add PM2 cluster mode to utilize both CPU cores

[Aiiion]

Summary

• Adds ecosystem.config.cjs to run 2 cluster workers via PM2 (matching the 2 vCPU server)
• Updates Dockerfile to install PM2 globally and startup.sh to use pm2-runtime (keeps process in foreground for correct Docker signal handling)
• Gates the purge cron job to the primary worker (NODE_APP_INSTANCE === '0') to prevent duplicate deletes across workers — the flush job is already protected by a Redis distributed lock

Test plan

• Build the Docker image and confirm container starts with 2 PM2 workers (pm2 list inside container)
• Confirm docker stop shuts down cleanly (no hanging processes)
• Confirm rate limiting still works correctly across workers (Redis-backed)
• Confirm auth flow (login + verify) works correctly across workers
• At 05:00 UTC, confirm purge job runs exactly once (check logs from only worker 0)

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Application now runs with multi-instance clustering for improved reliability and performance.

• Bug Fixes

  • Prevented duplicate cron job execution across clustered application instances.

• Chores

  • Updated application startup process to use PM2 process manager for better stability and resource management.

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR adopts PM2 as the process manager: PM2 is installed globally in the Dockerfile, a new ecosystem.config.cjs configures the Express app in cluster mode with two instances, startup.sh switches from npm start to pm2-runtime, and src/cron.mjs gates the daily log-purge cron job to the primary cluster worker only.

Changes

PM2 Cluster Mode Adoption

Layer / File(s)	Summary
PM2 installation, ecosystem config, and startup entrypoint Dockerfile, ecosystem.config.cjs, startup.sh	Dockerfile adds npm install -g pm2; ecosystem.config.cjs exports a two-instance cluster config for express-api running ./src/index.mjs; startup.sh replaces exec npm start with exec pm2-runtime ecosystem.config.cjs.
Cron purge gating to primary PM2 worker src/cron.mjs	registerCronJobs reads NODE_APP_INSTANCE to set isPrimaryWorker, conditionally creates the daily purge task only on the primary instance, and uses purgeTask?.stop() in the returned stop() handler.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• Aiiion/express-api#40: Introduced the daily log-purge cron job in registerCronJobs that this PR now gates to the primary PM2 cluster worker.

Poem

🐇 Two workers now hum in the cluster's embrace,
But only one rabbit purges logs in that space.
PM2 spins up the show with a nod,
ecosystem.config — a tidy new prod!
Hop hop, no duplicates, all is in place. 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'add PM2 cluster mode to utilize both CPU cores' directly and clearly summarizes the main change—adding PM2 cluster mode support to leverage the server's dual-core capacity.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch infra/pm2-cluster

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[coderabbitai]

🧹 Nitpick comments (1)

Dockerfile (1)

15-16: Pin PM2 to an explicit version for reproducible and safer builds.

npm install -g pm2 will resolve whatever is latest at build time, so rebuilds can drift unexpectedly. Pinning a tested version avoids silent runtime changes.

Suggested patch

 # Install PM2 globally
-RUN npm install -g pm2
+ARG PM2_VERSION=7.0.1
+RUN npm install -g pm2@${PM2_VERSION}

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@Dockerfile` around lines 15 - 16, The RUN npm install -g pm2 command installs
the latest version of PM2 at build time, making builds non-reproducible across
different runs. Modify the command to pin PM2 to a specific tested version by
appending `@version-number` to pm2, for example npm install -g pm2@5.x.x, ensuring
consistent builds and preventing unexpected runtime changes from silent version
updates.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@Dockerfile`:
- Around line 15-16: The RUN npm install -g pm2 command installs the latest
version of PM2 at build time, making builds non-reproducible across different
runs. Modify the command to pin PM2 to a specific tested version by appending
`@version-number` to pm2, for example npm install -g pm2@5.x.x, ensuring
consistent builds and preventing unexpected runtime changes from silent version
updates.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 4cea12ac-745d-4e3b-a811-bdfc6e83505f

📥 Commits

Reviewing files that changed from the base of the PR and between fbfef08 and 1361c2f.

📒 Files selected for processing (4)

• Dockerfile
• ecosystem.config.cjs
• src/cron.mjs
• startup.sh