Partial Order Plan Execution Slides with help from

Outline • Plan execution • Plan monitoring

Execution Start Can perform a step when all predecessors have been executed. At(Home) Sells(HWS,

Plan Execution Initialize the agenda, called Ready, with action Start Mark all actions as

Outline • Partial order planning in a nutshell • Plan execution • Plan monitoring

Action Monitoring Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) At(Home) Fail if

Plan Execution with Action Monitoring Initialize the agenda, called Ready, with action Start Mark

Execution Monitoring Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) • Check if

Plan Execution with Execution Monitoring Initialize agenda Ready with action Start Initialize agenda Active.

Plan Execution with Execution Monitoring (cont). Loop • … • Mark a as executed.

N IO PL UT AN NI EC EX Self-commanding Self-diagnosing Self-repairing NG Autonomous Agents:

Outline • Plan execution • Plan monitoring • Appendix: Partial order planning in a

Simple Operator-based Planning What assumptions are implied? • Input – Initial state of world

STRIPS Representation • Initial state: • ((block a) (block b) (block c) (on-table a)

Action Schemata • Instead of defining: pickup-A and pickup-B and … • Define a

A Solution: Complete and Consistent Plan • Complete Plan IFF every precondition of every

POP(<A, O, L>, agenda, actions) • <A, O, L>, A partial plan to expand

POP(<A, O, L>, agenda, actions) 1. 2. 3. 4. 5. 6. Termination: If agenda

Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) At(x) At(Home) Go(SM) Go(HWS) At(HWS)

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Partial Order Plan Execution Slides with help from: Dan Weld Stuart Russell & Peter Norvig 6/16/2021 Brian C. Williams 16. 412 J/6. 834 J Sept. 16 th, 2002 1

Outline • Plan execution • Plan monitoring

Execution Start Can perform a step when all predecessors have been executed. At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) At(Home) Go(HWS) At(HWS) Sells(HWS, Drill) Buy(Drill) At(HWS) Go(SM) At(SM), Sells(SM, Milk) At(SM), Sells(SM, Ban. ) Buy(Milk) Buy(Ban. ) At(SM) Go(Home) Have(Milk) At(Home) Have(Ban. ) Have(Drill) Finish

Plan Execution Initialize the agenda, called Ready, with action Start Mark all actions as not executed. Loop • If Ready is empty then terminate. • Dequeue action a from Ready and execute • When completed, mark a as executed. • For each action b such that a < b or linked(a, b, p). – If every action c is marked executed, such that c < b or linked(c, b, p’) – Then queue b on Ready.

Outline • Partial order planning in a nutshell • Plan execution • Plan monitoring

Action Monitoring Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) At(Home) Fail if preconditions Go(HWS) of ready action are not satisfied. At(HWS) Sells(HWS, Drill) Buy(Drill) At(HWS) Go(SM) At(SM), Sells(SM, Milk) At(SM), Sells(SM, Ban. ) Buy(Milk) Buy(Ban. ) At(SM) Go(Home) Have(Milk) At(Home) Have(Ban. ) Have(Drill) Finish

Plan Execution with Action Monitoring Initialize the agenda, called Ready, with action Start Mark all actions as not executed. Loop • If Ready is empty then terminate. • Dequeue action a from ready • If preconditions of action are satisfied – Then execute – Else return failure • When completed, mark a as executed. • For each action b such that a < b or linked(a, b, p). – If every action c has been executed, such that c < b or linked(c, b, p’) – Then queue b on Ready.

Execution Monitoring Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) • Check if precondition of remaining plan is violated. At(Home) Go(HWS) At(HWS) Sells(HWS, Drill) • Check if causal link crossing current time is violated. Buy(Drill) At(HWS) Go(SM) At(SM), Sells(SM, Milk) At(SM), Sells(SM, Ban. ) Buy(Milk) Buy(Ban. ) At(SM) Go(Home) Have(Milk) At(Home) Have(Ban. ) Have(Drill) Finish

Execution Monitoring Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) • Check if precondition of remaining plan is violated. • Check if causal link crossing current time is violated. At(Home) Go(HWS) At(HWS) Sells(HWS, Drill) Buy(Drill) At(HWS) Go(SM) At(SM), Sells(SM, Milk) At(SM), Sells(SM, Ban. ) Buy(Milk) Buy(Ban. ) At(SM) Go(Home) Have(Milk) At(Home) Have(Ban. ) Have(Drill) Finish

Plan Execution with Execution Monitoring Initialize agenda Ready with action Start Initialize agenda Active. Links to empty Mark all actions as not executed. Loop • If Ready is empty then terminate. • For each link on Active. Links – If the condition of link doesn’t hold, Then return failure • Dequeue action a from Ready • If preconditions of action are satisfied – Then execute – Else return failure • …

Plan Execution with Execution Monitoring (cont). Loop • … • Mark a as executed. • For each action c such that linked(c, a, p). – dequeue <c, a, p> from Active. Links. • For each action d such that linked(a, d, p). – queue <a, d, p> on Active. Links. • For each action b such that a < b or linked(a, b, p). – If every action c has been executed, such that c <b or linked(c, b, p’) – Then queue b on Ready.

