1. Rationale & Strategies Foundations of Constraint Processing
Least Commitment
Rationale & Strategies
Foundations of Constraint Processing
CSCE421/821, Fall 2015
Berthe Y. Choueiry (Shu-we-ri)
Avery Hall, Room 360
Tel: +1(402)
Weak Commitment

2. Background Constraint propagation Search 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
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
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)
Weak Commitment

5. Example: scheduling Context: Propagation Search Least commitment
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
Weak Commitment