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README.md

task-planner

  1. Summary
  2. Dependencies
  3. Design principles
  4. Usage examples
  5. API description
    1. Planner API
    2. Knowledge base API
      1. KnowledgeBaseInterface
      2. Predicate
      3. Fluent
      4. PredicateParams
      5. AssertionTypes

Summary

Defines a package (task_planner) that exposes interfaces to a knowledge base and a task planner.

Only the Metric-FF planner is currently supported.

Parameters for the planner are specified in config/planner_config.yaml; in particular, the following parameters need to be specified:

  • planner_name: Name of the used planner
  • domain_file: Absolute path of a planning domain file
  • planner_cmd: Command used for running the task planner executable; the words "DOMAIN" and "PROBLEM" are expected to be in the command so that they can be appropriately replaced with the paths of domain and problem files
  • plan_file_path: Directory where generated plan files should be saved

Note: The package is developed for Python 3.5+ since it makes use of Python typing.

Dependencies

Design principles and assumptions

The task planner is based on the following assumptions:

  • A planner working with PDDL domains is used
  • The parameters of the domain predicates, fluents, and actions are explicitly typed
  • Only single-robot planning is done

The following main design principles were followed in the development of this package:

  • Knowledge is stored in a MongoDB database since having persistent storage of the knowledge makes it possible to recover from software failures
  • To simplify the use of domain predicates, fluents, and actions within the application, an appropriate mapping needs to be defined for these (see task_planner/knowledge_models.py for the mapping of fluents and predicates and task_planner/action_models.py for the mapping of actions)
  • The design of the knowledge base interface is based on mas_knowledge_base

Usage examples

Knowledge base

The knowledge base can be used by creating an instance of the task_planner.knowledge_base_interface.KnowledgeBaseInterface class:

from task_planner.knowledge_base_interface import KnowledgeBaseInterface
kb_interface = KnowledgeBaseInterface('ropod_kb')

Let's say we want to add and remove certain facts (for instance, we want to state that the robot "frank" is at the location "BRSU_L0_C1" and that the load "mobidik_123" is not at "BRSU_L0_C1" anymore); we can specify these facts as follows:

facts_to_add = [('robot_at', [('bot', 'frank'), ('loc', 'BRSU_L0_C1')])]
facts_to_remove = [('load_at', [('load', 'mobidik_123'), ('loc', 'BRSU_L0_C1')])]

Note that we represent a predicate as a tuple in which the first entry is the predicate name and the second is a list of predicate parameters ("name" and "value" pairs).

We can add and remove these facts from the knowledge base as follows:

kb_interface.insert_facts(facts_to_add)
kb_interface.remove_facts(facts_to_remove)

or with one function call:

kb_interface.update_kb(facts_to_add, facts_to_remove)

If the facts are fluents instead of predicate assertions (e.g. we want to insert or remove the fact that the robot "frank" is on the second floor), we can insert/remove them as follows:

fluents = [('robot_floor', [('bot', 'frank')], 2)]

# adding the fluents to the knowledge base
kb_interface.insert_fluents(fluents)

# or removing them from it
kb_interface.remove_fluents(fluents)

Note that a fluent is also represented as a tuple, but with three entries instead of two, such that the first entry is the fluent name, the second a list of predicate parameters ("name" and "value" pairs), and the third the fluent value.

Planning goals can be inserted/removed just as facts, only that the function calls change (insert_goals and remove_goals respectively). Note that only predicates can be inserted as planning goals.

