EUROPA An open source platform for Planning, Scheduling and Constraint Programming

1. EUROPA An open source platform for Planning, Scheduling and Constraint Programming Javier Barreiro

2. Talk Outline • EUROPA Overview • Brief History • Overview of EUROPA architecture and modules • What's new in EUROPA 2.1 • Client Interface • Interpreted NDDL • Constraint Violation reporting • UI components • Open source release and EUROPA portal • What's next for EUROPA • Road Map • Using / Contributing to EUROPA

3. EUROPA – Brief History • Timeline • HSTS (1998) - Initial DDL and planning paradigm. • RAX (1999) – first deployment • EUROPA (2002)‏ - Initial implementation of current approach • EUROPA 2 (2005)‏ - Modular architecture, more robust implementation • EUROPA 2.1 (2007) - Open source, architecture evolution. ‏ • Project Sample • Science activity planning • Airborne observatory SOFIA • Remote Agent Experiment (RAX)‏ • Mars Exploration Rovers(MER)‏ • Phoenix Mars Mission. • Power Systems Control of the International Space Station through the Solar Array Constraint Engine (SACE). • Planning and scheduling support for experiments • Life in the Atacama (LITA)‏ • Bed Rest study.

4. EUROPA - Overview • EUROPA can be used to model and solve problems in : • Constraint Satisfaction : See Nqueens example • Scheduling : See RCPSP Example • Planning : See Rover Example • Combinatorial Optimization • Typical EUROPA workflow : • Model Problem • Create/Invoke Solver • Modify problem, iterate. • Embed model/solver into larger application • See Light Switch Example

5. EUROPA - Overview Plan Works PSUI User Apps Client API C++ Java ... Temporal Network Resources Modeling Languages NDDL ANML ... Constraint Engine Plan Database Rules Engine Solver

6. Constraint Engine NDDL // Domains typedef int [0 8] Queen; typedef double [0.0 100.0] myRange; // Variables Queen q1,q2, ... , q8; // Constraints neq(q1,q2); allDiff(q1,q2, ... , q8); • Domains • int,float,bool,string,object • Enumeration, Range • Variables • Have base and current domain • Constraints • Built-in constraint library (42 constraints, see EUROPA portal)‏ • Constraint Propagation • Each constraint has a propagator • Constraint Engine allows propagators to perform domain reduction

7. Plan Database NDDL class LightSwitch extends Timeline { predicate turnOn { duration=1; } predicate turnOff { duration=1; } } class LightBulb extends Timeline { LightSwitch mySwitch_; LightBulb(LightSwitch b) { mySwitch_ = b; } predicate On {} predicate Off {} } • Classes • Inheritance • Constructor • Objects • Contain Tokens, Variables, other objects • Tokens • Object-scoped • Temporal entities (start, end, duration). Typically represent activities or states. • Variables • Object attribute • Token parameter • Global variable

8. Rules Engine NDDL LightBulb::On { // Bulb must be Off to be turned On met_by(object.Off); met_by(object.mySwitch_.turnOn); } LightBulb::Off { // Bulb must be On to be turned Off met_by(object.On); // Must be turned off through the switch met_by(object.mySwitch_.turnOff); } goal(LightBulb.Off g1); eq(g1.start,10); • Subgoals • Token Master-Slave relationships • Allen Relations • before, after, meets,met_by,equals, contains, contained_by, paralleled_by, parallels, starts, ends, ends_after, ends_before, ends_after_start, starts_before_end, starts_during, contains_start, ends_during, contains_end, starts_after, starts_before, any • Token Activate-Merge-Reject mechanisms

9. Temporal Network • Temporal Constraints are mirrored in Distance Graph • temporalDistance, precedes, concurrent • Negative cycle detection provides earlier conflict detection NDDL activity4.start, [ -inf 25 ], activity9.start ); temporalDistance( activity10.start, [ 5 +inf ], activity11.start ); precedes(0,activity0.start); precedes(activity0.end,maxDuration); precedes(0,activity1.start); precedes(activity1.end,maxDuration); mporalDistance(