Outline • Plan execution • Plan monitoring

N IO PL UT AN NI EC EX Self-commanding Self-diagnosing Self-repairing NG Autonomous Agents: What is missing? RECOVERY Monitors Command dispatch Fault protection Attitude control Mission Goal Scenario

Outline • Plan execution • Plan monitoring • Appendix: Partial order planning in a nutshell

Simple Operator-based Planning What assumptions are implied? • Input – Initial state of world – Goal • Atomic time. • partial state – Operators (actions) • Fn: world-state • Output • Agent is sole cause of change. – Sequence of actions north 11 a W 0 a W 1 north 12 • Agent is omniscient (no sensing necessary). a W 2 • Actions have deterministic effects. • No indirect effects. ] STRIPS Assumptions

STRIPS Representation • Initial state: • ((block a) (block b) (block c) (on-table a) (on-table b) (clear a) (clear b) (clear c) (arm-empty)) • goal (partial state): • ((on a b) (on b c))) • Available actions • Strips operators a W 0 north 11 a W 1 precond: (and (agent-at 1 1) (agent-facing north)) North 11 effect: (and (agent-at 1 2) (not (agent-at 1 1))) • Effects specify how to change the set of propositions.

Action Schemata • Instead of defining: pickup-A and pickup-B and … • Define a schema: ? var denotes a free variable (: operator pick-up : parameters ((block ? ob 1)) : precondition (and (clear ? ob 1) (on-table ? ob 1) (arm-empty)) : effect (and (not (clear ? ob 1)) (not (on-table ? ob 1)) (not (arm-empty)) (holding ? ob 1))) } No all te: yo ow stri u m de ps us rive doe t b d sn e c eff ’t om ect pl s; ete !

A Solution: Complete and Consistent Plan • Complete Plan IFF every precondition of every step is achieved Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) At(Home) Go(HWS) At(HWS) Sells(HWS, Drill) A step’s precondition is achieved iff • its the effect of a preceding step, • no possibly intervening step At(SM), Sells(SM, Milk) undoes it. Buy(Drill) At(HWS) Go(SM) At(SM), Sells(SM, Ban. ) Buy(Milk) • Consistent Plan IFF there is no contradiction in the ordering constraints (I. e. , never si < sj and sj < si. ) Buy(Ban. ) At(SM) Go(Home) Have(Milk) At(Home) Have(Ban. ) Have(Drill) Finish

POP(<A, O, L>, agenda, actions) • <A, O, L>, A partial plan to expand • Agenda: A queue of open conditions still to be satisfied: <p, aneed > • Actions: A set of actions that may be introduced to meet needs. • aadd: an action that produces the needed condition p for aneed • Athreat : an action that might threaten a causal link from aproducer to aconsumer

POP(<A, O, L>, agenda, actions) 1. 2. 3. 4. 5. 6. Termination: If agenda is empty, return plan <A, O, L>. Goal Selection: select and remove open condition <p, aneed > from agenda. Action Selection: Choose new or existing action aadd that can precede aneed and whose effects include p. Link and order actions. Update Agenda: If aadd is new, add preconditions to agenda. Threat Detection: For every action athreat that might threaten some causal link from aproduce to aconsume, choose a consistent ordering: a) Demotion: Add athreat < aproduce b) Promotion: Add aconsume < athreat Recurse: on modified plan and agenda Choose is nondeterministic Select is deterministic

Start At(Home) Sells(HWS, Drill) Sells(SM, Milk) Sells(SM, Ban. ) At(x) At(Home) Go(SM) Go(HWS) At(HWS) Sells(HWS, Drill) Buy(Drill) At(SM), Sells(SM, Milk) At(SM), Sells(SM, Ban. ) Buy(Milk) Buy(Ban. ) At(SM) Go(Home) Have(Drill) Have(Milk) Have(Ban. ) at(Home) Finish