Task planner

An example of using the MetricFF task planner to create a plan for a Mobidik transportation task is given below:

from task_planner.metric_ff_interface import MetricFFInterface

# reading the planner configuration parameters
domain_file = ''
planner_cmd = ''
plan_file_path = ''
with open('config/planner_config.yaml', 'r') as config_file:
    planner_config = yaml.load(config_file)
    domain_file = planner_config['domain_file']
    planner_cmd = planner_config['planner_cmd']
    plan_file_path = planner_config['plan_file_path']

planner = MetricFFInterface('ropod_kb', domain_file, planner_cmd, plan_file_path, debug=True)

# creating a task request
task_request = TaskRequest()
task_request.load_id = 'mobidik_123'
task_request.delivery_pose.id = 'BRSU_L0_C0'

robot_name = 'frank'

# creating the list of task goals
task_goals = [('load_at', [('load', task_request.load_id),
                           ('loc', task_request.delivery_pose.id)]),
              ('empty_gripper', [('bot', robot_name)])]

# requesting a plan
plan_found, plan = planner.plan(task_request, robot_name, task_goals)

API description

Planner API

The task planner exposes functionalities for creating a task plan given a task request and a robot task assignment. The package includes a generic interface definition - the abstract TaskPlannerInterface class in task_planner/planner_interface.py - as well as a specific implementation that uses the Metric-FF planner - the MetricFFInterface class in task_planner/metric_ff_interface.py.

TaskPlannerInterface

An abstract class containing the following fields:

  • kb_database_name: Name of a database for storing the knowledge base
  • domain_file: Absolute path of a planning domain file
  • domain_name: Name of the planning domain (extracted from the domain file)
  • planner_cmd: Command used for running a task planner; the words "DOMAIN" and "PROBLEM" are expected to be in the command so that they can be appropriately replaced with the paths of domain and problem files
  • plan_file_path: Directory where generated plan files should be saved
  • debug: A Boolean indicating whether to run the planner in debug mode (thus providing more detailed debugging output)

The following abstract methods are declared in the interface:

  • plan: Returns a list of ropod.structs.action.Action objects representing a task plan for a task request and robot
  • generate_problem_file: Generates a PDDL problem file given a list of predicate and fluent assertions and task goals
  • parse_plan: Parses a generated plan from a file. Returns a tuple of type Tuple[bool, list], the first entry of which indicates whether the plan was found and the second of which is a list of ropod.structs.action.Action objects (an empty list if no plan was found)
  • process_action_str: Converts an action string read from a plan file to a ropod.structs.action.Action object

Knowledge base API

The knowledge base API defines various functionalities for working with a knowledge base and a planning domain. The primary interface for the knowledge base is the KnowledgeBaseInterface class in [task_planner/knowledge_base_interface.py], which allows inserting, retrieving, and removing positive assertions (both predicate and fluent assertions), as well as inserting and removing planning goals.

We distinguish between predicates, (numerical) fluents, and planning goals in the knowledge base. A predicate p is defined as a binary function p(x_1, ..., x_n) with named parameters; a fluent f is also a function f(x_1, ..., x_n) with named parameters, but can take a numerical value. Only predicates are supported as planning goals.

The interface uses custom-defined Python objects for working with predicates and fluents (see Predicate and Fluent below); however, to reduce the code verbosity for users of the interface, the methods for inserting and removing predicates/fluents/planning goals take tuple rather than the custom objects:

  • A predicate tuple has two entries, the first representing the name of the predicate and the second a list of ("name", "value") pairs for the predicate parameters; in other words p(x_1, ..., x_n) is equivalent to ('p', [('name-x_1', 'value-x_1'), ..., ('name-x_n', 'value-x_n')]) in tuple form
  • A fluent tuple takes three entries, the first representing the name of the predicate, the second a list of ("name", "value") pairs for the predicate parameters, and the third the fluent value; in other words f(x_1, ..., x_n) = k is equivalent to ('f', [('name-x_1', 'value-x_1'), ..., ('name-x_n', 'value-x_n')], k) in tuple form

KnowledgeBaseInterface

The knowledge base interface exposes functionalities for working with a knowledge base, such as inserting, retrieving, and removing assertions (both predicate and fluent assertions), as well as inserting and removing planning goals.

