo tracking
 Result of svn diff command is the fully functioning
patch file, but with little bit more limited representation
abilities (subset of patch format)
 Patch is also often called changeset 17

18. PATCH
svn diff > patchfile.patch
create patch
patch
apply patch
WC (working copy)
patch -p0 < patchfile.patch
18

19. COMMON BUZZWORDS OF
MERGING MACHINERY
diff merge has no sense
patch
without branches
branch merge
conflict cherrypicking
19

20. BRANCH
No merge without branch
No branch without merge
20

21. BRANCH
You can think about branch as:
 Separate isolated parallel
development line (codebase)
 Operation of separate development
line creation (branching)
 Set of files having common ancestor
with another set of files
 Another folder in repository (usually in
/branches parent directory) of
centralized VCS
 Dedicated private place in distributed
21
VCS

22. BRANCH TYPES
svn merge sourceURL1[@N] sourceURL2[@M] [WCPATH]
svn merge -r N:M SOURCE [PATH]
Merging operation is highly dependent on source
and target branches
Purpose of the branches, their contents and types
directly affect the merging operation workflow and
its results 22

23. BRANCH TYPES
Purpose
Duration Maturity
Branch
types
features
Codebase
Independence 23
disparity

24. BRANCH TYPES
Trunk Experimental
Branch
types
Release Support 24

25. COMMON BUZZWORDS OF
MERGING MACHINERY
diff patch
branch merge
merge is the most
advanced operation
conflict
VCSs have cherrypicking
25

26. MERGE
You can think about merge as:
 Reconciliation of multiple changes made to a revision-
controlled collection of files
 Application of the differences between two sources to a
working copy path (svn merge, svn update)
 Operation of joining two or more development histories
together (git merge)
 Reintegration of the whole branch (or its part) into
another branch
 Process of two or three files comparison: 2-way merge
and 3-way merge
 Automatically generated result of 3-way merge:
26
automerge

27. MERGE. USE CASE
27

28. MERGE. USE CASE
28

29. COMMON BUZZWORDS OF
MERGING MACHINERY
diff patch
conflicts happen
when there are difficulties
branch
with merging
merge
conflict cherrypicking
29

30. CONFLICT
It is difficult to avoid conflict situation
during team work
30

31. CONFLICT
 Conflict – is a state of one or more working copy
artifacts assuming that:
 The same source code lines have been modified by different
users
 Artifact properties have been modified by different users
 VCS cannot resolve modifications of directory structure have
been made by different users
 Conflict situations are potentially produced by:
 svn update
 svn merge
31

32. GENERAL CONFLICT RESOLUTION
WORKFLOW
Or edit files
Update manually saving
working copy final version and
with running svn
conflicting resolved
changes command
Use one of Commit
the conflict resolved
resolution changes
options
suggested by
VCS (svn 32
resolve)

33. CONFLICT RESOLUTION
OPTIONS
p • postpone
df • diff-full
e • edit
r • resolved
mc • mine-conflict
tc • theirs-conflict
33
s • show all options

34. COMMON BUZZWORDS OF
MERGING MACHINERY
diff patch
cherrypicking means
replication of single
change
branch merge
conflict cherrypicking
34

35. MERGING IN ACTION
35 Practical example of merging

36. REVISIONS HISTORY
>: svn log
svn://localhost/ParseCmdLine/branches/experi
mental/experimentalOptions --stop-on-copy
------------------------------------------------------------------------
r42 | svn | 2011-02-26 20:29:59 +0200 (Sat, 26 Feb 2011)
------------------------------------------------------------------------
r41 | svn | 2011-02-25 20:49:10 +0200 (Fri, 25 Feb 2011)
------------------------------------------------------------------------
r40 | svn | 2011-02-25 20:44:40 +0200 (Fri, 25 Feb 2011)
------------------------------------------------------------------------
r38 | svn | 2011-02-25 00:39:06 +0200 (Fri, 25 Feb 2011)
------------------------------------------------------------------------
r37 | svn | 2011-02-25 00:24:47 +0200 (Fri, 25 Feb 2011)
------------------------------------------------------------------------
r36 | svn | 2011-02-25 00:20:28 +0200 (Fri, 25 Feb 2011)
------------------------------------------------------------------------
>: svn log svn://localhost/ParseCmdLine/trunk
r43 | svn | 2011-02-28 16:26:44 +0200 (Mon, 28 Feb 2011)
------------------------------------------------------------------------
r39 | svn | 2011-02-25 20:41:01 +0200 (Fri, 25 Feb 2011)
------------------------------------------------------------------------
r33 | svn | 2011-02-24 13:24:31 +0200 (Thu, 24 Feb 2011)
------------------------------------------------------------------------
r32 | svn | 2011-02-24 13:21:44 +0200 (Thu, 24 Feb 2011)
------------------------------------------------------------------------
r31 | svn | 2011-02-24 13:16:34 +0200 (Thu, 24 Feb 2011) 36
------------------------------------------------------------------------
r30 | svn | 2011-02-24 12:25:47 +0200 (Thu, 24 Feb 2011)
------------------------------------------------------------------------
r28 | svn | 2011-02-24 12:14:14 +0200 (Thu, 24 Feb 2011)

