# Callbacks with xpress in JuMP
**URL:** https://discourse.julialang.org/t/callbacks-with-xpress-in-jump/116889
**Category:** Optimization (Mathematical)
**Tags:** jump, optimization
**Created:** [July 10, 2024, 11:04pm UTC](https://discourse.julialang.org/t/callbacks-with-xpress-in-jump/116889 "2024-07-10T23:04:12Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Popeye](https://discourse.julialang.org/u/Popeye)
#### Post date: [July 10, 2024, 11:04pm UTC](https://discourse.julialang.org/t/callbacks-with-xpress-in-jump/116889/1 "2024-07-10T23:04:12Z")
Hi everyone,
I’m currently working on a MILP problem using the Xpress solver in JuMP. Some instances of this problem are taking considerably longer to solve to optimality, and I would like to implement solver-specific callbacks that dynamically adjust the solver’s runtime/time limit based on the optimality gap. Examples of rules can be:
- For a 10% gap, allow 10 minutes before terminating if no significant improvement to the objective function is made, with a 9% improvement threshold.
- For a 5% gap, allow 5 minutes before terminating if no significant improvement is made, with a 4% improvement threshold.
- For a 1% gap, allow 3 minutes before terminating if no significant improvement is made, with a 0.7% improvement threshold.
I have tried using callbacks but without success. I have read about the use of solver-specific callbacks in Xpress, but I am not entirely sure if this is the best way to achieve my goal. How should I correctly set up these callbacks in JuMP? Are there any examples or code available for such callbacks, or alternative approaches to achieve the same objective?
Thank you for your assistance!
---
### Author:  [@odow](https://discourse.julialang.org/u/odow)
#### Post date: [July 13, 2024, 12:23am UTC](https://discourse.julialang.org/t/callbacks-with-xpress-in-jump/116889/2 "2024-07-13T00:23:52Z")
> Are there any examples or code available for such callbacks, or alternative approaches to achieve the same objective?
Not yet, and I don’t know how to achieve this without looking into the details.
I’ve opened an issue to look into this: [Add callback example to README · Issue #270 · jump-dev/Xpress.jl · GitHub](https://github.com/jump-dev/Xpress.jl/issues/270)
---
### Author:  [@odow](https://discourse.julialang.org/u/odow)
#### Post date: [July 15, 2024, 5:11am UTC](https://discourse.julialang.org/t/callbacks-with-xpress-in-jump/116889/3 "2024-07-15T05:11:30Z")
I’ve opened a PR to update the docs: [Add callback example to README by odow · Pull Request #271 · jump-dev/Xpress.jl · GitHub](https://github.com/jump-dev/Xpress.jl/pull/271)
I guess you want something like this:
```Julia
using JuMP, Xpress
model = direct_model(Xpress.Optimizer())
start_time, log_times = time(), Any[]
function my_callback_function(cb_data)
prob = cb_data.model
p_obj, p_bound = Ref{Cdouble}(), Ref{Cdouble}()
Xpress.Lib.XPRSgetdblattrib(prob, Xpress.Lib.XPRS_MIPBESTOBJVAL, p_obj)
Xpress.Lib.XPRSgetdblattrib(prob, Xpress.Lib.XPRS_BESTBOUND, p_bound)
rel_gap = abs((p_obj[] - p_bound[]) / p_obj[])
push!(log_times, (time = time() - start_time, gap = rel_gap))
if log_times[end].gap < 0.1 # or some analysis of the gap
Xpress.Lib.XPRSinterrupt(cb_data.model, 1234)
end
return
end
set_attribute(model, Xpress.CallbackFunction(), my_callback_function)
```