Chapter 6: Ambient Calculus

1
Chapter 6 Ambient Calculus

• Programming Distributed Computing Systems A
  Foundational Approach
• Carlos Varela
• Rensselaer Polytechnic Institute

2
Mobile Ambients

• Locations represented by a topology of boundaries
• Not identified with globally vague names
• Process mobility represented as crossing of
  boundaries
• Not as communication of processes or process
  names over channels
• Security represented as ability/inability to
  cross boundaries
• Interaction between processes is by shared
  location within a common boundary

3
Mobile Ambients

• n names
• P,Q processes
• (n)P
• 0
• P Q
• !P
• nP
• M.P
• M capabilities
• in n
• out n
• open n

4
Entry Capability

• n in m.P Q m R
• m n P Q R
• n m m

in m.P Q
R
n
R
P Q
5
Exit Capability

• m n out m.P Q R
• n P Q m R
• m n m

n R
P Q
R

out m.P Q
6
Open Capability

• open m.P m Q
• P Q
• open m.P P Q

m
Q
7
Subjective/Objective Moves

• Subjective moves
• n in m.P Q m R m n P Q
  R
• m n out m.P Q R m R n
  P Q
• open m.P m Q P Q
• Objective moves
• mv in m.P m R m P R
• m mv out m.P R P m R
• Ambient I/O
• (x).P input
• ltxgt output

8
Encoding Objective MovesWith Subjective Moves

• allow n !open n
• mv in n.P (k) k in n.enter out k.open k.P
  
• mv out n.P (k) k out n.exit out k.open k.P
  
• n P n P allow enter
• n P n P allow exit
• n P n P allow enter allow
  exit

9
Exercises

• Prove
• mv in n.P n Q n P Q
• n mv out n.P Q n Q P

10
Reference Cell

• cell c w c ltwgt
• get c (x).P mv in c.(x).(ltxgt mv out c.P)
• set cltwgt.P mv in c.(x).(ltwgt mv out c.P)

11
Mutual Exclusion

• acquire n.P open n.P
• release n.P n P
• acquire l.P release l.Q open l.P l Q
• P 0 Q P Q

12
Dining Philosophers

• phil l r !(acquire l.acquire r.(release l
  release r))
• fork f release f
• 2-table phil f1 f2 phil f2 f1 fork f1
  fork f2
• 2-table-nd phil f1 f2 phil f1 f2 fork f1
  fork f2
• 3-table phil f1 f2 phil f2 f3 phil f3 f1
• fork f1 fork f2 fork f3