37. REVISIONS HISTORY
>: svn log
svn://localhost/ParseCmdLine/branches/experi
mental/experimentalOptions --stop-on-copy
>: svn log svn://localhost/ParseCmdLine/trunk
37

38. MERGING RANGE OF REVISIONS
>: svn merge svn://localhost/ParseCmdLine/trunk
svn://localhost/ParseCmdLine/branches/experimental/experim
entalOptions –r 39:43
38

39. REINTEGRATING A BRANCH
>: svn merge
svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions
svn://localhost/ParseCmdLine/trunk --reintegrate
39

40. MERGING TWO TREES
>: svn merge
svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions
svn://localhost/ParseCmdLine/trunk --ignore-ancestry
40

41. DIFF3 VS MERGE
 diff3 always ignores ancestry
 merge takes into account ancestry by default
 Therefore, repository should store information about:
 Branching (svn copy operations)
 Merging (svn merge operations, revisions, baselines, etc)
 Tree modifications (files copying, deletion, movement, etc)
 Operations sequence
 Before the release of Subversion 1.5 this functionality
was implemented in svnmerge.py
 In Subversion 1.5 it is called merge tracking
41

42. MERGE TRACKING
Reduces the bookkeeping overhead for branch maintenance
Allows to avoid common cases of the "repeated merge" problem
Allows cherry-picking of changes
Allows outputting list of revisions available for merging
Support of bidirectional merges
Support of blocking changes
Support of merge rollbacks 42

43. CHERRYPICKING
 svn merge -c REV SOURCE WC
 svn merge svn://localhost/ParseCmdLine/trunk
svn://localhost/ParseCmdLine/branches/experim
ental/experimentalOptions –c 43
/trunk
-c 43
/experimental
Revision
number 22 23 24 25 … 39 40 41 42 43 43

44. END OF SECTION
44

45. MERGE. MATURITY TESTING
45 Proving that nothing is so simple as someone says

46. MERGE. MATURITY TESTING
There are several cases proving that…
you cannot fully trust automerge
46

47. MERGE. MATURITY TESTING
Merging operation maturity test cases:
Standard conflict handling
BOF/EOF Stingers
Identical Twins
X-Tuplets
Hungry Blobs
Unique anchors
47

48. MERGE. MATURITY TESTING
Merging operation maturity test cases:
Standard conflict handling
BOF/EOF Stingers
Identical Twins sure same source code
Making
lines have been modified by
X-Tuplets different users
Hungry Blobs
Unique Diff3
anchors Subversion Git
Works as expected Works as expected Works as expected 48

49. MERGE. MATURITY TESTING
Merging operation maturity test cases:
Standard conflict handling
BOF/EOF Stingers
Identical Twins
Sometimes problems emerge
X-Tuplets at the beginning and ending of
the file
Hungry Blobs
Unique Diff3
anchors Subversion Git
Works as expected Works as expected Works as expected 49

50. MERGE. MATURITY TESTING
Merging operation maturity test cases:
Standard conflict handling
BOF/EOF Stingers
Identical Twins
X-Tuplets handling identical files from
Hungry Blobsdifferent working copies
Unique Diff3
anchors Subversion Git
Fails Works as expected Works as expected 50

51. MERGE. MATURITY TESTING
Merging operation maturity test cases:
Diff3 Subversion Git
Standard conflict Works as expected
Fails handling Fails
BOF/EOF Stingers
handling identical files from different working
Identical Twins with minor adjacent changes
copies, but
X-Tuplets
Hungry Blobs
Unique anchors
51

52. MERGE. MATURITY TESTING
Merging operation maturity test cases:
Diff3 Subversion Git
Standard expected handling
Works as conflict Fails Works as expected
BOF/EOF Stingers
adjacent changes from
Identical Twins
different working copies
X-Tuplets
Hungry Blobs
Unique anchors
52

