# Optimal Routing with Mixed Integer Programming

The problem seems simple: a telecommunications firm has jobs that must be completed, and must assign them to its available technicians. Not all technicians are qualified to do all jobs, they are based out of depots a certain distance away from the jobs, and each job has a deadline.

Below, we will formulate this as a Mixed Integer Programming problem with the objective of minimizing lateness and, if possible, completing all the jobs. We will then demonstrate both a custom, hand-coded solution, and a solution designed to use industry-leading MIP package Gurobi.

The original version of this problem is borrowed from a Gurobi example application, which you can find here.

## Problem details

A telecom firm has a list jobs to be completed in a given day. The jobs are of different types, and each type has a priority: urgent equipment repairs are higher priority than routine repairs. Each customer is located in a particular city, has a windown of time in which they would prefer the job to be started, and deadline for the job to be completed.

To complete each job, one qualified technician must travel to the customer location. Each technician is qualified to do some types of jobs and not others. They must begin and end their day in their home depot which is located in a particular city. Each technician has a maximum number of allocated work hours for the day. The total time spent traveling and working on jobs cannot exceed this number.

The company would like to start all jobs in the specified window and complete all jobs on time, if possible. If this is not possible, the company prefers to “triage” the situation in the following order:

1. First, complete as many jobs as possible. Skip low-priority jobs before high-priority jobs.
2. Second, deviate as little as possible from the customer’s preferred arrival time, especially for higher-priority jobs.
3. Third, minimize late completions of jobs, especially for higher-priority jobs.

## Formal specification

### Time and Space

Suppose that the planning day starts at time 0 and ends after $T$ minutes. There are a set of $N$ locations which are the sites of customers and technician depots. Let $\tau_{mn}$ represent the travel time in minutes from location $m$ to location $n$ for $m,n \in \{1,2,…, N\}$.

### Jobs

There are $J$ jobs to be completed. Each job $j \in \{1, 2, …, J\}$ has a priority level $\pi_j$ and requires $p_j$ minutes to be completed. Each customer has specified that they would prefer work to begin between time $a_j$ and $b_j$, and that the job should be fully complete by time $c_j$. Let the location of each job be given by $L_j \in \{1,2,…,N\}$.

### Technicians

There are $K$ technicians available for work. Each must begin and end his or her work day at their “home” depot, located at $O_k \in \{1,2,…,N\}$ for $k \in \{1,2,…,K\}$. Total time spent traveling and working for technician $k$ may not exceed $W_k$ minutes. Each technician is qualified to do some jobs, and may be unqualified to do others: let $q_{jk}$ be an indicator function, taking a value of 1 if technician $k$ is qualified to do job $j$, and taking a value of 0 otherwise.

### Company’s problem

The company must choose which technicians will do which jobs and in which order they will do them. The order of the jobs defines a route that the technician will travel, from the depot to the first job, then to the second and so on, and finally returning from the last job to the depot. Additinally, the company must specify a start time for each job.

The company wishes to minimize total weighted lateness, out-of-bounds start times, and no-shows. The weight the company places on each minute of lateness is normalized to 1, and multiplied by the priority $\pi_j$ for each job. The company places a weight $C_0 \times \pi_j$ on each minute that a job $j$’s start time is outside the customer’s preferred window, and values the cost of not completing job $j$ at all at $C_1 \times \pi_j$.

Let $x_{jkr}$ be an indicator variable taking a value of 1 if job $j$ is the $r^{\text{th}}$ job which the company chooses to assign to technician $k$, and zero otherwise. For example, if technician 1 is assigned two jobs, first job $7$, and then job $3$, then $x_{711} = 1$ and $x_{312} = 1$, while $x_{j1r} = 0$ for all other combinations of $j$ and $r$. Let $t_j \in [0,T]$ represent the start time that the company chooses for job $j$.

For ease of presentation, let us also define $z_j \equiv \sum\limits_{r = 1}^J \sum\limits_{k = 1}^{K} x_{jkr}$. Notice that if the job $j$ is assigned in any order to any technician, $z_j$ will take a value of 1; but it will take a value of 0 otherwise. Thus, $z_j$ serves as an indicator of whether the job will be completed or not.

