Metaheuristic approach in a VRP

[Bruno Moreira]

I'm currently using OR-Tools to solve a Vehicle Routing Problem with a metaheuristic approach (TABU_SEARCH). Here’s a snippet of my code:

1. By default, it seems the solver keeps running until it hits the maximum time limit (total_time). Is there a way to configure an additional stopping criterion (like a certain number of iterations or a gap tolerance) so that it doesn’t necessarily run for the entire duration?

2. Another issue I’m facing is that the solution I get is always the same. I was under the impression that using a metaheuristic (TABU_SEARCH) would produce different solutions because it’s supposed to be somewhat stochastic. Am I missing some parameter or configuration that would introduce randomness into the search process?

Any tips?

Thank you in advance.

[Laurent Perron]

No, the solver is supposed to be deterministic.

Laurent Perron | Operations Research | lpe...@google.com | (33) 1 42 68 53 00

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[Bruno Moreira]

So none of this methods have any random component?

I can't change the seed or something?

[Vincent Furnon]

About randomness, randomly re-ordering nodes in the model can help.

About stopping criteria, you can specify a max number of solutions generated (solution_limit in RoutingSearchParameters). You can also define a custom stopping criterion using RoutingModel::AddAtSolutionCallback, passing a callback which will call RoutingModel::CancelSearch when you want to interrupt the search. 

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[Bruno Moreira]

I am only using Tabu_Search for 300 seconds. Is there anyway i can print all the solutions found each second? Basically, i want to do a graph to see how it works until it finds the best solution.