[PPT] - Version Control Systems (Part 1) Devin J. Pohly PowerPoint Presentation

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CMPSC 311: Introduction to Systems Programming Page 1

Institute for Networking and Security Research Department of Computer Science and Engineering Pennsylvania State University, University Park, PA

Systems and Internet Infrastructure Security

i i

Version Control Systems (Part 1)

Devin J. Pohly <djpohly@cse.psu.edu>

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Page 2 CMPSC 311: Introduction to Systems Programming

A version control system is a system for keeping track of the changes made to a document (or collection of documents) over time

Version control systems

Any kind of document...
Resumes
Source code
TPS reports

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Page 3 CMPSC 311: Introduction to Systems Programming

Why?

It’s a time machine!
Look at old versions
Never lose anything
Revert your mistakes
Code fearlessly!
Collaboration
Work in parallel
Merge changes
Social coding

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Page 4 CMPSC 311: Introduction to Systems Programming

A word about words

These terms all refer to

the same thing:

Version control system
Revision control system
Source code/control

management

VCS/RCS/SCM
For this lecture: “version

control” and VCS

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Page 5 CMPSC 311: Introduction to Systems Programming

Basic concepts

Revision: one meaningful

change or set of changes

Repository: where all of

the revisions are stored

Working copy: copy of
ne revision, where the

user makes changes

Check out: get a working

copy from repository

Check in/commit: add a

new revision to repository

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Page 6 CMPSC 311: Introduction to Systems Programming

Basic concepts

Branches: parallel lines of

development

Trunk/master: main

development branch

Tip/head: latest revision
n a branch
Tag: special name given

to an important revision

Often used for numbered

releases like “v4.0”

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Page 7 CMPSC 311: Introduction to Systems Programming

The first generation

Local VCSes
1970s and 80s
SCCS, RCS
Repository stored in a

shared local directory

User must lock a file

before making changes

Lock-edit-unlock model

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Page 8 CMPSC 311: Introduction to Systems Programming

RCS usage

Extremely simple
Check out (read-only)
rcs co foo.h
Check out and lock
rcs co -l foo.c
Check in and commit

changes

rcs ci foo.c

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Page 9 CMPSC 311: Introduction to Systems Programming

How RCS works

Each file has its own repository in the RCS directory,

in which all of that file’s revisions are stored

Repository main.c

1.1–1.50

foo.c

1.1–1.12

foo.h

1.1–1.5

Alice Bob

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Page 10 CMPSC 311: Introduction to Systems Programming

How RCS works

Anyone can check out a read-only working copy of a

file.

Repository main.c

1.1–1.50

foo.c

1.1–1.12

foo.h

1.1–1.5

Alice Bob foo.h

1.5

foo.h

1.5

rcs co foo.h rcs co foo.h

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Page 11 CMPSC 311: Introduction to Systems Programming

How RCS works

If Alice wants to make changes to foo.c, she must lock

the file for writing when she checks it out.

Repository main.c

1.1–1.50

foo.c

1.1–1.12

foo.h

1.1–1.5

Alice Bob foo.c

1.12

foo.h

1.5

foo.h

1.5

rcs co -l foo.c

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Page 12 CMPSC 311: Introduction to Systems Programming

How RCS works

Once Alice has locked foo.c, nobody else may lock it.
Alice can now safely edit her local copy of the file.

Repository main.c

1.1–1.50

foo.c

1.1–1.12

foo.h

1.1–1.5

Alice Bob foo.c

1.12

foo.h

1.5

foo.h

1.5

rcs co -l foo.c vim foo.c

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Page 13 CMPSC 311: Introduction to Systems Programming

How RCS works

You can only commit changes to a file if you hold the lock.
Committing foo.c checks in Alice’s changes as a new revision,

then unlocks the file so others can lock it.

Repository main.c

1.1–1.50

foo.c

1.1–1.12

foo.h

1.1–1.5

Alice Bob foo.c

1.12

foo.h

1.5

foo.h

1.5

rcs ci foo.c vim foo.h rcs ci foo.h

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Page 14 CMPSC 311: Introduction to Systems Programming

How RCS works

When Alice commits her modified foo.c, the

repository creates the new revision number 1.13.

Repository main.c

1.1–1.50

foo.c

1.1–1.13

foo.h

1.1–1.5

Alice Bob foo.h

1.5

foo.h

1.5

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Page 15 CMPSC 311: Introduction to Systems Programming

The second generation

Centralized VCSes
1980s to 2000s
CVS, Subversion

(SVN)

Still widely used
Repository on a server

with many clients

Copy-modify-merge

model

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Page 16 CMPSC 311: Introduction to Systems Programming

How Subversion works

Spot the differences!