The company’s problem can then formally be given as $$\min\limits_{x_{jkr} ~ , ~ t_j} \left \{ \sum\limits_{j=1}^J \pi_j \times \left [ \begin{array}{l} \left ( 1 - z_j \right ) ~ \times \overbrace{C_1}^{\begin{array}{c}\text{penalty} \\ \text{for} \\ \text{no-show} \end{array}} + \\ \\ \quad + ~ z_j \times \left ( \begin{array}{l} \small \quad \quad \underbrace{\max \{ 0 ~ , ~ t_j + p_j - c_j \}}_{\begin{array}{c} \text{lateness} \end{array}} \\ \small + ~ C_0 \cdot \underbrace{\max \{ 0 ~ , ~ a_j - t_j \}}_{\begin{array}{c} \text{start} \\ \text{too early} \end{array}} \\ \small + ~ C_0 \cdot \underbrace{ \max \{ 0 ~ , ~ t_j - b_j \}}_{\begin{array}{c} \text{start} \\ \text{too late} \end{array}} \end{array} \right ) \end{array} \right ] \right \},$$

subject to the following constraints:

1. At most one technician per job: \begin{flalign}\sum\limits_{r = 1}^J \sum\limits_{k = 1}^{K} & x_{jkr} \leq 1 \\ & \quad \quad {\small \text{ for } j \in \{1,2,…,J\}} \end{flalign}
2. Technicians not assigned if not qualified: \begin{flalign} \left (\sum\limits_{r = 1}^J x_{jkr} \right )& \cdot \left ( 1 - q_{jk} \right ) = 0 \\ & \quad \quad {\small \text{ for } j,k \in \{1,2,…,J \} \times \{1,2,…,K \} } \end{flalign}
3. For each technician, order of jobs is assigned sequentially with no gaps: $$\sum\limits_{j=1}^J x_{jkr} \leq \left \{ \begin{array}{l l} 1 & {\small \text{for } r = 1 , k \in \{ 1,2,…,K \} } \\ \\ \sum\limits_{j=1}^J x_{jk,r-1} & {\small \text{for } r \in \{2, 3, …, J\}, k \in \{ 1,2,…,K \} } \end{array} \right.$$
4. Job starts allow sufficient time to arrive at job site from previous location: \begin{flalign}t_j &\leq \sum\limits_{k = 1}^K x_{jk0} \tau_{ \small O_k,L_j} + \sum\limits_{r=2}^J \sum\limits_{i = 1}^J \sum\limits_{k=1}^K S_{ijkr} \left [ t_i + p_i + \tau_{\small L_i,L_j} \right ] \\ \\ & \quad \quad {\small \text{ for } j \in \{1,2,…,J \} } \end{flalign}
• here we define $S_{ijkr} \equiv x_{ik,r-1} x_{jkr}$. Thus, $S_{ijkr}$ takes a value of 1 if job $j$ is technician $k$’s $r^{\text{th}}$ job, and is preceded in Sequence by job $i$.
1. No technician works longer than time allocation: \begin{flalign}\sum\limits_{r = 1}^J \sum\limits_{j = 1}^{J} & x_{jkr} p_j + \sum\limits_{j = 1}^{J} x_{jko} \tau_{\small O_k, L_j } \\ & \quad + \sum\limits_{r=2}^{J} \sum\limits_{i=1}^J \sum\limits_{j=1}^J S_{ijkr} \tau_{\small L_i, L_j} + \sum\limits_{r=1}^{J} \sum\limits_{j = 1}^J F_{jkr} \tau_{\small L_j,O_k} \leq W_k \\ \\ & {\small \text{ for } k \in \{1, 2, …, K \} } \end{flalign}
• here we define $F_{jkr} \equiv \left \{ \begin{array}{r l}\small \left ( \sum\limits_{i = 1}^J x_{ikr} \right ) \cdot \left (1 - \sum\limits_{i=1}^J x_{ik,r+1} \right ) \cdot x_{ikr} & {\scriptsize \text{ for } r \in \{1, 2, …, R-1 \} } \\ \small \\ x_{ikr} & {\scriptsize \text{ for } r = R } \end{array} \right.$. Thus, $F_{jkr}$ takes a value of 1 if $j$ is the $r^{\text{th}}$ and Final job assigned to technician $k$.

