Screen Time Guardian — Distributed Process Control

Overview#

A screen-time management system in two parts: an enforcement agent running on the child's computer, and a control application the parents run from their own devices. The agent authenticates the user, checks the current time against an allowed-usage policy, monitors an active session, and shuts the machine down when the allowance is exhausted.

The reason this is a systems project rather than an application project is a single line in the requirements: both parents may run the control app simultaneously, from different machines, on different operating systems, editing the same policy file. That turns a scheduling tool into a distributed mutual-exclusion problem, and getting it wrong corrupts the policy that everything else depends on.

The Enforcement Agent#

Runs at startup and cannot be trivially dismissed. Its control flow:

• Authenticate — read a password from the keyboard
• Parent override — if the parent password is entered, suspend enforcement for 60 minutes, since the parent using the machine is not the case being policed
• Blocked period — if the current time falls outside the allowed window, announce when use resumes, then run two things concurrently: a 15-second countdown to an unconditional shutdown, and a second authentication attempt, so a parent can still intervene before the machine powers off
• Failed authentication — three wrong attempts blocks the machine for 10 minutes and shuts it down, so the lockout cannot be brute-forced
• Active session — with a valid child password inside an allowed window, the agent runs a monitoring loop performing three tasks concurrently: periodic activity capture, live re-reading of the policy so a parent's change applies without a restart, and countdown warnings at the one-minute mark before shutdown

The Policy Format#

Allowed usage is expressed in a compact line-based format, one rule per line:

Where F/T are the window bounds, D is the maximum continuous session, I the mandatory break between sessions, and S the total allowance within the window. The three rules above read as: free use from 06:00 to 06:45; between 07:30 and 11:30, 150 minutes total in blocks of at most 60 with 20-minute breaks; and between 19:00 and 21:30, 90 minutes total taken at any time.

Expressing the policy declaratively rather than as code is what lets both applications — written for different platforms — share one source of truth without shipping shared logic.

The Concurrency Problem#

The policy file is synchronised through cloud storage so control apps on any platform can reach it. That is also what creates the hazard: two parents editing concurrently is a textbook critical section, and a naive read-modify-write loses one parent's change or corrupts the file outright.

The system coordinates access explicitly rather than hoping the window is too small to matter:

• Mutual exclusion on the policy file — a parent's edit acquires exclusive access before the read-modify-write and releases it afterwards, so the two updates serialise instead of racing
• Coordination across machines and operating systems, which rules out any single-host primitive and forces the lock into the shared medium itself
• Deadlock avoidance — the acquisition protocol is designed so a control app that crashes mid-edit cannot leave the policy permanently locked, which is the failure mode that turns a safety mechanism into a denial of service
• Concurrency within the agent — the monitoring loop's three tasks share state and are coordinated so a policy update landing mid-countdown is applied consistently rather than partially

The Control Application#

Runs on the parent's phone or computer and provides policy viewing and editing, plus access to the activity history the agent has recorded — the current day at minimum, with historical access as an extension.

This project is operating-systems fundamentals applied to a real scenario: mutual exclusion, deadlock avoidance, inter-process communication, and concurrent task coordination — where the correctness requirement comes from an ordinary use case rather than a contrived one.

📄 Monitoring Intervention System