05 - Merge Management

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

3

TRAINING PLAN

1. What is merging?2. Why do we need to manage merging?3. Merging machinery4. Branch types5. Merging operation – maturity testing6. Merging in action7. Advanced merge management8. Criteria of successful merge

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INTRODUCTION TO MERGE MANAGEMENT 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

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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

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LET’S DIVE INTO THE MERGING MACHINERY

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COMMON BUZZWORDS OF MERGING MACHINERY

diff patch

branch merge

conflict cherrypicking

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diff patch

branch merge


There are two diffs:diff and diff3

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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

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DIFF (TWO-WAY MERGE)

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DIFF3 (THREE-WAY MERGE)

Manual comparison of three files

Automatic merging

Generation of conflict situations

Textual representation of the conflicts

Diff3 utility is the basis for:

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or

… as merging into mine the changes that would turn older into theirs

You can think of diff3 as subtracting older from theirs and adding the result to mine …

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diff patch

branch merge


patch is often complementary to

the diff

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PATCH

Useful utility for applying specific source code change set to the working copy

Reversed diff Imitation of distributed version control in

centralized VCSs

You can think about patch – as a:

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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

Interesting facts about patch:

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PATCH

WC (working copy)

patch

create patch

apply patch

svn diff > patchfile.patch

patch -p0 < patchfile.patch

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diff patch

branch merge


merge has no sense without branches

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BRANCH

No merge without branch

No branch without merge

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BRANCH

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 VCS

You can think about branch as:

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BRANCH TYPES

svn merge sourceURL1[@N] sourceURL2[@M] [WCPATH]svn merge -r N:M SOURCE [PATH]

Purpose of the branches, their contents and types directly affect the merging operation workflow and its

results

Merging operation is highly dependent on source and target branches

BRANCH TYPES

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Purpose

Maturity

Codebase disparity

Independence

Duration Branch types

features

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BRANCH TYPES

Branch types

Experimental

SupportRelease

Trunk

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diff patch

branch merge


merge is the most advanced operation

VCSs have

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MERGE

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: automerge

You can think about merge as:

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MERGE. USE CASE

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MERGE. USE CASE

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diff patch

branch merge


conflicts happen when there are difficulties with

merging

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CONFLICT

It is difficult to avoid conflict situation during team work

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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

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GENERAL CONFLICT RESOLUTION WORKFLOW

Update working

copy with conflicting changes

Use one of the conflict resolution

options suggested

by VCS (svn

resolve)

Or edit files manually

saving final version and running svn resolved command

Commit resolved changes

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CONFLICT RESOLUTION OPTIONS

• postponep• diff-fulldf• edite• resolvedr• mine-conflictmc• theirs-conflicttc• show all optionss

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diff patch

branch merge



cherrypicking means replication of single

change

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MERGING IN ACTIONPractical example of merging

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REVISIONS HISTORY>: svn log

svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions --stop-on-copy

>: svn log svn://localhost/ParseCmdLine/trunk

------------------------------------------------------------------------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)------------------------------------------------------------------------

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)------------------------------------------------------------------------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)

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REVISIONS HISTORY>: svn log

svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions --stop-on-copy

>: svn log svn://localhost/ParseCmdLine/trunk

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MERGING RANGE OF REVISIONS

>: svn merge svn://localhost/ParseCmdLine/trunk svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions –r 39:43

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REINTEGRATING A BRANCH

>: svn merge svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions svn://localhost/ParseCmdLine/trunk --reintegrate

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MERGING TWO TREES

>: svn merge svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions svn://localhost/ParseCmdLine/trunk --ignore-ancestry

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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

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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

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svn merge -c REV SOURCE WC svn merge svn://localhost/ParseCmdLine/trunk svn://localhost/ParseCmdLine/branches/experimental/experimentalOptions –c 43

CHERRYPICKING

/experimental

22 23 24 25 … 39 40 41 42 43

-c 43

/trunk

Revision number

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END OF SECTION

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MERGE. MATURITY TESTINGProving that nothing is so simple as someone says

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MERGE. MATURITY TESTING

There are several cases proving that…you cannot fully trust automerge

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Standard conflict handling

BOF/EOF Stingers

Identical Twins

X-Tuplets

Hungry Blobs

Unique anchors

Merging operation maturity test cases:

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BOF/EOF Stingers

Identical Twins

X-Tuplets

Hungry Blobs

Unique anchors


Making sure same source code lines have been

modified by different users

Diff3 Subversion Git

Works as expected

Works as expected

Works as expected

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BOF/EOF Stingers

Identical Twins

X-Tuplets

Hungry Blobs

Unique anchors



Works as expected

Works as expected

Works as expected

Sometimes problems emerge at the beginning

and ending of the file

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BOF/EOF Stingers

Identical Twins

X-Tuplets

Hungry Blobs

Unique anchors



Fails Works as expected

Works as expected

handling identical files from

different working copies

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BOF/EOF Stingers

Identical Twins

X-Tuplets

Hungry Blobs

Unique anchors

Merging operation maturity test cases:Diff3 Subversion Git


Fails

handling identical files from different working copies, but with minor adjacent

changes

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BOF/EOF Stingers

Identical Twins

X-Tuplets

Hungry Blobs

Unique anchors

Merging operation maturity test cases:Diff3 Subversion Git

Works as expected


adjacent changes from different working copies

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BOF/EOF Stingers

Identical Twins

X-Tuplets

Hungry Blobs

Unique anchors


using additional heuristics during diff for covering

special cases of merging


Fails Fails Fails

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One more example of faulty automerging:

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ADVANCED MERGE MANAGEMENTMore complex issues related to merging