## Solution approaches

### Features of this problem that make it tricky

This problem contains a mixture of discrete decision variables (whom to send to each job, in which order) with continuous variables (what exact time to start each job). The presence of discrete decisions is what makes this and other Mixed-Integer-Programming problems relatively difficult to solve: the number of permutations of possible combinations of discrete choices will be massive even for a relatively small-scale problem. This means that a brute-force approach which blindly explores all the permutations will probably be computationally infeasible even for a toy problem.

We should also note that the objective function is not continuously differentiable in the continuous choice variables. This means that any simple derivative-based approach to dealing with this part of the problem will also likely fail.

There are two practical approaches that may be useful in solving this kind of problem:

1. Formulate a customized approach that exploits simplifying features specific to the structure of this problem.
2. Formulate the problem in a way that it can be fed into a general MIP solver such as Gurobi, which has been optimized to automatically exploit features that commonly occur in the structure of problems of this general type.

In what follows, we will try both.

### A customized algorithm

In our custom algorithm, the decision process will be explicitly broken down into three stages:

1. The company chooses which technicians do which jobs.
2. For each technician assigned more than one job, the company chooses which order to do the jobs in.
3. Given the assignment and order of the jobs, the compnay chooses the start time for each.

We will solve the problem starting at stage 3 and working backwards. When determining the choices or stage 1, we will use a classic “branch and bound” approach, in which entire branches of possible combinations of decisions are iteratively ruled out based on the current “best-yet” objective function value.

Two key features we will exploit are:

1. Effectively discrete choice of start time. For a given ordering of jobs, there are only a limited number of start times for each job that are likely to be optimal. This allows us to treat each continuous time choice as just another discrete choice with a limited number of possible values, and avoid possibly unreliable non-derivative-based continuous optimization algorithms.
2. Independence across technicians. For a given allocation of jobs, the contribution of each technician to the objective function is independent of each other. This will mean that while working backwards through the problem, we only have to solve each stage once.

The following are the specific steps the algorithm takes:

1. Calculate the partial independent contribution of each possible assignment of jobs, in each possible ordering, for each technician.
• a. For each combination and ordering of jobs, check first if it will satisfy the total work time constraint for the technician. If not, skip it.
• b. If a combination and ordering satisfies the total work time constraint, cycle through the relevant permutations of possible starting times in order.
• c. For each job, try at most 7 possible starting times: (1) as early as possible, (2) at the beginning of the “start window”, (3) at the tail end of the “start window,” (4) just in time to finish the job at the due time, (5) just in time for the following job to start at the beginning of its “start window,” (6) just in time for the following job to start at the end of its “start window,” (7) just in time for the following job to finish just in time, at its due time. There are many cases where some of these are not relevant and are skipped. For example, if a technician is only assigned one job, it is unnecessary to try more than one start time: that which allows her to finish right on time, or as close to it as possible.
• d. For each combination of assigned jobs, discard all but the one with the lowest partial contribution to the objective function. If there is more than one such ordering, keep only the first one found–ordering is irrelevant to the optimality of the assignments of other technicians.
1. Perform an undirected search through the possible combinations of job assignments to technicians:
• a. Cycle through technicians one at a time making assignments.
• b. Use the already-prepared dictionary of partial contributions to the objective function to calculate total objective function value.
• c. If at some point when only some technicians have received an assignment, the accumulated objective value exceeds the best-yet objective function value, abort, and skip all permutations involving the already-made assignments. This is the “branch and bound” part of the algorithm.
• d. Count any left-over jobs as unfilled, and add to objective function accordingly.
• e. If an allocation is found which yields an objective value of 0, stop. This is the theoretical minimum, and we only need one solution–no need to look further.

The part of this algorithm which probably has the most room for improvement is step 2: in general, an undirected search might be inefficient. For the particular problem instance we will review below, however, it turns out to be quite fast.

### Gurobi-based solution

The problem as specified above is already nearly ready to be introduced to the Gurobi solver. It only requires the definition of a few auxiliary variables so that the constraints and objective function can meet Gurobi’s particular requirements. The most important of these is that no more than two decision variables can ever be multiplied by one another in a single term.