10. Resources • Multiple Resource types • Reservoirs : Independent produce/consume • Reusables : Matching produce/consume • Unary : Reusable specialization • Profile Envelope Computation • Resource Profile internally transformed into flow network representation. Efficient Max Flow computation provides earlier conflict detection NDDL class Allocation { int m_actId; float m_qty; Reusable m_resource; Allocation( Reusable r, int id, float qty) { m_actId = id; m_resource = r; m_qty = qty; } } class ProblemInstance { predicate Activity { int m_actId; } } ProblemInstance::Activity { Allocation allocations; eq( allocations.m_actId, m_actId ); foreach( a in allocations ) { equals( a.m_resource.uses u ); eq( u.quantity, a.m_qty ); } }

11. Built-in Solver • Flaws • Open Condition • Unbound Variable • Threat • XML-Configuration • Flaw Managers • Flaw Filters • Priorities • This is just one kind of solver that can be built using EUROPA Chronological Backtracking algorithm Result plan(PlanDatabase db, FlawManagers fms) { if(!isConsistent(db)) return INITIALLY_INCONSISTENT; Stack s = {}; DecisionPoint dp = selectDecisionPoint(db, fms); // Uses priorities, filters. while(dp != NULL) { while(dp.hasNext()) { dp.tryNext(); if (!isConsistent(db)) dp.undo(); } if (!isConsistent(db)) { // Exhausted choices if(s.isEmpty() return SEARCH_EXHAUSTED; else dp = backtrack (s); } else { s.push(dp); dp = selectDecisionPoint(db, fms); } } return PLAN_FOUND; }

12. Toolkit • Plan Works (see EUROPA portal)‏ • PSUI Components • Gantt • Resource Chart • Table views • Mouse Listeners

13. What's new in EUROPA 2.1 • What's new in EUROPA 2.1 • Client Interface • Interpreted NDDL • Constraint Violation reporting • UI components • An example to tie everything together : RCPSP Benchmarks • Open source release and EUROPA portal

14. What's New – Client Interface Plan Works PSUI User Apps Client API C++ Java ... Temporal Network Resources Modeling Languages NDDL ANML ... Constraint Engine Plan Database Rules Engine Solver

15. What's New – Client Interface • EUROPA packaged as an engine • Clean startup/shutdown • Standard pieces all preconfigured (constraint library, built-in data types, etc)‏ • User can still do fine-grained configuration, but only if desired • PSEngine interface • Defined in C++. • Using SWIG makes it easy to map to Java, Python, C#, LISP, and many others. Only Java binding provided with release for now. • SACE, A4O already using it. • Assembly interface still supported • Has been migrated to package EUROPA as an engine • Hard to map to SWIG • Exposes internal interfaces • Will eventually be unified with PSEngine

16. What's New – Client Interface PSVariable PSVarType getType(); PSEntity* getParent(); bool isSingleton(); bool isInterval(); bool isEnumerated(); PSVarValue getSingletonValue(); double getLowerBound(); double getUpperBound(); PSList<PSVarValue> getValues(); void specifyValue(PSVarValue& v); void reset(); double getViolation() const; std::string getViolationExpl() const; PSEngine PSObject string getObjectType() const; PSList<PSVariable*> getMemberVariables(); PSVariable* getMemberVariable(name); PSList<PSToken*> getTokens(); Constraint Engine PSVariable* getVariableByKey/Name(key/name)‏ bool/void get/setAutoPropagation() const bool propagate()‏ bool/void get/setAllowViolations() const double getViolation() const string getViolationExpl() const PSToken bool isFact(); PSObject* getOwner(); PSToken* getMaster(); PSList<PSToken*> getSlaves(); PSTokenState getTokenState() const; PSVariable* getStart/End/Duration(); double getViolation() const; std::string getViolationExpl() const; PSList<PSVariable*> getParameters(); PSVariable* getParameter(name); void activate(); void reject(); void merge(PSToken* activeToken); void cancel(); Plan Database PSList<PSObject*> getObjectsByType(type) PSObject* getObjectByKey/Name(key/name) PSList<PSToken*> getTokens() PSToken* getTokenByKey(PSEntityKey key) PSList<PSVariable*> getGlobalVariables()‏ PSResource ResourceProfile getLimits/Levels()‏ PSSolver void step(); void solve(int maxSteps,int maxDepth); void reset(); PSList<std::string> getFlaws(); bool isExhausted(); bool isTimedOut(); bool isConstraintConsistent(); Solver PSSolver* createSolver(config)‏

17. What's new – Interpreted NDDL • Interpreted NDDL • Code generation still supported • NDDL supported in java, NDDL-XML supported in C++. • Significant time savings during development. • Allows model run-time modification/generation. This is useful for application and tool development. • Small performance penalty at object creation (<10%)‏ • Currently used by SACE, Bed Rest, A4O.

