distributed version
control with mercurial
or: how I learned to stop worrying and love the merge
Presented by Ted Naleid
Wednesday, November 4, 2009

overview
robust and mature
“centralized” and “distributed”
advantages over subversion
why svn merging is broken
subversion’s advantages
comparison to git
concepts
basic usage
how to get started
Wednesday, November 4, 2009

mercurial is robust
and mature
Wednesday, November 4, 2009

large projects
Python
Mozilla
NetBeans
OpenOffice
OpenSolaris
OpenJDK
Symbian OS
Xen Hypervisor
Wednesday, November 4, 2009

cross platform support
great command line on win/mac/linux
standalone GUIs
windows - TortiseHG
mac - murky
linux - hgk
Wednesday, November 4, 2009

IDE support
IntelliJ
Eclipse
NetBeans
Visual Studio
TextMate
BBEdit
emacs
vi
Wednesday, November 4, 2009

continuous integration and
bug tracking
Hudson
Cruise Control
BuildBot
Bamboo
TeamCity
JIRA
Trac
FogBugz/Kiln
+ support in maven and ant
Wednesday, November 4, 2009

internet hosting
bitbucket
Google Code
SourceForge
kenai (Sun)
Wednesday, November 4, 2009

Build Server
commit
Remote
update Repo
centralized version
control systems
Wednesday, November 4, 2009

Build Server
Build
Repo
sh
pu
ll
pu
commit
Alice's Carl's
update Repo Repo
Bob's
Repo
distributed version
control system
Wednesday, November 4, 2009

Build Server
Build
Repo
sh
pu
ll
pu
commit push
Alice's Carl's
update Repo pull Repo
pu
sh
pu
ll
Bob's
Repo
distributed version
control system
Wednesday, November 4, 2009

subversion limitations
mercurial strengths
Wednesday, November 4, 2009

shared
remote repository
owned
local repository
Wednesday, November 4, 2009

requires active net
connection
always available
Wednesday, November 4, 2009

slow over-the-wire
access
speedy local access
Wednesday, November 4, 2009

single point of failure
multiple redundant copies
Wednesday, November 4, 2009

working with branches
is slow and painful
simple, in-place
branch switching
hg update branch_name
Wednesday, November 4, 2009

expensive public
branches discourage
experimentation
cheap, easy
branches encourage
experimentation
Wednesday, November 4, 2009

saving state is
intertwined with releasing
save whenever, push out
when it’s ready
Wednesday, November 4, 2009

would you use a word
processor that only let you
save at the end of a
paragraph?
Wednesday, November 4, 2009

litters workspace
with .svn directories
single .hg directory
at project root
Wednesday, November 4, 2009

time to set up new
repository limits use
instant setup
enables new uses
hg init
hg add
hg commit -m “first commit”
Wednesday, November 4, 2009

merging is messy,
dangerous and often
avoided
merging is the core
operation so it’s done well
Wednesday, November 4, 2009

why is subversion
merging so broken
and scary?
Wednesday, November 4, 2009

#1
when there are conflicts,
you’re forced to merge into
an unsaved working copy
Wednesday, November 4, 2009

#2
when there are no conflicts,
you aren’t forced to merge
this is a bug, not a feature
Wednesday, November 4, 2009

in mercurial, you commit
before pulling new code
only fully formed changes
get pushed out
history is always preserved
Wednesday, November 4, 2009

merging is a common, trivial
operation
a merge occurs every time
you pull down new code
Wednesday, November 4, 2009

subversion strengths
mercurial
limitations
Wednesday, November 4, 2009

familiar
new ideas require
understanding
Wednesday, November 4, 2009

very strong
tool support
tool support not
as mature
Wednesday, November 4, 2009

better choice for lots of
huge binary files
not optimal for
huge binary files
Wednesday, November 4, 2009

everyone knows where
the canonical repository is
convention defines
where to push/pull
from
Wednesday, November 4, 2009

checking out part of a
repository is easy
no easy way to get
only part of a
repository
Wednesday, November 4, 2009

allows empty directories
tracks files, so empty
directories aren’t
saved
Wednesday, November 4, 2009

subversion is the clear
leader in free
centralized version control
annoying holy war
raging over
hg vs. git
Wednesday, November 4, 2009

mercurial git
Wednesday, November 4, 2009

easy local md5 is easy,
revision numbers right?
Wednesday, November 4, 2009

plugin design with kitchen sink
basic default
functionality design
Wednesday, November 4, 2009

better slower http
performance over performance, no
http, built-in quick easy built-in
server serverhg serve
Wednesday, November 4, 2009

works well on windows support
windows, mac, is lacking and
and linux low priority
Wednesday, November 4, 2009

history is rewriting history
sacred is a badge of
(though you can commit sacrilege with plugins) honor
Wednesday, November 4, 2009

mercurial concepts
Wednesday, November 4, 2009

change sets are stored as
nodes in a DAG
(directed acyclic graph)
tip
newest r3
file system version
r2
time
r1
oldest r0
Wednesday, November 4, 2009

a change set with no
children is a “head”
r3
r2
r1
r0
Wednesday, November 4, 2009

