Routing Options

This section describes some of the options for the routing solver.

Search limits

Search limits terminate the solver after it reaches a specified limit, such as the maximum length of time, or number of solutions found. You can set a search limit through the solver's search parameters. See Time limits for an example.

The following table describes the most common search limits.

Name Type Default Description
solution_limit int64 kint64max Limit to the number of solutions generated during the search.
time_limit.seconds int64 kint64max Limit in seconds to the time spent in the search.
lns_time_limit.seconds int64 100 Limit in seconds to the time spent in the completion search for each local search neighbor.

First solution strategy

The first solution strategy is the method the solver uses to find an initial solution. The following table lists the options for first_solution_strategy.

Option Description
AUTOMATIC Lets the solver detect which strategy to use according to the model being solved.
PATH_CHEAPEST_ARC Starting from a route "start" node, connect it to the node which produces the cheapest route segment, then extend the route by iterating on the last node added to the route.
PATH_MOST_CONSTRAINED_ARC Similar to PATH_CHEAPEST_ARC, but arcs are evaluated with a comparison-based selector which will favor the most constrained arc first. To assign a selector to the routing model, use the method ArcIsMoreConstrainedThanArc().
EVALUATOR_STRATEGY Similar to PATH_CHEAPEST_ARC, except that arc costs are evaluated using the function passed to SetFirstSolutionEvaluator().
SAVINGS Savings algorithm (Clarke & Wright). Reference: Clarke, G. & Wright, J.W.: "Scheduling of Vehicles from a Central Depot to a Number of Delivery Points", Operations Research, Vol. 12, 1964, pp. 568-581.
SWEEP Sweep algorithm (Wren & Holliday). Reference: Anthony Wren & Alan Holliday: Computer Scheduling of Vehicles from One or More Depots to a Number of Delivery Points Operational Research Quarterly (1970-1977), Vol. 23, No. 3 (Sep., 1972), pp. 333-344.
CHRISTOFIDES Christofides algorithm (actually a variant of the Christofides algorithm using a maximal matching instead of a maximum matching, which does not guarantee the 3/2 factor of the approximation on a metric travelling salesman). Works on generic vehicle routing models by extending a route until no nodes can be inserted on it. Reference: Nicos Christofides, Worst-case analysis of a new heuristic for the travelling salesman problem, Report 388, Graduate School of Industrial Administration, CMU, 1976.
ALL_UNPERFORMED Make all nodes inactive. Only finds a solution if nodes are optional (are element of a disjunction constraint with a finite penalty cost).
BEST_INSERTION Iteratively build a solution by inserting the cheapest node at its cheapest position; the cost of insertion is based on the global cost function of the routing model. As of 2/2012, only works on models with optional nodes (with finite penalty costs).
PARALLEL_CHEAPEST_INSERTION Iteratively build a solution by inserting the cheapest node at its cheapest position; the cost of insertion is based on the arc cost function. Is faster than BEST_INSERTION.
LOCAL_CHEAPEST_INSERTION Iteratively build a solution by inserting each node at its cheapest position; the cost of insertion is based on the arc cost function. Differs from PARALLEL_CHEAPEST_INSERTION by the node selected for insertion; here nodes are considered in their order of creation. Is faster than PARALLEL_CHEAPEST_INSERTION.
GLOBAL_CHEAPEST_ARC Iteratively connect two nodes which produce the cheapest route segment.
LOCAL_CHEAPEST_ARC Select the first node with an unbound successor and connect it to the node which produces the cheapest route segment.
FIRST_UNBOUND_MIN_VALUE Select the first node with an unbound successor and connect it to the first available node. This is equivalent to the CHOOSE_FIRST_UNBOUND strategy combined with ASSIGN_MIN_VALUE (cf. constraint_solver.h).

Search status

You can return the status of a search using the routing model's status method. Here's the Python code to print the status of a search:

print("Solver status: ", solver.status())

This prints an integer with the following meanings:

Value Description
0 ROUTING_NOT_SOLVED: Problem not solved yet.
1 ROUTING_SUCCESS: Problem solved successfully.
2 ROUTING_FAIL: No solution found to the problem.
3 ROUTING_FAIL_TIMEOUT: Time limit reached before finding a solution.
4 ROUTING_INVALID: Model, model parameters, or flags are not valid.

Local search options

The following table lists the options for local search strategies (also called metaheuristics). See Changing the search strategy for examples of setting these options.

Option Description
AUTOMATIC Lets the solver select the metaheuristic.
GREEDY_DESCENT Accepts improving (cost-reducing) local search neighbors until a local minimum is reached.
GUIDED_LOCAL_SEARCH Uses guided local search to escape local minima (cf. http://en.wikipedia.org/wiki/Guided_Local_Search); this is generally the most efficient metaheuristic for vehicle routing.
SIMULATED_ANNEALING Uses simulated annealing to escape local minima (cf. http://en.wikipedia.org/wiki/Simulated_annealing).
TABU_SEARCH Uses tabu search to escape local minima (cf. http://en.wikipedia.org/wiki/Tabu_search).
OBJECTIVE_TABU_SEARCH Uses tabu search on the objective value of solution to escape local minima

Propagation control

Name Type Default Description
use_light_propagation bool true Use constraints with light propagation in routing model. Extra propagation is only necessary when using depth-first search or for models which require strong propagation to finalize the value of secondary variables. Changing this setting to true will slow down the search in most cases and increase memory consumption in all cases.