53. MERGE. MATURITY TESTING
Merging operation maturity test cases:
Diff3 Subversion Git
Standard conflict handling
Fails Fails Fails
BOF/EOF Stingers
Identical Twins additional heuristics
using
during diff for covering special
X-Tuplets cases of merging
Hungry Blobs
Unique anchors
53

54. MERGE. MATURITY TESTING
One more example of faulty automerging:
54

55. ADVANCED MERGE MANAGEMENT
56 More complex issues related to merging

56. BRANCHES INHERITANCE
There are certain branch creation and inheritance
restrictions which will be described by filling in table
shown below
RELATION child
BRANCH TYPE trunk support release experimental
trunk
parent support
release
experimental
57

57. BRANCHES INHERITANCE
Trunk cannot be the child of any other branch
RELATION child
BRANCH TYPE trunk support release experimental
trunk
parent support
release
experimental
58

58. BRANCHES INHERITANCE
Support branch can be inherited only from trunk
RELATION child
BRANCH TYPE trunk support release experimental
trunk +
parent support -
release -
experimental -
59

59. BRANCHES INHERITANCE
Release branches can be inherited both from trunk
and support branches
RELATION child
BRANCH TYPE trunk support release experimental
trunk + +
parent support - +
release - -
experimental - -
60

60. BRANCHES INHERITANCE
Experimental branches can be inherited from any
type of branch except release branch
RELATION child
BRANCH TYPE trunk support release experimental
trunk + + +
parent support - + +
release - - -
experimental - - +
61

61. SUPPORT AND RELEASE
BRANCHES
/1.1.x /2.0.x
/trunk
/1.2.x
/1.0.x
/1.x.x
release branch
62
support branch

62. EXPERIMENTAL BRANCHES
/trunk
/1.x.x
/db_refactoring
/l18n_embedding
/UI_lazy_loading
/UI_integration
/Layout_rework
63
support branch
/3-column
experimental branch

63. TO MERGE OR NOT TO MERGE?
There are certain merging restrictions which will be
described by filling in table shown below
We will fill in it step by step in order to understand all
aspects of the advanced merge management.
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent
parent
support
from child
parent
release
child 64
parent
experimental
child

64. TO MERGE OR NOT TO MERGE?
Final result will look as follows
 Logically excluded cases of merging
 Cases excluded by branch inheritance and creation rules
+  Allowed case of merging
-  Disallowed case of merging operation
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - - +
parent - +
support
from child - - -
parent
release
child + + - - 65
parent +
experimental
child + + - - + +

65. TO MERGE OR NOT TO MERGE?
Trunk can be only parent branch
RELATION child
BRANCH TYPE trunk support release experimental
trunk + + +
parent support - + +
release - - -
experimental - - +
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent
parent
support
from child
parent
release
child 66
parent
experimental
child

66. TO MERGE OR NOT TO MERGE?
Merging from parent to parent of the same branch
seems to be merged into itself. It makes sense only
as a conflict which is not the case of merge we're
considering
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent
parent
support
from child
parent
release
child 67
parent
experimental
child

67. TO MERGE OR NOT TO MERGE?
Branch cannot be parent to itself
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent
parent
support
from child
parent
release
child 68
parent
experimental
child

68. TO MERGE OR NOT TO MERGE?
There are cases automatically excluded by branch
creation and inheritance rules
RELATION child
BRANCH TYPE trunk support release experimental
trunk + + +
parent support - + +
release - - -
experimental - - +
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent
parent -
support
from child - -
parent -
release
child - - 69
parent +
experimental
child + +

69. TO MERGE OR NOT TO MERGE?
There are cases automatically excluded by branch
creation and inheritance rules
RELATION child
BRANCH TYPE trunk support release experimental
trunk + + +
parent support - + +
release - - -
experimental - - +
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent
parent
support
from child - -
parent - -
release
child - 70
parent - -
experimental
child -

70. TO MERGE OR NOT TO MERGE?
This is the result of exclusion merging cases by
logic and branches creation and inheritance rules.
We will be filling in empty cells
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent
parent
support
from child
parent
release
child 71
parent
experimental
child

71. TO MERGE OR NOT TO MERGE?
Support branches are fully independent, they are
not assumed to be merged with trunk or other
support branches
/trunk
/1.x.x
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent -
parent
support
from child -
parent
release
child 72
parent
experimental
child

72. TO MERGE OR NOT TO MERGE?
Purpose of release branch is maturing the release
It is expected that only bugfixes will be merged to
parent branch and no changes will be merged from
parent branch
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - -
parent -
support
from child -
parent
release
child + + 73
parent
experimental
child

