robertz.co

JWT auth lifecycle — stateless verify, refresh, and the edge cases

Wed, 27 May 2026 00:00:00 GMT

Pure-JS standalone of the full JSON Web Token auth lifecycle: login → HS256 sign → Bearer call → stateless verify → access expiry → refresh-token rotation. Walks every edge case — tampered signature, alg:none forgery, missing header, nbf-in-the-future, wrong audience, leaked secret, refresh-token replay with reuse detection, and the revocation gap — with a REAL HMAC-SHA256 computed in the browser. Five panels on one screen: the request flow, token anatomy, the stateless verification pipeline (0 DB queries), token lifetimes on a timeline, and horizontal scaling vs shared session stores.

GB10 Grace Blackwell — heterogeneous CPU + GPU task distribution

Wed, 27 May 2026 00:00:00 GMT

A single-file in-browser visualizer of the NVIDIA GB10 Grace Blackwell Superchip (DGX Spark): a 20-core Arm Grace CPU and a Blackwell GPU sharing one coherent 128 GB LPDDR5X pool over NVLink-C2C. Write Python tasks and watch a distributor route each to the CPU or the GPU by its measured traits — data-parallel & regular to the GPU, branchy/irregular/low-parallel to the CPU — with no explicit memory copies. Reuses the same per-lane interpreter as the RTX 4090 SIMT sim.

GPU SIMT execution — warps, latency hiding, divergence & coalescing

Wed, 27 May 2026 00:00:00 GMT

A single-file in-browser GPU visualizer: write a Python kernel and watch every lane of a 32-thread warp actually run it on a simulated NVIDIA RTX 4090 SM. Five views show warp scheduling and latency hiding, branch divergence with per-lane masking, memory coalescing and shared-memory bank conflicts, the grid/block/thread hierarchy, and a CPU-vs-GPU roofline. Every visual is driven by a real per-lane interpreter, not a canned animation.

A local-HPC workflow-management broker, built on just Postgres

Wed, 27 May 2026 00:00:00 GMT

queue_workflows is a workflow-management broker for a local, self-hosted HPC whose only moving part is Postgres — SKIP LOCKED claims woken by LISTEN/NOTIFY, lease-based reclaim, a DAG dispatcher with a durable outbox, and a GPU warm-model cache. Here's why making Postgres the only broker is the whole point.

Concurrency battle! Which language wins?

Wed, 20 May 2026 00:00:00 GMT

How Ruby, Python, Elixir, Java, C++, JavaScript — and HTML — each handle doing many things at once, with diagrams, a head-to-head table, and a final scorecard.

PostgreSQL on-disk filesystem visualizer

Tue, 19 May 2026 00:00:00 GMT

Pure-JS standalone of the PostgreSQL on-disk layout. Walks a predefined SQL script through a psql terminal while showing PGDATA, base//, 1 GB segments, 8 KB pages, line-pointer arrays, HeapTupleHeader, MVCC xmin/xmax/ctid chains, the _fsm and _vm forks, pg_wal records, pg_xact CLOG bits, B-tree pages and declarative partitioning, all updating in real time as the data flows from terminal → backend → WAL → buffer cache → disk file.

CQRS + event sourcing — plane tickets from multiple vendors

Tue, 19 May 2026 00:00:00 GMT

Pure-JS standalone of a CQRS / event-sourcing plane-ticket platform: Rails command + query APIs, PostgreSQL append-only event store, RabbitMQ fanout to five projectors, Redis L1 cache with single-flight, saga orchestrator across three GDS-style vendors. Walks the canonical edge cases: concurrent seat race, vendor timeout + compensation, payment-OK-issue-fails refund saga, out-of-order delivery, hold expiration, at-least-once redelivery, multi-vendor saga, Redis stampede, GDPR tombstones, projection replay, burst load.

mmWave 5G FR2 phased-array RF path — live 28 GHz beamformer

Tue, 19 May 2026 00:00:00 GMT

Pure-JS standalone of an mmWave 5G FR2 phased-array RF path. Walks a 28 GHz 8×8 array through cold start → boot cal → P1 SSB sweep → P2/P3 refinement → 256-QAM TX → body blockage → beam-failure recovery → NLOS reflected beam, with live polar beam pattern, per-element state, EVM constellation against 3GPP TS 38.141-2 thresholds, LO phase-noise PSD anchored to TR 38.803, and a full link-budget waterfall.

Instagram — system-design tick-by-tick request flow

Tue, 19 May 2026 00:00:00 GMT

Pure-JS visualiser of the Instagram architecture (GFG reference): DNS, Client, Load Balancer, API Gateways, App Servers (read/write), Cache + Shard Manager + Metadata DB shards, Notification Service + Queue, Search Aggregators + Elastic Search Cache, CDN + Blob Storage, Video Processing Queue + Workers + Service, Feed Generation Queue + Service. Tick = 0.5 s. Walks five real user scenarios — app launch, photo upload, search, CDN fetch, push notification — with every operation listed in a left-side event log.