change sets have zero, one
or two parents
Wednesday, November 4, 2009

the root has zero parents
r4
r3
r2
r1
r0
Wednesday, November 4, 2009

normal commits have
one parent
r4
r3
r2
r1
r0
Wednesday, November 4, 2009

merges have two parents
r4
r3
r2
r1
r0
Wednesday, November 4, 2009

branch name is determined
by the first parent
r4 branch: default r4 branch: release_1.x
r3 branch: release_1.x r3 branch: release_1.x
r2 branch: default r2 branch: default
r1 r1
r0 r0
first parent is r2 first parent is r3
Wednesday, November 4, 2009

history is immutable
Wednesday, November 4, 2009

merging only happens in
your local repository
Wednesday, November 4, 2009

that’s it!
Wednesday, November 4, 2009

mercurial usage
Wednesday, November 4, 2009

hg status <-> svn status
hg log <-> svn log
hg diff <-> svn diff
hg cat <-> svn cat
familiar basic hg
hg
blame
add
<->
<->
svn
svn
blame
add
commands hg
hg
hg
remove
rename
update
<->
<->
<->
svn
svn
svn
remove
rename
update
hg resolve <-> svn resolve
hg commit <-> svn commit
hg merge <-> svn merge
Wednesday, November 4, 2009

hg init
hg add
hg commit -m “project start”
create a new r0 “project start”
repository
Wednesday, November 4, 2009

<changes ...>
hg commit -m “created domain”
commit changes
to your r1 “created domain”
local repository r0 “project start”
Wednesday, November 4, 2009

<more changes...>
hg commit -m “updated user”
commit changes
to your r2 “updated user”
local repository r1 “created domain”
r0 “project start”
Wednesday, November 4, 2009

hg update -r 1
switch your file
system to a r2 “updated user”
different revision r1 “created domain”
r0 “project start”
Wednesday, November 4, 2009

hg branch release_1.0.x
<changes...>
hg commit -m “fix role”
“fix role”
r3
branch: release_1.x
create a branch r2
“updated user”
branch: default
“created domain”
r1
branch: default
“project start”
r0
branch: default
Wednesday, November 4, 2009

hg update default
“fix role”
r3
branch: release_1.x
merge changes r2
“updated user”
branch: default
from a branch r1
“created domain”
branch: default
“project start”
r0
branch: default
Wednesday, November 4, 2009

hg merge release_1.x
<resolve any conflicts>
hg commit -m “merged branch”
r4 “merged branch”
“fix role”
r3
branch: release_1.x
merge changes r2 “updated user”
from a branch r1 “created domain”
r0 “project start”
Wednesday, November 4, 2009

hg clone http://path.to.repo
get an existing b b
repository a a
remote local
Wednesday, November 4, 2009

hg pull
get the latest c
changes from b b
another
repository a
remote
a
local
Wednesday, November 4, 2009

hg pull
pull adds c c
changes but b b
doesn’t modify
file system a
remote
a
local
Wednesday, November 4, 2009

hg update
update your c c
working directory b b
with the changes a a
remote local
Wednesday, November 4, 2009

what if you’ve already made
a change in your repository?
Wednesday, November 4, 2009

hg pull
we added c to b d c
b b
they added d to b a a
remote local
Wednesday, November 4, 2009

hg pull
d
d c
pulling in d gives b b
us a new head a a
remote local
Wednesday, November 4, 2009

hg update
# abort, crosses branches
d
when we try to d c
update to the tip, b b
hg says you need
to merge a
remote
a
local
Wednesday, November 4, 2009

hg merge
<resolve any conflicts>
hg commit -m “merge done”
e
d
d c
merge the b b
changes together
a a
remote local
hg fetch does all this in one command
(pull, update, merge, commit) use that instead
Wednesday, November 4, 2009

hg push
e
d
d c
push changes b b
back out
a a
remote local
Wednesday, November 4, 2009

hg push
e e
d d
c c
push changes b b
back out
a a
remote local
Wednesday, November 4, 2009

how to get started
with mercurial
Wednesday, November 4, 2009

watch the peepcode
screencast
$9, cheap!
Wednesday, November 4, 2009

read the book
free online!
Wednesday, November 4, 2009

install it
Wednesday, November 4, 2009

start small
Wednesday, November 4, 2009

convert your existing
repository
Wednesday, November 4, 2009

commit and share often
Wednesday, November 4, 2009

install and learn to use a
3-way merge tool
Wednesday, November 4, 2009

profit!
Wednesday, November 4, 2009

links
mercurial wiki/download
http://mercurial.selenic.com
O’Reilly’s Mercurial - The Definitive Guide
http://hgbook.red-bean.com/ (free!)
peepcode intro screencast ($9)
http://peepcode.com/products/meet-mercurial
mercurial branching info
http://stevelosh.com/blog/entry/2009/8/30/a-guide-to-branching-in-mercurial/
ACM article on DVCS
http://preview.tinyurl.com/qezmta
Wednesday, November 4, 2009

Questions?
Wednesday, November 4, 2009