Callback Codes#

The Gurobi callback routines make use of a pair of arguments: where and what. When a user callback function is called, the where argument indicates from where in the Gurobi optimizer it is being called (presolve, simplex, barrier, MIP, etc.). When the user callback wishes to obtain more detailed information about the state of the optimization, the what argument can be passed to an appropriate get method for your language to obtain additional information (e.g., GRBcbget in C, GRBCallback::getIntInfo in C++, GRBCallback.getIntInfo in Java, GRBCallback.GetIntInfo in .NET, and Model.cbGet in Python).

More detailed information on how to use callbacks in your application can be found in the reference manuals for the different Gurobi language interfaces (C, C++, Java, .NET, and Python).

A few parameters are callback settable. These parameters can be modified from within a callback call. How to do that for the different APIs is illustrated here.

Possible values for the where and what arguments are listed in the following tables. Note that these values are referred to in slightly different ways from the different Gurobi interfaces. Consider the SIMPLEX value as an example. You would refer to this constant as follows from the different Gurobi APIs:

Language

Callback constant

C

GRB_CB_SIMPLEX

C++

GRB_CB_SIMPLEX

Java

GRB.Callback.SIMPLEX

.NET

GRB.Callback.SIMPLEX

Python

GRB.Callback.SIMPLEX

Possible where values are:

where

Numeric value

Optimizer status

POLLING

0

Periodic polling callback

PRESOLVE

1

Currently performing presolve

SIMPLEX

2

Currently in simplex

MIP

3

Currently in MIP

MIPSOL

4

Found a new MIP incumbent

MIPNODE

5

Currently exploring a MIP node

MESSAGE

6

Printing a log message

BARRIER

7

Currently in barrier

MULTIOBJ

8

At the end of a multi-objective iteration

IIS

9

Currently computing an IIS

Allowable what values depend on the value of the where argument. Valid combinations are:

what

where

Result type

Description

RUNTIME

Any except POLLING

double

Elapsed solver runtime (seconds).

WORK

Any except POLLING

double

Elapsed solver work (work units).

MEMUSED

Any except POLLING

double

Allocated memory (GB).

MAXMEMUSED

Any except POLLING

double

Maximum memory ever allocated (GB).

PRE_COLDEL

PRESOLVE

int

The number of columns removed by presolve to this point.

PRE_ROWDEL

PRESOLVE

int

The number of rows removed by presolve to this point.

PRE_SENCHG

PRESOLVE

int

The number of constraint senses changed by presolve to this point.

PRE_BNDCHG

PRESOLVE

int

The number of variable bounds changed by presolve to this point.

PRE_COECHG

PRESOLVE

int

The number of coefficients changed by presolve to this point.

SPX_ITRCNT

SIMPLEX

double

Current simplex iteration count.

SPX_OBJVAL

SIMPLEX

double

Current simplex objective value.

SPX_PRIMINF

SIMPLEX

double

Current primal infeasibility.

SPX_DUALINF

SIMPLEX

double

Current dual infeasibility.

SPX_ISPERT

SIMPLEX

int

Is problem currently perturbed?

MIP_OBJBST

MIP

double

Current best objective.

MIP_OBJBND

MIP

double

Current best objective bound.

MIP_NODCNT

MIP

double

Current explored node count.

MIP_SOLCNT

MIP

int

Number of solutions that were presented through the MIPSOL callback. For multiobjective models, this counter is reset to 0 at the beginning of each multiobjective pass.

MIP_CUTCNT

MIP

int

Current count of cutting planes applied.

MIP_NODLFT

MIP

double

Current unexplored node count.

MIP_ITRCNT

MIP

double

Current simplex iteration count.

MIP_OPENSCENARIOS

MIP

int

Number of scenarios that are still open in a multi-scenario model.

MIP_PHASE

MIP

int

Current phase in the MIP solution process. Possible values are 0 (in the NoRel heuristic), 1 (in the standard MIP search), or 2 (performing MIP solution improvement). Predefined constants are available (e.g., GRB.PHASE_MIP_SEARCH).

MIPSOL_SOL

MIPSOL

double *

Solution vector for new solution (C only). The resultP argument to C routine GRBcbget should point to an array of doubles that is at least as long as the number of variables in the user model.

Use callback methods getSolution in C++, getSolution in Java, GetSolution in .NET, and cbGetSolution in Python.

MIPSOL_OBJ

MIPSOL

double

Objective value for new solution. Not necessarily better than the current incumbent.

MIPSOL_OBJBST

MIPSOL

double

Current best objective.

MIPSOL_OBJBND

MIPSOL

double

Current best objective bound.

MIPSOL_NODCNT

MIPSOL

double

Current explored node count.

MIPSOL_SOLCNT

MIPSOL

int

Number of solutions that were presented through the MIPSOL callback before the current invocation. It may happen, particularly at the beginning of the root node, that some invocations of the MIPSOL callback are not accounted for immediately, and the value presented for MIPSOL_SOLCNT is temporarily lower than what could be expected. For multiobjective models, this counter is reset to 0 at the beginning of each multiobjective pass.

MIPSOL_OPENSCENARIOS

MIPSOL

int

Number of scenarios that are still open in a multi-scenario model.

MIPSOL_PHASE

MIPSOL

int

Current phase in the MIP solution. Possible values are 0 (in the NoRel heuristic), 1 (in the standard MIP search), or 2 (performing MIP solution improvement). Predefined constants are available (e.g., GRB.PHASE_MIP_SEARCH).

MIPNODE_STATUS

MIPNODE

int

Optimization status of current MIP node (see the Status Code section for further information).

