KM Kai MercerSECURITY · BACKEND · TOOLING
Open to freelance & contract

I build systems
that stay up
when it matters.

I'm Kai Mercer — a security-minded backend engineer. I design edge gateways, abuse-prevention layers and the boring, reliable plumbing behind products that can't afford to fall over. Eight years across fintech, messaging and infra.

View selected work ↓ Get in touch
Illustrated portrait of Kai Mercer
8 yrsbuilding production systems
40+services shipped to prod
99.99%uptime on core gateway
4.2Brequests filtered / month
// about

Engineer first, tinkerer always.

I started out patching nginx configs for a hosting company at nineteen and never really stopped. Today I work mostly in Go and Rust on things that sit between the internet and an application — reverse proxies, bot scoring, proof-of-work challenges, rate limiters, the unglamorous stuff that decides whether your launch survives a Reddit thread.

My rule of thumb: make failure boring. Canary deploys, idempotent consumers, structured logs you can actually grep, and dashboards that tell the truth at 3am. I care a lot about the boring parts because the boring parts are what break.

Outside work I reverse-engineer old game ROMs, maintain a couple of small CLI tools, and brew coffee that's measurably better than it has any right to be.

  • 📍Based in Lisbon, Portugal — working remotely across EU / US timezones
  • 🎯Focused on edge security, backend reliability, developer tooling
  • 💼Currently independent; taking on 1–2 contract engagements per quarter
  • 🗣Languages English (fluent), Portuguese (working), Mandarin (basic)
Illustration of a terminal with a deploy running
// selected work

Six projects worth showing.

A mix of open-source and client work. Every one of these went to production and got paged about at least once.

Sentinel dashboard screenshot

Sentinel 2024 →

Edge gateway doing real-time bot scoring — UA fingerprinting, proof-of-work challenges and per-IP rate shaping. Sits in front of three client apps, adds ~0ms at p50.

4.2B req/mo filtered · 0ms p50 latency
GoNginxRedisPoW
Orbit orchestration screenshot

Orbit 2023

A tiny orchestrator for long-running background jobs. Leases, retries and exactly-once side effects without dragging in a message broker. Runs 1.8M jobs a day on two boxes.

1.8M jobs/day · 2 nodes
RustPostgresgRPC
Prism event pipeline screenshot

Prism 2023

Streaming event pipeline with a pluggable transform stage. Backpressure is a first-class citizen; if a downstream slows down, upstream slows down instead of dropping data.

12.4k evt/s · p99 = 38ms
GoKafkaClickHouse
Relay webhook delivery screenshot

Relay 2022

Webhook delivery service you can reason about. Idempotency keys on every consumer, exponential backoff, dead-letter inspection UI, and a replay button that actually works.

99.98% delivery · replay-safe
TypeScriptSQSS3
Atlas anycast routing screenshot

Atlas 2022

Anycast routing layer with deterministic replay — reroute a region's traffic to a shadow cluster, watch it, and roll back in under half a second if the numbers look wrong.

12 PoPs · failover 400ms
RustHAProxyBGP
Uptime status dashboard screenshot

Beacon 2021

Self-hosted status page + synthetic checker. Probes your endpoints from six regions, writes to a tiny timeseries store, and tweets less than the alternatives.

30d availability 100%
GoSQLiteMastodon
// stack

What I reach for.

Opinions are strong, attachment is low — I'll write TypeScript if it's the right tool.

⚙ Languages

Goexpert
Rustadvanced
Pythonexpert
TypeScriptstrong

🛡 Infra & Security

Linux / networkingexpert
WAF / bot mitigationexpert
Kubernetesadvanced
Threat modellingstrong

🧰 Tooling

PostgreSQLRedisKafkaClickHouse Nginx / CaddyTerraformAnsiblePrometheus GrafanaOpenTelemetryDockerGitHub Actions CloudflareWireGuardValgrindGhidra

📚 Also

Technical writingIncident reviewMentoring Reverse engineeringLoad testing
// experience

The road so far.

2024 — PRESENT

Independent engineer · Contract / freelance

Embedded with product teams that need an edge layer, a migration, or someone to own the pager for a quarter.

  • Cut abusive traffic 99% on a client's login API with adaptive PoW + IP reputation
  • Led a monolith → service extraction for a payments team; zero-downtime cutover
  • Wrote the on-call runbook that took MTTR from 47min to 9min
2021 — 2024

Staff Backend Engineer · Nimbus Messaging

Core messaging infrastructure serving 180M monthly active users.

  • Owned the ingest path: 2.4M msg/s peak, p99 under 30ms end-to-end
  • Introduced shadow deploys — caught 14 regressions before customers did
  • Mentored four engineers; two now lead their own teams
2019 — 2021

Backend Engineer · Larkspur Fintech

Ledger and fraud platform for a European neobank.

  • Rebuilt the ledger reconciler; discrepancies went from weekly to never
  • Passed two external audits with zero critical findings
2016 — 2019

Software Engineer · various startups

The formative years — full-stack, on-call too often, learned what not to do.

  • Three seed-stage products shipped; two acquired, one beautifully failed
// contact

Let's build something solid.

Got a gateway that's melting? A service that needs to survive its first real traffic spike? Or just want to compare notes on reverse engineering?

✉️ [email protected] 🐙 GitHub 💼 LinkedIn 📄 Résumé (PDF)
Prefer plain text? Mail me at [email protected] — I reply within a day.