The portrait: my avatar as 48 keys, one strip each. Writes land in the memtable, flush to L0, compact into L1 and finally into one sorted run: the face. The level chart above shows where each key is.
Compaction · an LSM-tree of a career: written fast, merged later
Arthur Costa
> SCAN result:* · the only quantified outcomes
mem
Memtable · now
In memory and still changing: open pull requests, ongoing community work, what I am looking for.
mutable · sorted by keyflushes to L0 when it fills
L0
Level 0 · 2026
Flushed, immutable runs. L0 runs may overlap in key range, so a read checks each one. Most of this is work on my own time.
L1
Level 1 · 2022–2025
Compacted into one sorted run, no overlaps. The production results live here.
L2
Level 2 · 2016–2022
The last level: the largest, the oldest, and the only place a tombstone may finally be dropped. Foundations.
GET
The read path
Ask for one technology. The read checks the memtable, then each level, newest first; a Bloom filter lets it skip levels that cannot hold the key. The first level that answers is the most recent place I used it.
try
Text equivalent: the status line names the newest level that holds the key and every older version. The Bloom filters are real: 64 bits and three FNV-1a hashes per level, built from this page's own entries, so a “maybe” can turn out to be a false positive.
The stack, as a sorted string table
Keys sorted by name; the value is where I group it.
PUT
Write to me
Why this design
The concept is the log-structured merge tree from DDIA chapter 3: new writes land in memory, flush as sorted runs, and are merged into larger, older levels in the background.
The tree is the information architecture, from the first screen down: the memtable holds what is still changing, L0 holds what is still active in 2026, L1 2022–2025, L2 the foundations, and the level rail at the top is the navigation.
Reading is compaction: when you finish a level, the rail's tick counts and the side diagram merge it into the next, and the portrait under it turns the strips of the levels you have read from faint to solid; at the end, the profile is one sorted run, and the same merged portrait closes the page.
Retiring my own agent platform (AC-005) is written as tombstones that disappear only at the last level, which is exactly when an LSM-tree may drop them.
The read path turns the stack into a lookup that answers when I last used a technology, not how good I am at it, which is why there are no skill bars.
Azeret Mono carries keys and levels, and its weight is the level; Source Serif 4 carries the prose, as a storage paper sets its argument around its code.
Warm grey paper and ink from the avatar; ochre marks only fresh writes and rust only tombstones, the plate's own pair.
Motion is the mechanism, plays once and holds; every animation has a text equivalent, Pause stops all motion, and reduced motion shows the compacted end state.
Facts on this page come from my records; employers and clients are deliberately not named. Undated work sits in L1 until it has a date.
Complete profile
Senior backend & platform engineer · Porto, Portugal
I build platforms that hold up in production: moving fleets to the cloud, turning batch into event streams, and, on my own time, execution platforms and safety tooling for coding agents.
Open to senior backend or platform roles, remote or hybrid
Nine years in production (since 2017)
Decision records
2023–2025 · At work · Platform · Delivered
200+ instances to cloud local zones, with autoscaling
Compute cost fell by ~70% across 200+ instances.
~70% lower compute cost — 200+ instances moved from on-prem and edge sites to cloud local zones, with autoscaling
200+ instances migrated — with autoscaling, for live sporting-event peaks
Context and decision
Context. A real-time platform for live sporting events ran its workloads on on-prem and edge environments.
Decision. Lead the infrastructure modernization: migrate the workloads to cloud local zones and enable autoscaling, with multi-region load tests before peak events.
Stack. Cloud local zones · Autoscaling · Go · Python · Locust
2022–2023 · At work · Payments · Delivered
Payment settlement, from batch to event-driven
Processing success rose from 60% to 90%+.
60% → 90%+ processing success — payment settlement moved from batch to event streams with change data capture
Context and decision
Context. A legacy payment and settlement platform ran as batch processing on a monolith.
