🚚 Delivery Truck Loading System Concurrency Problem

📝 Problem Setup

A logistics company has multiple loading docks where workers load packages into delivery trucks.

• Each truck has a weight/volume limit.

• Packages arrive continuously from the warehouse.

• Multiple workers can attempt to load packages in parallel.

• Once a truck is full, it should be dispatched immediately, and a new empty truck should arrive at the dock.

⚠️ Concurrency Challenge

Naive approach:
Each worker checks whether the truck has space left and then puts the package in.

Problems

• Two workers check simultaneously → both see “space available.”

• Both load packages → truck exceeds capacity.

• Or, one worker sees “truck is full,” but another worker just dispatched it → confusion: packages might get lost or assigned incorrectly.

🔑 Solution Approaches

1. Naive Locking

• Place a synchronized lock on the truck while loading.

• Multiple workers try to load packages onto a dock’s current truck.

  • Correctness first: never overfill a truck; never dispatch twice; never lose a package.

    • Only one worker can load at a time.

• We’ll serialize critical operations with a single lock so only one worker mutates truck state at a time.

🔒 Locking plan (coarse-grained / “monitor”)

• Use one monitor lock per dock (i.e., synchronized on the Dock instance).

  • Why lock the dock (not the truck)?