The following methods are exposed by the interface:

  • get_predicate_names: Returns a list with the names of all predicates stored in the knowledge base
  • get_fluent_names: Returns a list with the names of all fluents stored in the knowledge base
  • get_predicate_assertions: Returns a list of Predicate objects representing all assertions of a given predicate in the knowledge base. If no predicate name is given, returns all predicate assertions in the knowledge base
  • get_fluent_assertions: Returns a list of Fluent objects representing all fluent assertions in the knowledge base
  • get_fluent_value: Returns the value of a given fluent in the knowledge base (the fluent is passed as a tuple). Returns None if an assertion for the fluent is not found
  • update_kb: Inserts a list of facts (predicate assertions) into the knowledge base and removes a list of facts (also predicate assertions) from it. The predicate assertions are expected to be passed as tuples
  • insert_facts: Inserts a list of facts (predicate assertions) into the knowledge base. The facts are expected to be passed as tuples
  • remove_facts: Removes a list of facts (predicate assertions) from the knowledge base. The facts are expected to be passed as tuples
  • update_predicate: Updates a given predicate. The predicate is expected to be passed as a tuple
  • insert_fluents: Inserts a list of fluents (fluent assertions) into the knowledge base. The fluents are expected to be passed as tuples
  • remove_fluents: Removes a list of fluents (fluent assertions) from the knowledge base. The fluents are expected to be passed as tuples
  • update_fluent: Updates a given fluent. The fluent is expected to be passed as a tuple
  • insert_goals: Inserts a list of goals (predicate assertions) into the knowledge base. The goals are expected to be passed as tuples
  • remove_goals: Removes a list of goal (predicate assertions) from the knowledge base. The goals are expected to be passed as tuples

Predicate

Predicate is a class representing a predicate p(x_1, ..., x_n). The class has the following two fields:

  • name: The name of the predicate
  • params: A list of PredicateParams objects representing the predicate parameters

The following methods are exposed by the Predicate class:

  • to_dict: Converts a Predicate object to a dictionary with two keys - "name" and "params", where the value of "params" is a list of PredicateParams dictionaries
  • from_tuple (static): Creates a Predicate object from a given tuple
  • from_dict (static): Creates a Predicate object from a given predicate dictionary (in the form returned by to_dict)
  • __eq__: The comparison operator is overridden for comparing two Predicate objects; returns True if both the names and all parameters are the same

Fluent

The Fluent class represents a fluent f(x_1, ..., x_n) = k. The class has the following three fields:

  • name: The name of the fluent
  • params: A list of PredicateParams objects representing the fluent parameters
  • value: The value taken by the fluent at the current time instant

The following methods are exposed by the Fluent class:

  • to_dict: Converts a Fluent object to a dictionary with three keys - "name", "params", and "value", where the value of "params" is a list of PredicateParams dictionaries
  • from_tuple (static): Creates a Fluent object from a given tuple
  • from_dict (static): Creates a Fluent object from a given predicate dictionary (in the form returned by to_dict)
  • __eq__: The comparison operator is overridden for comparing two Fluent objects; returns True if both the names and all parameters are the same

PredicateParams

PredicateParams represents an argument of a predicate or a fluent, namely the argument's name and value. The following fields are defined in the class:

  • name: The name of an argument
  • value: The argument value

The following methods are exposed by the PredicateParams class:

  • to_dict: Converts a PredicateParams object to a dictionary with two keys - "name" and "value"
  • from_tuple (static): Creates a PredicateParams object from a given tuple
  • from_dict (static): Creates a PredicateParams object from a given predicate dictionary (in the form returned by to_dict)
  • __eq__: The comparison operator is overridden for comparing two PredicateParams objects; returns True if both the names and values of two parameters are the same
  • __neq__: The comparison operator is overridden for comparing two PredicateParams objects; returns True if either the names or the values of two parameters differ

AssertionTypes

A class defining constants for working with assertions. The following two constants are defined in the class:

  • PREDICATE = 'predicate'
  • FLUENT = 'fluent'
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