Decision. Move the core components to Java and Go, replace batch with streaming and change data capture, containerize on Kubernetes, and centralize logs and dashboards for the SLAs.
Stack. Java · Go · Streaming · CDC · Kubernetes
2026 · Own time · Platform · Working system
Horus: remote execution for builds and agent jobs
A changed worktree is a new job, never a silent retry; a worktree without a data class never leaves the machine.
Context and decision
Context. Builds and coding-agent jobs start on a workstation but need the CPU, isolation and uptime of a remote fleet, without leaking a private worktree or losing work halfway.
Decision. A local daemon with a socket API and a written durability contract; content-hash sync so a job runs on exactly the bytes submitted; recovery bound to that content; a versioned accounting ledger; gVisor sandboxes on VPS runners behind a data-class gate.
Stack. Go · Rust · gVisor
2026 · Own time · Virtualization · Spike
Firecracker microVMs as a sandbox compute driver
A failure-injection suite on a KVM host (kill mid-create, simulated reboot, duplicate RPCs, kill points 0–50 ms, a leak counter) and 23 security probes on a 22-item adversarial matrix. An unimplemented check reports SKIP, never PASS. Proposed upstream as OpenShell#4351.
Context and decision
Context. Can Firecracker microVMs, with the jailer, back an open-source agent sandbox runtime (OpenShell) through the whole lifecycle, and stay correct when things crash?
Decision. A compute driver with its own guest kernel, images from a verified catalog, reclamation of the previous run's VMs at start-up, and a guest flush before Stop.
Stack. Go · Firecracker · KVM
2026 · Own time · Platform design · Accepted
Retire my own agent platform; keep the native clients
Far less to operate; contribution rules now refuse new routers, schedulers and dashboards.
Context and decision
Context. My agent-workflow repository had grown a router, a scheduler, a coordinator hierarchy, a provider dispatcher and a dashboard in front of three coding-agent clients.
Decision. Retire it (19 October 2026). Each client runs natively; the repository keeps shared skills, a small configuration, standalone safety tools and 26 dated decision records.
Stack. Python · Bash
2026 · Own time · Platform safety · Accepted
Safety tools fail closed
Documented as a tripwire, not a sandbox, so nobody mistakes it for isolation.
Context and decision
Context. Coding agents run shell commands in real repositories from three clients with three hook systems; secrets once reached a model through process arguments.
Decision. One command-safety hook for all three clients; output redaction; long-lived credentials kept out of worker reach; a missing safety dependency refuses instead of being caught and allowed.
Stack. Python
2026 · Own time · Distributed systems · Lab
Course labs on microVMs, behind an mTLS control plane
A lab's user id stays reserved until its cleanup has succeeded; CA rotation cannot be bypassed by session resumption.
Context and decision
Context. A course platform gives every learner a disposable lab machine, created and destroyed constantly by many people at once.
Decision. A microVM lab runtime with a chaos test that kills VMs mid-session; idempotent resets by key; mutual TLS on a control-plane CA with reload without restart; pinned keyless signing that fails closed.
Stack. Go · Kotlin · protobuf
2026 · Own time · Storage & consensus · Study
A relational database, from storage to Raft
Built layer by layer as a study; each phase is a commit history.
Context and decision
Context. The fastest way to understand a distributed database is to build its layers in order and see where each guarantee comes from.
Decision. Disk manager, buffer pool, B+ tree, heap file and write-ahead log; SQL parser, planner and Volcano executor; 2PL then MVCC; Raft, Multi-Raft with range sharding, Percolator-style transactions, SSI with hybrid logical clocks, an S3 backend and the PostgreSQL wire protocol.
Stack. Rust
2026 · Own time · Sync & security · Working system
Offline-first sync with signed form-fill
The extension never fills a form the owner did not approve.
Context and decision
Context. A personal job-search tool runs on several desktops plus a browser extension; data must sync without a central source of truth.
Decision. Tombstones pruned only once every active desktop has applied them; fill tokens signed on the desktop with a P-256 key from the OS keyring; an encrypted vault export; an LLM judge measured against blind readings.