Repository

1–56 foo.c foo.h main.c

Alice Bob

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Page 17 CMPSC 311: Introduction to Systems Programming

How Subversion works

Files are stored in one repository rather than individual ones.
Repository and users can all be on different hosts.

Repository

1–56 foo.c foo.h main.c

Alice Bob

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Page 18 CMPSC 311: Introduction to Systems Programming

How Subversion works

Checkout does the same thing: gets a working copy of the latest

revision (which now contains all the files) from the repository.

You don’t have to lock files to change them.

Alice Bob foo.c foo.c foo.h main.c foo.h main.c

svn checkout URL svn checkout URL

Repository

1–56 foo.c foo.h main.c

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Page 19 CMPSC 311: Introduction to Systems Programming

How Subversion works

If Alice modifies some files and commits her changes...

Alice Bob foo.c foo.c foo.h main.c foo.h main.c

vim foo.c vim foo.h svn commit

Repository

1–56 foo.c foo.h main.c

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Page 20 CMPSC 311: Introduction to Systems Programming

How Subversion works

... a new revision of the repository (r57) is created.

Alice Bob foo.c foo.c foo.h main.c foo.h main.c Repository

1–57 foo.c foo.h main.c

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Page 21 CMPSC 311: Introduction to Systems Programming

How Subversion works

If Bob makes changes that don’t overlap with Alice’s, Subversion

can merge them automatically.

This is what happens most of the time.

Alice Bob foo.c foo.c foo.h main.c foo.h main.c

vim main.c svn commit

Repository

1–57 foo.c foo.h main.c

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Page 22 CMPSC 311: Introduction to Systems Programming

How Subversion works

If Subversion can’t merge the changes automatically, it

notifies Bob that there is a merge conflict.

Alice Bob foo.c foo.c foo.h main.c foo.h main.c

vim foo.c svn commit

Repository

1–57 foo.c foo.h main.c

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Page 23 CMPSC 311: Introduction to Systems Programming

How Subversion works

So Bob updates his repository with Alice’s changes,

merges them with his, and tries to commit again.

Alice Bob foo.c foo.c foo.h main.c foo.h main.c

svn update vim foo.c svn commit

Repository

1–57 foo.c foo.h main.c

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Page 24 CMPSC 311: Introduction to Systems Programming

How Subversion works

Bob’s commit succeeds this time, creating revision 58,

which is then stored in the repository.

Alice Bob foo.c foo.c foo.h main.c foo.h main.c Repository

1–58 foo.c foo.h main.c

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Page 25 CMPSC 311: Introduction to Systems Programming

Commit graphs

By convention, the arrow points from the child

revision to the parent revision.

Every branch in Subversion or RCS has an entirely

linear commit graph.

(Branches are linearized when you merge them.)

56 57 58 SVN repository

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Page 26 CMPSC 311: Introduction to Systems Programming

The third generation

Distributed VCSes
2000s to today
Bazaar, Git, Mercurial
Seeing widespread use
Everyone has a full

repository

Highly collaborative
Linux development
GitHub and other “social

coding” sites

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Page 27 CMPSC 311: Introduction to Systems Programming

Sharing your revisions

When you commit your

changes, the revision is stored in your local repository

All communication is

between repositories

You push local revisions

to a remote repository...

and you pull revisions

from a remote repository into the local one.

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Page 28 CMPSC 311: Introduction to Systems Programming

How Git works

Spot the differences

“Official” repository ..A

foo.c foo.h main.c

Alice Bob

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Page 29 CMPSC 311: Introduction to Systems Programming

How Git works

Alice and Bob will both have their own repositories,

no different from the “official” one.

“Official” repository ..A

foo.c foo.h main.c

Alice Bob

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Page 30 CMPSC 311: Introduction to Systems Programming

How Git works

First step is to clone the repository, not check it out.
This gives you a local clone of the entire repo!

git clone URL git clone URL

“Official” repository ..A

foo.c foo.h main.c

Alice’s repository ..A

foo.c foo.h main.c

Bob’s repository ..A

foo.c foo.h main.c

Alice Bob

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Page 31 CMPSC 311: Introduction to Systems Programming

How Git works

Alice can now work locally (and offline), without

worrying about other repositories.

Alice

A

foo.c foo.h main.c

git checkout

Alice’s repository ..A

foo.c foo.h main.c

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Page 32 CMPSC 311: Introduction to Systems Programming

How Git works

Alice edits foo.c as usual, adds her changes to the new

revision, and commits it.

