# Gurobi callback function not accessing / not loading

**URL:** <https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501>\
**Category:** Optimization (Mathematical)\
**Tags:** jump, gurobi\
**Created:** [September 17, 2024, 2:00pm UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501 "2024-09-17T14:00:48Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![cskx](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cskx/32/212010_2.png) [@cskx](https://discourse.julialang.org/u/cskx)\
**Post date:** [September 17, 2024, 2:00pm UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501/1 "2024-09-17T14:00:48Z")

</div>

Hello,

I would like to access the values during each iterations, then plot the objective function (maybe more info) against iterations, I wrote a callback function as below. I found that my cb\_where is never equal to GRB\_CB\_MIP or GRB\_CB\_SIMPLEX, so that this "I’m here… " phrase is never printed…

Then, I want to try something solver-independent with my\_callback\_function2, but it’s not working as well…

Thanks ahead for help.

```julia
env = Gurobi.Env()
ot_model = direct_model(Gurobi.Optimizer(env))
# ot_model = Model(Gurobi.Optimizer)

@variable(ot_model, p[1:I, 1:J] >= 0)
@variable(ot_model, z[1:I])
@constraint(ot_model, p * ones(J) .== ones(I))
@constraint(ot_model, transpose(p) * μ .== ν)
@constraint(ot_model, [i = 1:I], z[i] >= (X[i] - (p * Y)[i])^2)
@objective(ot_model, Min, sum(μ[i] * z[i] for i = 1:I))

function my_callback_function1(cb_data, cb_where::Cint)
    # GRB_CB_SIMPLEX GRB_CB_MIP
        
    if cb_where == GRB_CB_MIP || cb_where == GRB_CB_SIMPLEX ||cb_where == GRB_CB_MIPSOL
        
        print("I'm here... ")
        Gurobi.load_callback_variable_primal(cb_data, cb_where)
        z_val = callback_value(cb_data, z)
        print("The currenct z value is ", z_val)
    end
end

function my_callback_function2(cb_data)
    z_val = callback_value(cb_data, z)
    print("The currenct z value is ", z_val)
end

MOI.set(ot_model, Gurobi.CallbackFunction(), my_callback_function2)

optimize!(ot_model)

```

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<div class="post-metadata">

**Author:** ![eduardosalaz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/eduardosalaz/32/22287_2.png) [@eduardosalaz](https://discourse.julialang.org/u/eduardosalaz)\
**Post date:** [September 17, 2024, 3:56pm UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501/2 "2024-09-17T15:56:09Z")

</div>

If you want to get the current best objective on each new MIP incumbent node, you should use the GRB\_CB\_MIPSOL\_OBJBST code on new MIP nodes (GRB\_CB\_MIPSOL) as follows:

```julia
function my_callback_function(cb_data, cb_where)
    if cb_where == GRB_CB_MIPSOL
        current_best_obj = Ref{Cdouble}()
        GRBcbget(cb_data, cb_where, GRB_CB_MIPSOL_OBJBST, current_best_obj)
        current_best_obj_val = current_bestobj[]
        println("Current best objective: $current_best_obj_val")
    end
end

```

As for solver independent callbacks, it depends on which solvers you will be working with, personally I find it better to write solver specific callbacks.

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<div class="post-metadata">

**Author:** ![eduardosalaz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/eduardosalaz/32/22287_2.png) [@eduardosalaz](https://discourse.julialang.org/u/eduardosalaz)\
**Post date:** [September 17, 2024, 3:56pm UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501/3 "2024-09-17T15:56:46Z")

</div>

If you want, you can provide a MWE and I can try to run your model to see what’s happening

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<div class="post-metadata">

**Author:** ![eduardosalaz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/eduardosalaz/32/22287_2.png) [@eduardosalaz](https://discourse.julialang.org/u/eduardosalaz)\
**Post date:** [September 17, 2024, 4:00pm UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501/4 "2024-09-17T16:00:34Z")

</div>

Actually I am just looking at your model and you have a quadratic term there so that may have something to do, not sure, will look into it, but I definitely can run the model if you want to.

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<div class="post-metadata">

**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [September 17, 2024, 9:42pm UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501/5 "2024-09-17T21:42:24Z")

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Hi @cskx, welcome to the forum.

The Gurobi `cb_where` codes are [Callback Codes - Gurobi Optimization](https://www.gurobi.com/documentation/current/refman/cb_codes.html)

You could see where the callback is being called from with

```Julia
function my_callback_function1(cb_data, cb_where::Cint)
    print("Calling from $cb_where")
end

MOI.set(ot_model, Gurobi.CallbackFunction(), my_callback_function1)

```

The answer for your problem is probably that it isn’t a MIP, so `GRB_CB_MIP` and `GRB_CB_MIPSOL ` are never called, and Gurobi doesn’t solve this with the simplex method, so `GRB_CB_SIMPLEX` is never called.

For the solver independent callback, you need to follow [Solver-independent Callbacks · JuMP](https://jump.dev/JuMP.jl/stable/manual/callbacks/). This likely won’t work with Gurobi because these callbacks are intended only for MIPs, and your problem does not have integer variables.

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<div class="post-metadata">

**Author:** ![cskx](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cskx/32/212010_2.png) [@cskx](https://discourse.julialang.org/u/cskx)\
**Post date:** [September 18, 2024, 10:55am UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501/6 "2024-09-18T10:55:50Z")

</div>

Thank you so much @odow !! Finally I found out that my callback is being called at Barrier, and then I adapted your response from another post…

```julia
GRBcbget(cb_data, cb_where, GRB_CB_BARRIER_PRIMOBJ, primal_val)

```

Thanks once a again, I appreciate it !! : )

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<div class="post-metadata">

**Author:** ![cskx](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cskx/32/212010_2.png) [@cskx](https://discourse.julialang.org/u/cskx)\
**Post date:** [September 18, 2024, 10:57am UTC](https://discourse.julialang.org/t/gurobi-callback-function-not-accessing-not-loading/119501/7 "2024-09-18T10:57:58Z")

</div>

Hi, thank you for helping, actually it was running on GRB\_CB\_BARRIER instead of GRB\_CB\_MIPSOL. I just solved the problem with odow’s response. Thanks