Stack. Go · Rust
2025–2026 · Own time · Developer tooling · Shipped v2.1.1
gitshift: many Git identities, one machine
Switching on a directory change went from about 3.8 s to about 0.5 s.
3.8 s → 0.5 s directory switch — gitshift identity switch on directory change, release v2.1.x
Context and decision
Context. Several GitHub and GitLab accounts on one machine means juggling keys, identities and signing by hand.
Decision. One command switches Git author, SSH key, commit signing and the GitHub CLI account, by alias or automatically by directory; releases through GoReleaser and a Homebrew tap.
Load tests to inform cost optimization and Kubernetes autoscaling; graceful shutdown and connection draining in a shared Helm chart; CI repaired with quality gates; codebase rules for agentic coding, shared with the team.
Software engineer, platform / backend
2023–2025 · Real-time platform for live sporting events
Led the move of on-prem and edge workloads to cloud local zones with autoscaling (AC-001); real-time, event-driven services on serverless and containers; Go and Python automation; Locust multi-region load tests for peak events; CI/CD across GoCD, GitHub Actions, Jenkins, Buildkite, Ansible and Packer; metrics, logs, traces and alerts.
Software engineer, payments / settlement
2022–2023 · Payment and settlement platform (consulting)
Led the modernization of a legacy payment and settlement platform to event-driven streaming and CDC (AC-002).
Software engineer, post-purchase
2021–2022 · E-commerce marketplace, several countries
Backend services in Go, Java, Node.js and Groovy for post-purchase and payment workflows; faster claim and dispute resolution.
Software engineer
2019–2021 · Core banking, Brazil
Monoliths and mainframe-based systems modernized into Java and Spring microservices; migration of microservices to the public cloud under regulatory SLAs.
Associate software engineer, CRM & integrations
2018–2019 · Fintech CRM
Real-time CRM features integrated with banking services such as loans, OTP and account management.
Associate software engineer
2017–2018 · Software house
CRM and warehouse management systems in PHP and JavaScript.
How I work
Fail closed: a missing safety dependency refuses; it is never caught and allowed.
Crash and recovery suites before claims: kill mid-create, simulated reboots, leak counters.
Evidence tied to a commit; a check that is not implemented says SKIP, never PASS.
Decision records for every significant choice, including retiring my own platform.
Load tests before peak events.
Also built
Drover
Watch and approve agent sessions from any device: a Rust core over UniFFI, pairing scopes bound to the consumed token.
Rust
JVM performance labs
Eleven on-heap versus off-heap lock-free scenarios with JMH, and 13 serialization frameworks behind one endpoint. Numbers not quoted until re-run with raw output.
Java · JMH
graphql-federation
Federation 2 across Java, Go and TypeScript subgraphs, gRPC between services, SLO burn-rate dashboards.
Java · Go · TypeScript · gRPC
Ponte
A management system for tutoring schools: live lessons with recording and consent, TOTP and lockout, a finance ledger ported to Go and checked by replaying a shared corpus.
TypeScript · Go · PostgreSQL
deep-engineering-knowledge-base
An LLM pipeline that writes and checks engineering lessons, with an independent critic that excludes the author.
Python
techish-visual
An engine for animated technical diagrams: a ThorVG WASM renderer and performance budgets in CI.
TypeScript
keyframe
A Linux studio for recording and editing technical videos, with cuts snapped to silence.
Rust
java-workflows
Reusable GitHub Actions for Java teams: a Java 11 to 25 matrix, security scanning and SBOMs.
GitHub Actions
Upstream
OpenShell#4351
Firecracker compute driver proposal, from AC-004 (open source). · open
OpenShell#4591
A gateway reporting "database is locked" under two concurrent clients. · open
Orca, 10 pull requests
SSH OAuth forwarding, pairing-offer rotation without restart, security events, isolation from external agent config. · 9 open, 1 closed