1. Repository Sally Harry Ira write read read
A centralized client-server system.

2. A A A A A’ A” A’ A” A’ A” A’ A” Repository Repository Repository
read
read
Harry
Sally
Harry
Sally A A A’ A”
Two users read the same file.
They both begin to edit their copies.
Repository
Repository A’ A”
THE PROBLEM
write
write
Harry
Sally
Harry
Sally A’ A” A’ A”
Harry publishes his version first.
Sally accidentally overwrites Harry’s version!

3. A A A A’ A’ A’ A’ A’ A’ Repository Repository Repository Repository
LOCK
LOCK
read
Harry
Sally
Harry
Sally A A’
Harry “locks” file A, then copies it for editing.
While Harry edits, Sally’s lock attempt fails.
Repository
Repository A’ A’
THE LOCK-MODIFY-UNLOCK MODEL
read
write
UNLOCK
LOCK
Harry
Sally
Harry
Sally A’ A’ A’
Harry writes his version, then releases his lock.
Now Sally can lock, read, and edit the latest version.

4. A A A A A’ A” A” A” A’ A” A’ A” Repository Repository Repository
read
read
Harry
Sally
Harry
Sally A A A’ A”
Two users copy the same file.
They both begin to edit their copies.
Repository
Repository A” A”
THE COPY-MODIFY-MERGE MODEL, page 1
write
write
Harry
Sally
Harry
Sally A’ A” A’ A”
Sally publishes her version first.
Harry gets an “out-of-date” error!

5. A” A” A’ A” A” A* A” A* A* A* A” A* A* Repository Repository
read
Harry
Sally
Harry
Sally
A’ A” A” A* A”
Harry compares the latest version to his own.
A new merged version is created.
Repository
Repository A* A*
THE COPY-MODIFY-MERGE MODEL, page 2
write
read
Harry
Sally
Harry
Sally A* A” A* A*
The merged version is published.
Now both users have each others’ changes.

6. /
calc
Makefile
integer.c
button.c
paint
Makefile
canvas.c
brush.c

7. 1 2 3