Vibecheck: Background Job Queue

The scenario

You're on-call. Support has escalated three tickets from the past week:

• Ticket #1841 — "I got the same welcome email three times"
• Ticket #2203 — "I never received my onboarding email"
• Ticket #2391 — "I received my step-3 email before my step-1 email"

The service has been running fine in staging. Production has multiple worker instances deployed behind a load balancer. The issues started around the same time the team scaled from one worker to three.

Your job is to diagnose what's going wrong and fix it.

Codebase overview

File	Responsibility
src/scheduler.ts	Entry point. Starts the poll loop.
src/jobRunner.ts	Fetches pending jobs and processes them.
src/jobRepository.ts	All database reads/writes for the jobs table.
src/emailService.ts	Calls the external email API.

Setup

# Install dependencies
npm install

# Start a local Postgres database (Docker)
docker run --rm -p 5432:5432 -e POSTGRES_PASSWORD=postgres -d postgres:16

# Create the schema and seed data
psql postgresql://postgres:postgres@localhost:5432/postgres -f schema.sql

# Run the tests
npm test

# Start the scheduler locally
DB_PASSWORD=postgres npm start

Environment variables

Variable	Default	Description
DB_HOST	localhost	Postgres host
DB_PORT	5432	Postgres port
DB_NAME	jobqueue	Database name
DB_USER	postgres	Database user
DB_PASSWORD	(empty)	Database password
EMAIL_API_KEY	(empty)	API key for email provider
EMAIL_FROM	onboarding@example.com	Sender address

Constraints

• Do not change the database schema in a way that breaks the existing SELECT index on (status, created_at).
• The fix must work correctly with N concurrent worker processes (N ≥ 1).
• Unit tests must continue to pass after your changes.
• Do not remove the attempts column — it will be used downstream.