To this end,

• $S_{ijkr}$ the “sequence” indicator variable, is defined exactly as given above and used to specify constraints 4 and 5.
• $F_{jkr}$ the “final job” indicator variable from constraint 5, is defined almost as given above, with a one additional intermediate auxiliary variable defined to meet the decision variable multiplication limit.
• Auxiliary variables are defined for lateness, for earliness of job start, and for lateness of job start, and these are used to specify the objective function.

## Performance comparison

We test the performance of the two algorithms on the base scenario given here in the original Gurobi example. You can also see these details by examining the data files in this Git repository. For the sake of brevity, we do not replicate all the details here. The basic features are thus: there are 7 jobs and 7 technicians with a planning horizon of 10 hours/600 minutes, from 7am to 5pm. As it turns out, there is enough slack in the technician roster for there to be multiple ways to start and complete all the jobs on time.

### Custom algorithm performance

The algorithm is implemented without any parallelization, even though it is eminently parallelizable. You may to the Jupyter notebook in this Git repository for the full code, and the data files for the scenario. It takes approximately 1.75 seconds to complete Step 1, and 20 milliseconds to complete Step 2 and find one of the on-time solutions.

### Gurobi performance

The same Jupyter notebook referenced above also contains code for the Gurobi problem implementation. Note that it might be a little bit tricky to get this part of the notebook to run, as the the problem may be bigger than what the standard free Gurobi license allows. Because of this, I replicate the model output messages below:

Gurobi Optimizer version 9.5.1 build v9.5.1rc2 (linux64)
Thread count: 24 physical cores, 48 logical processors, using up to 24 threads
Optimize a model with 182 rows, 5173 columns and 3731 nonzeros
Model fingerprint: 0xaf002fe6
Model has 1029 quadratic objective terms
Model has 4459 general constraints
Variable types: 28 continuous, 5145 integer (5145 binary)
Coefficient statistics:
Matrix range     [1e+00, 2e+02]
QMatrix range    [1e+00, 1e+00]
QLMatrix range   [1e+00, 2e+02]
Objective range  [6e+03, 2e+04]
QObjective range [2e+00, 5e+02]
Bounds range     [1e+00, 6e+02]
RHS range        [1e+00, 5e+02]
Presolve added 1759 rows and 0 columns
Presolve removed 0 rows and 4259 columns
Presolve time: 0.17s
Presolved: 8104 rows, 4108 columns, 22814 nonzeros
Presolved model has 762 SOS constraint(s)
Variable types: 1196 continuous, 2912 integer (2912 binary)
Found heuristic solution: objective 97600.000000

Root relaxation: objective 0.000000e+00, 548 iterations, 0.01 seconds (0.01 work units)

Nodes    |    Current Node    |     Objective Bounds      |     Work
Expl Unexpl |  Obj  Depth IntInf | Incumbent    BestBd   Gap | It/Node Time

0     0    0.00000    0    7 97600.0000    0.00000   100%     -    1s
H    0     0                    1674.0000000    0.00000   100%     -    1s
0     0    0.00000    0    7 1674.00000    0.00000   100%     -    1s
0     2    0.00000    0    7 1674.00000    0.00000   100%     -    1s
*   26    20               5       0.0000000    0.00000  0.00%  44.3    1s

Cutting planes:
Implied bound: 4

Explored 30 nodes (2482 simplex iterations) in 1.74 seconds (1.09 work units)
Thread count was 24 (of 48 available processors)

Solution count 3: 0 1674 97600

Optimal solution found (tolerance 1.00e-04)
Best objective 0.000000000000e+00, best bound 0.000000000000e+00, gap 0.0000%
Optimal objective value: 0.0


Note that the Gurobi solver is highly optimized and takes advantage of parallel processing where possible. Using multiple cores, it executes in about the same time, 1.75 seconds, as my custom algorithm.

### Compared to the original Gurobi example

In the original Gurobi example, the problem was formalized in a slightly different way from what I have done here. It ends up producing an instantiated model with fewer variables and constraints, and Gurobi reports that it executed from start finish in a fraction of a second.

## In conclusion

Here we have seen that there are at least three ways to implement a solver for this problem. All of them find a solution in an acceptably short period of time, at least for the simple base scenario we have tested here.