73. TO MERGE OR NOT TO MERGE?
Purpose of release branch is maturing the release
/0.2.x /1.0.x
/trunk /1.x.x
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - -
parent -
support
from child -
parent
release
child + + 74
parent
experimental
child

74. TO MERGE OR NOT TO MERGE?
Child to child relation represents sibling branches
No merge between sibling release and support
branches is allowed
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - -
parent -
support
from child - -
parent
release
child + + - 75
parent
experimental
child

75. TO MERGE OR NOT TO MERGE?
Child to child relation represents sibling branches
/trunk
/1.0.x
/1.x.x
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - -
parent -
support
from child - -
parent
release
child + + - 76
parent
experimental
child

76. TO MERGE OR NOT TO MERGE?
It is allowed to merge experimental branches with
any type of parent branch
/trunk
/UI_integration
/Layout_rework
/3-column
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - - +
parent - +
support
from child - -
parent
release
child + + - 77
parent +
experimental
child + + +

77. TO MERGE OR NOT TO MERGE?
No merge allowed between sibling experimental and
any type of other branch (except experimental)
/trunk /Layout_rework
/1.x.x /UI_integration
/1.0.x /3-column
/UI_integration
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - - +
parent - +
support
from child - - -
parent
release
child + + - - 78
parent +
experimental
child + + - - + +

78. TO MERGE OR NOT TO MERGE?
Final picture
49  Total number of cells
33  Number of cells with logically excluded cases
20  Number of cells with cases excluded logically and using
branch inheritance rules
9  Allowed case of merging
73%  Disallowed cases introduced by merging rules
DESTINATION to
BRANCH TYPE trunk support release experimental
RELATION parent parent child parent child parent child
trunk parent - - +
parent - +
support
from child - - -
parent
release
child + + - - 79
parent +
experimental
child + + - - + +

79. SUPPORT AND RELEASE
BRANCHES
/1.1.x /2.0.x
/trunk
/1.2.x
/1.0.x
/1.x.x
release branch allowed merge
operation 80
support branch disallowed merge
operation

80. EXPERIMENTAL BRANCHES
/trunk
/1.x.x
/db_refactoring
/l18n_embedding
/UI_lazy_loading
/UI_integration
allowed merge /Layout_rework
disallowed merge
82
support branch
/3-column
experimental branch

81. CONCLUSION
83

82. HOW WOULD YOU KNOW THE MERGE IS
APPROPRIATE?
84

83. HOW WOULD YOU KNOW THE MERGE IS
APPROPRIATE?
Make sure source and target branches are
compatible for merging
85

84. HOW WOULD YOU KNOW THE MERGE IS
SUCCESSFUL?
86

85. HOW WOULD YOU KNOW THE MERGE IS
SUCCESSFUL?
Don’t leave merges unfinished
87

86. HOW WOULD YOU KNOW THE MERGE IS
SUCCESSFUL?
Run unit-tests and inspections afterwards
88

87. HOW WOULD YOU KNOW THE MERGE IS
SUCCESSFUL?
 Formal criteria:
 There are no conflicts
 Code compiles
 Unit-tests run successfully
 Conceptual criteria:
 Implemented features do not have logical and architectural
contradictions
 Whole codebase correspond to the introduced concepts
 In case of intersection between merged features
functionality it should lead to additional development
89

88. 90

89. AFTERWORD
91

90. RECOMMENDED READING
1. Version control with Subversion By Ben Collins-Sussman,
Brian W. Fitzpatrick, C. Michael Pilato
92

91. RECOMMENDED READING
2. Version Control with Git by Jon Loeliger
93

92. RECOMMENDED READING
3. Mercurial: The Definitive Guide by Bryan O'Sullivan
94

93. USEFUL LINKS
 http://www.misuse.org/science/2007/02/24/3-way-merging/ -
three way merging
 http://svnbook.red-
bean.com/en/1.5/svn.branchmerge.advanced.html - advanced
merging in subversion
 http://codereflect.com/2010/09/21/versioning-systems-and-3-
way-merge-process/ - simple way to understand 3 way merge
process
 http://book.git-
scm.com/3_basic_branching_and_merging.html - basic
branching and merging
 http://book.git-
scm.com/5_advanced_branching_and_merging.html -
advanced branching and merging
 http://hgbook.red-bean.com/read/a-tour-of-mercurial-merging-
work.html - how mercurial merging works
 http://www.collab.net/community/subversion/articles/merge- 95
info.html - detailed description of merge tracking features