Alice

A

foo.c foo.h main.c

vim foo.c git add foo.c git commit

Alice’s repository ..A

foo.c foo.h main.c

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Page 33 CMPSC 311: Introduction to Systems Programming

How Git works

Revision B, based on A, is now in Alice’s repository.

Alice

B

foo.c foo.h main.c Alice’s repository ..B

foo.c foo.h main.c

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Page 34 CMPSC 311: Introduction to Systems Programming

How Git works

In our Git example, Alice has committed revision B “onto”

revision A.

A B Alice’s repository

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Page 35 CMPSC 311: Introduction to Systems Programming

How Git works

In our Git example, Alice has committed revision B “onto”

revision A.

She can then commit another revision C onto that.

A B C Alice’s repository

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Page 36 CMPSC 311: Introduction to Systems Programming

Push and pull in Git

Alice can push her new revisions to another repository, such as the

“official” one.

The revision being pushed must be a descendant of the remote one.

git push

A..C

“Official” repository ..A

foo.c foo.h main.c

Alice’s repository ..C

foo.c foo.h main.c

Bob’s repository ..B'

foo.c foo.h main.c

Alice Bob

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Page 37 CMPSC 311: Introduction to Systems Programming

Push and pull in Git

The official repository now contains Alice’s changes.
Notice Bob has also committed B' but not yet pushed!

“Official” repository ..C

foo.c foo.h main.c

Alice’s repository ..C

foo.c foo.h main.c

Bob’s repository ..B'

foo.c foo.h main.c

Alice Bob

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Page 38 CMPSC 311: Introduction to Systems Programming

Push and pull in Git

Bob’s commit graph has his new revision, but none of

Alice’s.

A B’ Bob’s repository

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Page 39 CMPSC 311: Introduction to Systems Programming

Push and pull in Git

Bob cannot push his changes yet, because B' is not a

descendant of C.

git push

“Official” repository ..C

foo.c foo.h main.c

Alice’s repository ..C

foo.c foo.h main.c

Bob’s repository ..B'

foo.c foo.h main.c

Alice Bob

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Page 40 CMPSC 311: Introduction to Systems Programming

Push and pull in Git

Bob needs to get Alice’s new changes and merge them.
First he pulls the revisions from the official repo...

git pull

“Official” repository ..C

foo.c foo.h main.c

Alice’s repository ..C

foo.c foo.h main.c

Bob’s repository ..B'

foo.c foo.h main.c

Alice Bob

A..C

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Page 41 CMPSC 311: Introduction to Systems Programming

Git merges

... and they are added to his repository.
He can now merge Alice’s changes with his.

A B B' C Bob’s repository

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Page 42 CMPSC 311: Introduction to Systems Programming

Git merges

This creates a new merge revision D which has two

parents: B' and C.

A B B' C D Bob’s repository

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Page 43 CMPSC 311: Introduction to Systems Programming

Coming full circle

Since D is a descendant of C, Bob can now push!

git push

“Official” repository ..C

foo.c foo.h main.c

Alice’s repository ..C

foo.c foo.h main.c

Bob’s repository ..D

foo.c foo.h main.c

Alice Bob

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Page 44 CMPSC 311: Introduction to Systems Programming

Coming full circle

The official repository now has revision D, which

contains both Alice’s and Bob’s changes.

“Official” repository ..D

foo.c foo.h main.c

Alice’s repository ..C

foo.c foo.h main.c

Bob’s repository ..D

foo.c foo.h main.c

Alice Bob

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Page 45 CMPSC 311: Introduction to Systems Programming

Coming full circle

Developers can also collaborate directly.
Here Alice gets Bob’s latest revisions directly from Bob himself

rather than from the “official” repository.

git pull BOB_URL

“Official” repository ..D

foo.c foo.h main.c

Alice’s repository ..C

foo.c foo.h main.c

Bob’s repository ..D

foo.c foo.h main.c

Alice Bob

C..D

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Page 46 CMPSC 311: Introduction to Systems Programming

Coming full circle

This could be used, for instance, to collaborate on

experimental features that aren’t ready for prime-time.

“Official” repository ..D

foo.c foo.h main.c

Alice’s repository ..D

foo.c foo.h main.c

Bob’s repository ..D

foo.c foo.h main.c

Alice Bob

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Page 47 CMPSC 311: Introduction to Systems Programming

Trends in version control

Isolated to collaborative
Serial to concurrent
Linear to branching
Centralized to

distributed

Limited workflows to