Mobile Agent Programming Concepts

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Mobile Agent ProgrammingConcepts

• FK

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Dimensions of mobility

• Moving between different execution servers
  (processes, hosts)
• Moving in time
• Moving with the user (execution server)
• Moving between contexts

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Properties of agent execution environments

• Static network (fixed host network address)
• Roaming network (dyn. host netw. address)
• Reliable real-time (stationary server computer)
• Intermittent real-time (laptop, iPhone)
• True persistence (Hard-disc, NV-RAM)
• Volatile persistence (battery RAM)

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Properties of agent execution environments
Dynamic network address Intermittent
realtime Volatile persistence
Static network address Reliable realtime True
persistence
Dynamic network address Intermittent
realtime True persistence
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Properties of agent execution environments
Dynamic
Cell phones
Early wearables
Network address
Laptops
Intermittent
Real-time
Servers
Static
Persistence
Volatile
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Dynamic network address

• Difficult to find a particular host
• Needs registration at a well-known name
• LAN local discovery techniques (eg Jini)
• WAN remote interaction (eg SIP)
• Network Address Translation (NAT) imposes similar
  problem
• IPv4 ? IPv6 needed but costly

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Intermittent real-time

• If the agent is suspended externally, eg by power
  management, it may not see the suspension.
• Can the agent control power mgmt when it needs
  to?
• Small devices (mobile phones) may be suspended
  without warning.

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Volatile persistence

• How much of the present state must be saved?
• To where can it be saved?
• How to (re)establish state after reanimation?
• How can the saved state be found?
• Time (how long was the agent asleep?)

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Planned discontinuity (1)
2. Set timer
3. Sleep
1. Save state
4. Wake up
5. Restore state
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Planned discontinuity (2)
3. External event
2. Terminate
1. Save state
4. Restart
5. Restore state
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Planned discontinuity (3)
1. Negotiate hosting
3. Terminate
2. Save state
6. Restore state
5. Restart
4. Transfer identity and state
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Mobile code (1)
Initial execution environment
Next execution environment
1. Locate exec. env.
2. Negotiate hosting
3. Transfer identity and state
5. Continue
4. Terminate
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Mobile code (2)
Initial execution environment
Next execution environment
1. Locate exec. env.
2. Negotiate hosting
3. Transfer identity and state
5. Continue
6. Acknowledge
7. Terminate
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What do we mean by state?

• Memory and cpu configuration of process
• Values of active variables (including call stack)
  and the execution point
• A classifiable subset of the true state

switch (myState) case 0 / state 0 /
break case 1 / state 1 / break //
etc...
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State (contd)

• For the programmer, state and code are often the
  same concept
• Mobile code (e.g. Java) may not be able to
  transfer the execution point
• This breaks the code-state relationship

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State (contd)
y1
START
y2
START
y y2
Migrate?
Yes
y y2
No
z y
Migrate?
z2
No
Variables evolve while code point is reset
z y
z4
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Goal-driven execution

• The task can be expressed as a sequence of
  discrete steps
• The accomplishment of each step can be tested
• What to do next is computable

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Goal-driven execution (contd)
Task Deliver a message (to a user) The message
is executable and must run on the recipients
current host.
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Goal-driven execution
while (true) do if (the current host is the user
host) then execute the message exit else lo
cate the user host move to user host endif done
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Goal-driven execution

• Gets complicated quite fast, e.g. remember
  failures, responses, a dialogue...
• State must be knitted into the task resolution
  structure
• Goal stack approach

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Goal stack interpreter

• Useful for intermittent real-time (wearable
  devices, power management)
• The goal stack is maintained as a data structure
• The goal on top of the stack is either replaced
  by a sequence of subgoals, or executed as an
  idempotent unit until it succeeds

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Goal stack interpreter
Execute goal-code
Goal expansion
Establish host of Ralph
Y
Fail?
Move to Ralphs host
Execute message code
Deliver message to Ralph
N
Pop stack
N
Empty stack?
Move commanded by external events has small
impact on top-most goal.
Y
Establish host of Ralph
Stop
Move to Ralphs host
Execute message code
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Goal stack interpreter
Execute goal-code
Pop
Establish host of Ralph
Y
Fail?
Move to Ralphs host
Move to Ralphs host
Execute message code
Execute message code
N
Pop stack
N
Empty stack?
Current goal
Y
Current goal
Stop
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Layers of abstraction
Task
Transport
Discovery
Communication
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Layers of abstraction
Task
Code for identifying and approaching the goal.
Transport
Discovery
Communication
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Layers of abstraction
Code for moving the agent between places of
execution.
Task
Code for identifying and approaching the goal.
Transport
Discovery
Communication
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Layers of abstraction
Code for moving the agent between places of
execution.
Task
Code for identifying and approaching the goal.
Transport
Discovery
Methods for finding (the addresses) to resources.
Communication
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Layers of abstraction
Code for moving the agent between places of
execution.
Task
Code for identifying and approaching the goal.
Transport
Discovery
Methods for finding (the addresses) to resources.
Sending the bits of the agent to another place.
Communication
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Agent enabling middleware

• Discovery mechanisms (execution hosts, other
  agents, negotiation partners ...)
• Addressing namespaces (IP, URLs, SIP, RMI)
• Communication means (netw. stack)
• Protocol (language and semantics)

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Game of Tag for agents

• Discover execution environments
• Query properties of the exec. env.
• Locate other agents
• Negotiate for status (being it or not)
• Transaction (reliably transferring it between
  players)
• Multiple roles and tasks dep. on being it.

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TAG player outline
Acquire state of game
Decide what to do
Execute choice
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TAG player outline
Locate all Bailiffs and players
Am I it?
YES
NO
YES
NO
YES
NO
Is the it player here?
Is there a player here?
Tag a player
Move to a Bailiff with players
Move to another Bailiff
Move or wait according to strategy
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TAG player outline
Locate all Bailiffs and players
Am I it?
YES
NO
YES
NO
YES
NO
Is the it player here?
Is there a player here?
Tag a player
Move to a Bailiff with players
Move to another Bailiff
Move or wait according to strategy
State information must be updated
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TAG player outline
Locate all Bailiffs and players
Am I it?
YES
NO
YES
NO
YES
NO
Is the it player here?
Is there a player here?
Tag a player
Move to a Bailiff with players
Move to another Bailiff
Move or wait according to strategy
Seek to be lonely Seek company Act randomly
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TAG player outline
Seek to be lonely Seek company Act randomly
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TAG player outline
Seek to be lonely Seek company Act randomly
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TAG player outline
Seek to be lonely Seek company Act randomly
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TAG player outline

• The lonely strategy
• increases the chance that the it player goes to
  another Bailiff
• makes the player faster
• needs as many Bailiffs as there are players, but
  no more
• with fewer Bailiffs it approaches the company
  strategy
• The company strategy
• increases the chance that the it player tags
  another player
• needs fewer Bailiffs
• makes the player slower

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End