Erich Laube

1

• Erich Laube
• Presentation based on
• A Language with Distributed Scope
• by Luca Cardelli

2
Problems with Passing References

• losing contexts
• losing structure of data
• for example what is 0xa500 at Site 2?

Site 1
Site 2
C.f(Obj B)
Obj A
Obj C
0xa500
0xa500
???
Obj B
3
Lexical vs. Dynamic Scoping

• Lexical free variables bound at compile time.
• Dynamic free variables bound at run time.

output g(4) (y4 f(2))
int y 3 int f(int x) return xy int
g(int y) return f(2) main() print(g(4))
lexical scoping
2y
2y
23 5
f(2)
dynamic scoping
2y
2y
24 6
f(2)
4
Distributed Lexical Scoping

• Lexical Scoping in Distributed Context
• identifiers are bound to their original site
• transmitted objects keep their bindings

5
Obliq Overview

• Distributed Lexical Scoping
• Interpreted
• Untyped
• Built on top of Modula3 Network Objects
• Object Oriented
• Garbage Collection

6
Obliq Objects

• Prototypical Approach
• have a state
• are local to a site
• example

o gt x gt 3, foo gt meth(a,b) b end, z gt
alias y of o2 end
7
Encoding Inheritance

• Inheritance

Class A int i ... Class B extends A
int j ...
o1 gt var i ... o2 gt
clone(o1) var j ...
8
Aliasing / References

• operations on a.x are redirected to b.y
• a.x now acts as a local stub
• like remote pointers
• alias for all fields of a

a.x alias y of b end
redirect a to b
9
Special Field Attributes

• protected
• prohibits external updates, cloning and aliasing
• serialized
• like synchronized in Java
• at most one thread operating on a field at a time

10
Example 1 Serialized Queue (1)

• standard example for concurrent programming
• two methods
• read reads and removes an item from the queue
• write puts an item on the queue
• a condition
• nonEmpty guard to prevent read from empty queue

11
Example 1 Serialized Queue (2)

• let queue
• (let nonEmpty condition()
• var q
• protected, serialized,
• write gt
• meth(s, elem)
• q q _at_ elem
• signal(nonEmpty)
• end

• read gt
• meth(s)
• watch nonEmpty
• until (q)gt0
• end
• let q0 q0
• q
• q1 for (q)-1
• q0
• end
• )

12
Network Objects
Namer
myObj
Site 2
Site 1
ObjA
ObjRemote
13
Example 2 Agent Migration (1)

• Idea
• write a program (agent)
• put it into a network
• disconnect
• agent travels around in the network and collects
  information
• come back online
• collect agent

14
Example 2 Agent Migration (2)

• Problem
• to collect information, an agent needs some kind
  of storage
• Obliq Solution
• one object can be attached to another
• active computations transmitted together with an
  evaluation stack (gt strong mobility)

15
Example 2 Agent Migration (3)
let state ... let agent proc(state, arg)
... end // migrate using a computation
engine let atSite1 net_importEngine(E1_at_Site1)
atSite1( proc(arg) agent(copy(state),
arg) end)
16
Example 2 Agent Migration (4)

• Before Migration
• After Migration

17
Conclusion

• consistent way of dealing with distribution
• at the cost of high traffic volume on network
• Obliq as testing suite for network-object
  libraries

18
Network Objects

• Server
• Client

net_export(obj, Namer, site1Obj)
let site1Obj net_import(obj,
Namer) site1Obj.opA(args) site3Obj.opB(site1O
bj)
19
Network Objects (2)

• Execution Engines
• Server
• Client

net_exportEngine(E1_at_Site1, Namer, arg)
let atSite1 net_importEngine(E1_at_Site1,
Namer) atSite1(proc(arg) 32 end)