"Node relaxation not Available"

Answered

Antonio Sassano

December 05, 2022 07:40

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In a Callback invoked while solving a MIP problem I wrote:

 if where == GRB.Callback.MIPNODE:
        x = model.cbGetNodeRel(model._x)

Gurobi gives the following answer:

File "src\gurobipy\model.pxi", line 6279, in gurobipy.Model.cbGetNodeRel

GurobiError: Node relaxation not available

Exception ignored in: 'gurobipy.callbackstub'
Traceback (most recent call last):
  File "src\gurobipy\callback.pxi", line 209, in gurobipy.CallbackClass.callback
  File "G:\Il mio Drive\MyCode.py", line 148, in mycallback x = model.cbGetNodeRel(model._x)
  File "src\gurobipy\model.pxi", line 6279, in gurobipy.Model.cbGetNodeRel
gurobipy.GurobiError: Node relaxation not available

BUT if I check the values of variable x, I found that they have been provided (I do not know whether the values are correct).

MOREOVER  the B&B proceeds and at the end it produces an (optimal!) solution which violates one of the constraints of the base formulation (NOT one of the lazy constraints added by the Callback).

Related to

• Callbacks

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  Jaromił Najman

  • Gurobi Staff

  December 05, 2022 10:03

  Hi Antonio,

  As described in the documentation of cbGetNodeRel

  Note that this method can only be invoked when the where value on the callback function is equal to GRB.Callback.MIPNODE, and GRB.Callback.MIPNODE_STATUS is equal to GRB.OPTIMAL

  You have to first check whether the node relaxation has been solved to optimality before accessing it.

  Best regards, 
  Jaromił

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• 

  David Torres Sanchez

  • Gurobi Staff

  December 05, 2022 12:11

  From Antonio:

  Many many thanks Jaromił, problem solved!
  Sorry! Antonio

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