MIPNODE_OBJBST

MIPNODE

double

Current best objective.

MIPNODE_OBJBND

MIPNODE

double

Current best objective bound.

MIPNODE_NODCNT

MIPNODE

double

Current explored node count.

MIPNODE_SOLCNT

MIPNODE

int

Number of solutions that were presented through the MIPSOL callback. For multiobjective models, this counter is reset to 0 at the beginning of each multiobjective pass.

MIPNODE_OPENSCENARIOS

MIPNODE

int

Number of scenarios that are still open in a multi-scenario model.

MIPNODE_PHASE

MIPNODE

int

Current phase in the MIP solution process. Possible values are 0 (in the NoRel heuristic), 1 (in the standard MIP search), or 2 (performing MIP solution improvement). Predefined constants are available (e.g., GRB.PHASE_MIP_SEARCH).

MIPNODE_REL

MIPNODE

double *

Relaxation solution for the current node, when its optimization status is GRB_OPTIMAL (C only). The resultP argument to C routine GRBcbget should point to an array of doubles that is at least as long as the number of variables in the user model.

Note that the relaxation solution retrieved at a node is not necessarily feasible for the user model.

Use callback methods getNodeRel in C++, getNodeRel in Java, GetNodeRel in .NET, and cbGetNodeRel in Python.

BARRIER_ITRCNT

BARRIER

int

Current barrier iteration count.

BARRIER_PRIMOBJ

BARRIER

double

Primal objective value for current barrier iterate.

BARRIER_DUALOBJ

BARRIER

double

Dual objective value for current barrier iterate.

BARRIER_PRIMINF

BARRIER

double

Primal infeasibility for current barrier iterate.

BARRIER_DUALINF

BARRIER

double

Dual infeasibility for current barrier iterate.

BARRIER_COMPL

BARRIER

double

Complementarity violation for current barrier iterate.

MULTIOBJ_OBJCNT

MULTIOBJ

int

Current count of objectives already optimized.

MULTIOBJ_SOLCNT

MULTIOBJ

int

Number of solutions that were presented through the MIPSOL callback for the current objective.

MULTIOBJ_SOL

MULTIOBJ

double *

Solution vector for new solution (C only). The resultP argument to C routine GRBcbget should point to an array of doubles that is at least as long as the number of variables in the user model.

Use callback methods getSolution in C++, getSolution in Java, GetSolution in .NET, and cbGetSolution in Python.

MULTIOBJ_STATUS

MULTIOBJ

int

Optimization status for current objective (see the Status Code section for further information).

MULTIOBJ_OBJBST

MULTIOBJ

double

Best value for current objective.

MULTIOBJ_OBJBND

MULTIOBJ

double

Best bound for current objective.

MULTIOBJ_MIPGAP

MULTIOBJ

double

MIP gap of the current objective (only available if the model is a MIP).

MULTIOBJ_ITRCNT

MULTIOBJ

double

Simplex iteration count for current objective.

MULTIOBJ_NODCNT

MULTIOBJ

double

Explored node count for current objective.

MULTIOBJ_NODLFT

MULTIOBJ

double

Unexplored node count for current objective.

MULTIOBJ_RUNTIME

MULTIOBJ

double

Elapsed solver runtime (seconds) for current objective.

MULTIOBJ_WORK

MULTIOBJ

double

Elapsed solver work (work units) for current objective.

IIS_CONSTRMIN

IIS

int

Minimum number of constraints in the IIS.

IIS_CONSTRMAX

IIS

int

Maximum number of constraints in the IIS.

IIS_CONSTRGUESS

IIS

int

Estimated number of constraints in the IIS.

IIS_BOUNDMIN

IIS

int

Minimum number of variable bounds in the IIS.

IIS_BOUNDMAX

IIS

int

Maximum number of variable bounds in the IIS.

IIS_BOUNDGUESS

IIS

int

Estimated number of variable bounds in the IIS.

MSG_STRING

MESSAGE

char *

The message that is being printed.

Remember that the appropriate prefix must be added to the what or where name listed above, depending on the language you are using.

Callback notes#

Note that the POLLING callback is an optional callback that is only invoked if other callbacks have not been called in a while. It does not allow any progress information to be retrieved. It is simply provided to allow interactive applications to regain control frequently, so that they can maintain application responsiveness.

The object-oriented interfaces have specialized methods for obtaining the incumbent or relaxation solution. While in C you would use GRBcbget, you use getSolution or getNodeRel in the object-oriented interfaces. Please consult the callback descriptions for C++, Java, .NET, or Python for further details.

Note that the MIPSOL callback might also be called for solutions that do not improve the incumbent. This has technical reasons and should only happen in rare cases. You might want to check the MIPSOL_OBJ value if your code relies on stricly improving solutions. The MIPSOL callback will also be called once for each MIP start.

Note that the MIPNODE callback will be called once for each cut pass during the root node solve. The MIPNODE_NODCNT value will remain at 0 until the root node is complete. If you query relaxation values from during the root node, the first MIPNODE callback will give the relaxation with no cutting planes, and the last will give the relaxation after all root cuts have been applied.

Note that the multi-objective optimization algorithm solves a sequence of optimization problems which we refer to as optimization passes. The MULTIOBJ callback will be called once at the end of each pass. Also, during each pass, MIP-related callbacks will be called if the original model is a MIP, and LP-related callbacks will be called if the original model is an LP.

Note that there are certain restrictions concerning the available callbacks when using the Gurobi Remote Services (Compute Server, Instant Cloud, etc.). Please refer to the Callbacks section of the Gurobi Remote Services Manual for more information.