Foundations of Constraint Processing, Spring 2009

April 9, 2009 Weak Commitment 1

Foundations of Constraint Processing

CSCE421/821, Spring 2009: www.cse.unl.edu/~choueiry/S09-421-821/

Berthe Y. Choueiry (Shu-we-ri)

Avery Hall, Room 360

choueiry@cse.unl.edu

Tel: +1(402)472-5444

Least Commitment

Rationale & Strategies

Foundations of Constraint Processing, Spring 2009

April 9, 2009 Weak Commitment 2

Background

• Constraint propagation– Reduces problem size – Eliminates inconsistent choices– Gets the problem closer to being solved– But does not eliminate any solutions– When it solves the problem, we keep all solutions

• Search– Solves the problem by making decisions– At each step, decisions ignore some perfectly acceptable

choices– But search focuses on finding one solution

Foundations of Constraint Processing, Spring 2009

April 9, 2009 Weak Commitment 3

Generalizing..

• Constraint propagation makes no commitment at all

• Search makes strong commitments

• Question: is there anything in the middle?

• Answer: least commitment – Common: planning, scheduling communities

Foundations of Constraint Processing, Spring 2009

April 9, 2009 Weak Commitment 4

Least commitment: rationale

• As long as you do not need to make a commitment, keep propagating

• If propagation is ‘stuck’ (no progress), – then make the least commitment you need to make – in order to enable more propagation – without ruling out ‘too’ many solutions

• Technique– typically by adding a constraint (which is weaker than

a variable assignment)

Foundations of Constraint Processing, Spring 2009

April 9, 2009 Weak Commitment 5

Example: scheduling

• Context: – scheduling tasks in time. You notice the 2 tasks must use the

same resource

• Propagation – adds a mutex constraint between both tasks without ordering

them

• Search – fixes the time for one or both tasks thus ruling out so many

possibilities

• Least commitment – adds a constraint committing one task to be before the other– tasks remain floating in time – and any task can be inserted between them