LLM model scaling — VRAM · RAM · SSD spillover (Ollama vs vLLM)

Tue, 19 May 2026 00:00:00 GMT

Pure-JS in-browser visualiser of how an LLM serves under real hardware constraints. Pick a model (3B…671B incl. MoE), a quantization (FP16/Q8/Q5/Q4/Q3/Q2), a context length, and a GPU. Two tabs — Ollama (llama.cpp, single-stream, layer offload, SSD mmap) and vLLM (PagedAttention KV pool, continuous batching, tensor parallelism). Live memory stack across VRAM → RAM → SSD plus a throughput estimator anchored to real bandwidth numbers (RTX 4090 = 1.0 TB/s, A100 = 2.0, H100 = 3.35, H200 = 4.8, MI300X = 5.3 TB/s, DDR5 = 90 GB/s, NVMe Gen4 = 7 GB/s).

Scaling Web Servers — latency, throughput, bandwidth, concurrency & parallelism

Tue, 19 May 2026 00:00:00 GMT

One unified flow — clients → network pipe → server with N workers → clients — with five live knobs (request rate, network delay, CPU time, bandwidth, worker count) and five live metrics. Watch how latency, throughput, bandwidth-used, concurrency, and parallelism emerge from the same physics. Little's Law (C = T × L) is checked on screen every frame.

5G NR — end-to-end packet journey & loss anatomy

Tue, 19 May 2026 00:00:00 GMT

Pure-JS standalone of a 5G NR end-to-end packet journey. One IP packet walks down the PDCP → RLC → MAC → PHY stack at the gNB, crosses a lossy 100 MHz n78 air interface with HARQ + RLC ARQ + PDCP discard timers, traverses an inter-gNB handover with Xn-U forwarding and PDCP SN recovery, and surfaces a live loss-cause attribution while TCP-Reno and TCP-BBR react differently on top. 15-phase scripted scenario, six failure-injection knobs.

AMD vs ARM — architecture differences

Sun, 17 May 2026 00:00:00 GMT

Memo: x86-64 (AMD64) vs ARM64 (AArch64) — ISA, registers, memory model, calling convention, SIMD, atomics, pages, market. Pure facts, tables, minimal prose.

exe32_js — Windows 32-bit JavaScript simulator

Sun, 17 May 2026 00:00:00 GMT

Pure-JavaScript in-browser port of the exe32_rb Ruby Windows-32 emulator. Parses real PE32 EXEs, decodes & executes a substantial i386 subset, resolves Win32 imports against stubbed DLLs, and visualises class invocations + data flow live. Ships with an interactive Snake game embedded as a real x86 PE binary; upload your own EXE.

DNS + network-journey visualizer

Sun, 17 May 2026 00:00:00 GMT

Pure-JS standalone of a real DNS resolution + the post-DNS protocol stack. Walks a query through home router → ISP → IXP → resolver → DE-CIX → submarine cable → root / TLD / authoritative / web server, with RFC-1035 wire format, EDNS0 OPT, TCP three-way handshake, TLS 1.3, HTTP/2 and WebSocket. Live queue, in-app preset transactions, mouse-wheel zoom + drag-pan.

PicoRV32 — live core visualizer

Fri, 15 May 2026 00:00:00 GMT

Pure-JS standalone of the PicoRV32 live core visualizer. Step through a multi-cycle RV32I CPU — FSM, register file, ALU datapath, and memory bus — entirely in the browser.

How do you manage state? — React implementation

Wed, 13 May 2026 00:00:00 GMT

The interview question has a wrong-shaped answer. There isn't 'state' — there are five different kinds of state, and the right React tool is different for each. Working code for all five.

Local Weather Cluster, how to run ICON DWD at home?

Tue, 12 May 2026 00:00:00 GMT

A two-box home HPC cluster that runs DWD's ICON limited-area model over Poland — multi-stage MPI image, MPICH over a 10 GbE direct link, NFS-shared workdirs, and the orchestration glue that keeps it boring.

INT4 vs NVFP4

Thu, 07 May 2026 00:00:00 GMT

What actually changes when you swap an INT4 weight format for NVFP4 on Blackwell — number grid, block scales, throughput, accuracy.

CQRS and Event Sourcing in Rails

Thu, 07 May 2026 00:00:00 GMT

Six months on rails_event_store and aggregate_root: what to repeat, what to skip, and the traps to avoid.

What the CPU does when there's nothing to do

Wed, 06 May 2026 00:00:00 GMT

The path from a naive idle loop to HLT, the kernel idle task, C-states, and MWAIT — how a modern CPU actually parks itself.

ACID in 2 minutes

Tue, 05 May 2026 00:00:00 GMT

What relational engines actually guarantee for a transaction on a single node — and where the sharp edges live.

Quantum gates in Python

Tue, 05 May 2026 00:00:00 GMT

Quantum gates are unitary matrices. With NumPy you can implement and apply them in a few lines — here's the canonical set.