HTN planning (Hierarchical Task Network)

Keywords: htn planning (hierarchical task network),htn planning,hierarchical task network,ai agent

HTN planning (Hierarchical Task Network) is a planning approach that decomposes high-level tasks into networks of subtasks hierarchically — using domain-specific knowledge about how complex tasks break down into simpler ones, enabling efficient planning for complex domains by exploiting task structure and procedural knowledge.

What Is HTN Planning?

• Hierarchical: Tasks are organized in a hierarchy from abstract to concrete.
• Task Network: Tasks are connected by ordering constraints and dependencies.
• Decomposition: High-level tasks are recursively decomposed into subtasks until primitive actions are reached.
• Domain Knowledge: Decomposition methods encode expert knowledge about how to accomplish tasks.

HTN Components

• Primitive Tasks: Directly executable actions (like STRIPS actions).
• Compound Tasks: High-level tasks that must be decomposed.
• Methods: Recipes for decomposing compound tasks into subtasks.
• Ordering Constraints: Specify execution order of subtasks.

HTN Example: Making Dinner

Compound Task: make_dinner

Method 1: cook_pasta_dinner
  Subtasks:
    1. boil_water
    2. cook_pasta
    3. make_sauce
    4. combine_pasta_and_sauce
  Ordering: 1 < 2, 3 < 4, 2 < 4

Method 2: order_takeout
  Subtasks:
    1. choose_restaurant
    2. place_order
    3. wait_for_delivery
  Ordering: 1 < 2 < 3

Planner chooses method based on context (time, ingredients available, etc.)

HTN Planning Process

1. Start with Goal: High-level task to accomplish.

2. Select Method: Choose decomposition method for current task.

3. Decompose: Replace task with subtasks from method.

4. Recurse: Repeat for each compound subtask.

5. Primitive Actions: When all tasks are primitive, plan is complete.

6. Backtrack: If decomposition fails, try alternative method.

Example: Robot Assembly Task

Task: assemble_chair

Method: standard_assembly
  Subtasks:
    1. attach_legs_to_seat
    2. attach_backrest_to_seat
    3. tighten_all_screws
  Ordering: 1 < 3, 2 < 3

Task: attach_legs_to_seat
Method: four_leg_attachment
  Subtasks:
    1. attach_leg(leg1)
    2. attach_leg(leg2)
    3. attach_leg(leg3)
    4. attach_leg(leg4)
  Ordering: none (can be done in any order)

Task: attach_leg(L)
  Primitive action: screw(L, seat)

HTN vs. Classical Planning

• Classical Planning (STRIPS/PDDL):
• Search: Searches through state space.
• Domain-Independent: General search algorithms.
• Flexibility: Can find novel solutions.
• Scalability: May struggle with large state spaces.

• HTN Planning:
• Decomposition: Decomposes tasks hierarchically.
• Domain-Specific: Uses expert knowledge in methods.
• Efficiency: Exploits task structure for faster planning.
• Constraints: Limited to decompositions defined in methods.

Advantages of HTN Planning

• Efficiency: Hierarchical decomposition reduces search space dramatically.
• Domain Knowledge: Encodes expert knowledge about how tasks are typically accomplished.
• Natural Representation: Matches how humans think about complex tasks.
• Scalability: Handles complex domains that classical planning struggles with.

HTN Planning Algorithms

• SHOP (Simple Hierarchical Ordered Planner): Total-order HTN planner.
• SHOP2: Extension with more expressive methods.
• SIADEX: HTN planner for real-world applications.
• PANDA: Partial-order HTN planner.

Applications

• Manufacturing: Plan assembly sequences, production workflows.
• Military Operations: Plan missions with hierarchical command structure.
• Game AI: Plan NPC behaviors with complex goal hierarchies.
• Robotics: Plan manipulation tasks with subtask structure.
• Business Process Management: Plan workflows with task decomposition.

Example: Military Mission Planning

Task: conduct_reconnaissance_mission

Method: aerial_reconnaissance
  Subtasks:
    1. prepare_aircraft
    2. fly_to_target_area
    3. perform_surveillance
    4. return_to_base
    5. debrief
  Ordering: 1 < 2 < 3 < 4 < 5

Task: prepare_aircraft
Method: standard_preflight
  Subtasks:
    1. inspect_aircraft
    2. fuel_aircraft
    3. load_equipment
    4. brief_crew
  Ordering: 1 < 2, 1 < 3, 4 < (all others complete)

Partial-Order HTN Planning

• Flexibility: Subtasks can be partially ordered — only specify necessary orderings.
• Advantage: More flexible than total-order plans — allows parallel execution.
• Example: attach_leg(leg1) and attach_leg(leg2) can be done in any order or in parallel.

HTN with Preconditions and Effects

• Hybrid Approach: Combine HTN decomposition with STRIPS-style preconditions and effects.
• Benefit: Ensures plan feasibility while exploiting hierarchical structure.
• Example: Check that preconditions are satisfied when selecting methods.

Challenges

• Method Engineering: Defining good decomposition methods requires domain expertise.
• Completeness: HTN planning may miss solutions not captured by defined methods.
• Flexibility: Limited to predefined decompositions — less flexible than classical planning.
• Verification: Ensuring methods are correct and complete is challenging.

LLMs and HTN Planning

• Method Generation: LLMs can generate decomposition methods from natural language descriptions.
• Task Understanding: LLMs can interpret high-level tasks and suggest decompositions.
• Method Refinement: LLMs can refine methods based on execution feedback.

Example: LLM Generating HTN Method

User: "How do I organize a conference?"

LLM generates HTN method:
Task: organize_conference
Method: standard_conference_organization
  Subtasks:
    1. select_venue
    2. invite_speakers
    3. promote_event
    4. manage_registrations
    5. arrange_catering
    6. conduct_conference
    7. follow_up
  Ordering: 1 < 3, 1 < 4, 2 < 6, 5 < 6, 6 < 7

Benefits

• Efficiency: Dramatically reduces search space through hierarchical decomposition.
• Knowledge Encoding: Captures expert knowledge about task structure.
• Scalability: Handles complex domains with many actions.
• Natural: Matches human problem-solving approach.

Limitations

• Method Dependency: Quality depends on quality of decomposition methods.
• Less Flexible: Cannot find solutions outside defined methods.
• Engineering Effort: Requires significant effort to define methods.

HTN planning is a powerful approach for complex, structured domains — it exploits hierarchical task structure and domain knowledge to achieve efficient planning, making it particularly effective for real-world applications where expert knowledge about task decomposition is available.

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