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BRANCHES INHERITANCE

RELATIONBRANCH TYPE trunk support release experimental

trunksupportrelease

experimental

child

parent

There are certain branch creation and inheritance restrictions which will be described

by filling in table shown below

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trunksupportrelease

experimental

child

parent

Trunk cannot be the child of any other branch

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trunk +support -release -

experimental -

child

parent

Support branch can be inherited only from trunk

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trunk + +support - +release - -

experimental - -

child

parent

Release branches can be inherited both from trunk and support branches

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trunk + + +support - + +release - - -

experimental - - +

child

parent

Experimental branches can be inherited from any type of branch except release branch

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SUPPORT AND RELEASE BRANCHES

/trunk

/1.0.x

/1.1.x

/1.x.x

/2.0.x

/1.2.x

support branch

release branch

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EXPERIMENTAL BRANCHES

/trunk

experimental branch

/db_refactoring

/UI_integration

/UI_lazy_loading

/Layout_rework

/3-column

/1.x.x

/l18n_embedding

support branch

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TO MERGE OR NOT TO MERGE?

DESTINATIONBRANCH TYPE trunk

RELATION parent parent child parent child parent childtrunk parent

parent

child

parent

child

parent

child

to

from

support release experimental

support

release

experimental

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.

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RELATION parent parent child parent child parent childtrunk parent - - +

parent - +child - - -

parent

child + + - -parent +

child + + - - + +

to

from


support

release

experimental

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

+-

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parent

child

parent

child

parent

child

to

from


support

release

experimental

Trunk can be only parent branch



experimental - - +

child

parent

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parent

child

parent

child

parent

child

to

from


support

release

experimental

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

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parent

child

parent

child

parent

child

to

from


support

release

experimental

Branch cannot be parent to itself

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RELATION parent parent child parent child parent child

trunk parent

parent -child - -

parent -child - -

parent +child + +

to

from


support

release

experimental

There are cases automatically excluded by branch creation and inheritance rules



experimental - - +

child

parent

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parent

child - -parent - -

child -parent - -

child -

to

from


support

release

experimental

There are cases automatically excluded by branch creation and inheritance rules



experimental - - +

child

parent

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parent

child

parent

child

parent

child

to

from


support

release

experimental

This is the result of exclusion merging cases by logic and branches creation and

inheritance rules.

We will be filling in empty cells

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RELATION parent parent child parent child parent childtrunk parent -

parent

child -parent

child

parent

child

to

from


support

release

experimental

Support branches are fully independent, they are not assumed to be merged with trunk or

other support branches

/trunk

/1.x.x

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RELATION parent parent child parent child parent childtrunk parent - -

parent -child -

parent

child + +parent

child

to

from


support

release

experimental

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

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parent -child -

parent

child + +parent

child

to

from


support

release

experimental

Purpose of release branch is maturing the release

/trunk

/0.2.x

/1.x.x

/1.0.x

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parent -child - -

parent

child + + -parent

child

to

from


support

release

experimental

Child to child relation represents sibling branchesNo merge between sibling release and support branches is allowed

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parent -child - -

parent

child + + -parent

child

to

from


support

release

experimental

Child to child relation represents sibling branches

/trunk

/1.x.x

/1.0.x

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parent - +child - -

parent

child + + -parent +

child + + +

to

from


support

release

experimental

It is allowed to merge experimental branches with any type of parent branch

/trunk

/UI_integration

/Layout_rework

/3-column

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parent


child + + - - + +

to

from


support

release

experimental

No merge allowed between sibling experimental and any type of other branch (except experimental)

/trunk

/UI_integration

/1.x.x/1.0.x

/UI_integration

/Layout_rework

/3-column

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parent


child + + - - + +

to

from


support

release

experimental

Final picture

Total number of cells Number of cells with logically excluded cases Number of cells with cases excluded logically and using

branch inheritance rules Allowed case of merging Disallowed cases introduced by merging rules

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20

973%

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SUPPORT AND RELEASE BRANCHES

/trunk

/1.0.x

/1.1.x

/1.x.x

/2.0.x

/1.2.x

support branch

release branch

disallowed merge operation

allowed merge operation

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EXPERIMENTAL BRANCHES

/trunk

/db_refactoring

/UI_integration

/UI_lazy_loading

/Layout_rework

/3-column

/1.x.x

/l18n_embedding

experimental branch

support branch

disallowed merge

allowed merge

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CONCLUSION

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HOW WOULD YOU KNOW THE MERGE IS APPROPRIATE?

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HOW WOULD YOU KNOW THE MERGE IS APPROPRIATE?

Make sure source and target branches are compatible for merging

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HOW WOULD YOU KNOW THE MERGE IS SUCCESSFUL?

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Don’t leave merges unfinished

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Run unit-tests and inspections afterwards

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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

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AFTERWORD

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RECOMMENDED READING

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

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RECOMMENDED READING

2. Version Control with Git by Jon Loeliger

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RECOMMENDED READING

3. Mercurial: The Definitive Guide by Bryan O'Sullivan

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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-info.html - detailed description of merge tracking features

http://www.misuse.org/science/2007/02/24/3-way-merging/

http://www.misuse.org/science/2007/02/24/3-way-merging/

http://www.infoq.com/news/2009/03/Continuous-Deployment



http://codereflect.com/2010/09/21/versioning-systems-and-3-way-merge-process/



http://book.git-scm.com/3_basic_branching_and_merging.html

http://book.git-scm.com/3_basic_branching_and_merging.html

http://book.git-scm.com/5_advanced_branching_and_merging.html

http://book.git-scm.com/5_advanced_branching_and_merging.html

http://hgbook.red-bean.com/read/a-tour-of-mercurial-merging-work.html



http://www.collab.net/community/subversion/articles/merge-info.html

http://www.collab.net/community/subversion/articles/merge-info.html

05 - Merge Management

Technology

Transcript of 05 - Merge Management