Flexible backlogs vs. regular backlogs?

What’s the difference between a flexible backlog and a regular backlog for a vehicle routing problem?

A vehicle backlog is a set of stops assigned to a vehicle that have yet to be serviced. They can sometimes be called a manifest or assigned tasks in various tooling. Nextmv thinks about vehicle backlogs for optimization in two ways: fixed and flexible.

In both cases, routes have already been assigned to vehicles. Choosing fixed or flexible backlogs is a question of how you want to handle assigning new stops to vehicles (because late orders came in, etc.). With fixed backlogs, any newly added stops always get appended to the end of the vehicle’s defined route. With flexible backlogs, any newly added stops can get inserted into the vehicle’s existing route in the best sequence.

So if a new stop gets assigned to a vehicle that happens to be 2 blocks away from the current stop it’s servicing, the flexible backlog will likely insert that new stop next on that vehicle’s route whereas the fixed backlog will add it to the end, even though the vehicle was right there. If that is too soon for a driver to react, you can also combine fixed and flexible so that the next few stops are fixed, and the remaining backlog is flexible.

We’ve got a nice walkthrough of the flexible backlog feature for VRPs on our blog. Hope this helps answer your question.