18. What's new – Constraint Violation Reporting • API • Enabled by calling ConstraintEngine::setAllowViolation(true)‏ • GetViolation() and GetViolationExpl()‏ • Granularity • Constraint Engine – Constraint – Variable • Token – Variable • See NQueens example

19. What's New – UI Components • Pre-packaged components for debugging/visualization • PSDesktop application • Drive EUROPA interactively through Java and NDDL listeners • Solver • Gantt • Resource Profile chart • Table views for Tokens/Objects

20. Example – RCPSP solver • An Example to put everything together. Solving the RCPSP • Minimize project makespan subject to temporal and resource constraints • Motivation : STAR, Crew Planning, Bed Rest. • NDDL generated from UBO benchmark problems (size 10,20,50, test sets available up to 1000 activities)‏ • PSUI used to visualize and debug solvers • Constraint Violation Reporting used to support search

21. UBO Benchmarks – Built-in Solver • NDDL generation • Problem instances auto generated from UBO files • See NDDL model and UI • Simple configuration <SAVHThreatManager order="most,earliest,lower"> <FlawHandler component="SAVHThreatHandler" filter="both" /> </SAVHThreatManager> • Scales poorly % Solved Avg Time (secs)‏

22. UBO Benchmarks – IFIR Solver • IFlatIRelax • Always respect temporal constraints. Iteratively add and remove precedence constraints to fix resource violations. • Record no-goods to deal with min/max temporal distance constraints • Custom solver written using PSEngine. Leverages Constraint Engine, Temporal Network, Resources. % Solved Avg Time in secs

23. UBO Benchmarks – Hybrid Solver • Local + Exhaustive Search • Use Local Search to quickly find good solution • Use exhaustive search to improve on good solution. Strong bound and typical clustering of good solutions should help • 2 Versions of Exhaustive search • Start from scratch and use IFIR solution as oracle • Start from IFIR solution, removing N precedences. • In both cases, IFIR solution used as strong upper bound % Solved Avg Time (secs)‏

24. UBO Benchmarks – Wrap up • Easy to use tool stack • EUROPA modules, API, SWIG-generated binding, UI tools • Real-life problems normally require • Interleaving search, inference and relaxation • composition/interleaving of solvers • Adding RCPSP solver to Built-in solvers • Improve branch selection (mine existing B&B solvers)‏ • See if breaking temporal constraints leads to faster initial solution % Solved within 10% of BKB Avg Time (secs) for Solved Problems

25. What's new – Open Source • Available under NOSA since Oct-07 • Release 2.1.0 made available to small group in Nov-07 • Release 2.1.1 announced more widely 1/14/08 • EUROPA portal contains everything • Code, docs, examples, publications, road map. • External europa-users group created Jan-08 • 34 non-NASA members as of 1/30/08 • MBARI already using EUROPA for AUV

26. Europa Road Map • Drivers • Component-based Architecture with state of the art components • Ease of Use : Creating, debugging, embedding. • Industrial Strength : Fast, Robust, Easy to deploy and manage.

27. EUROPA Road Map • Search support • Refine solver API to facilitate composition and extensions • Expose flaws more cleanly in solver client interface • Smarter • Provide hooks/implementation for back-jumping, dynamic backtracking • Domain-independent heuristics from (Smith & Bernardini)‏ • Explicit support for relaxations (SIMPL)‏ • Easier : • Solver checkpoints (Comet)‏ • Faster • more efficient backtracking implementation

28. EUROPA Road Map • Architecture • Refine PSEngine interface • Plug-in architecture • Refine module interfaces (extension points)‏ • Debug/Visualization tools • Improve PSUI (auto-refresh, improve gantt, table components)‏ • Provide better built-in search visualization tools • Industrial strength • Add systematic performance measurement and profiling. Model and run Benchmark problems from CP, Scheduling and Planning. • Windows support • Continuous Integration server • Standard logging using Log4Cpp • Binary release

29. EUROPA Road Map • Modeling • Streamline Plan Database so that parsers/interpreters can be supported directly by it. • NDDL • move parser to C++ • add infix notation for constraints • add new kinds of propagators (state, setup, etc)‏ • ANML • Plan Database and search (native POCL solver?) extensions needed to fully support it • PDDL (through ANML?)‏

30. THANKS!