https://jakubhyl.dev Interactive engineering tutorials and practical software notes by Jakub Hýl. en https://jakubhyl.dev/tutorials/git-internals-visualized Manipulate a simplified commit graph to see how Git objects, branches, HEAD, merge, rebase, reset, and cherry-pick fit together. Sat, 05 Sep 2026 10:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/git-internals-visualized https://jakubhyl.dev/tutorials/rate-limiting-algorithms-visualized Send traffic through fixed-window, sliding-window, and token-bucket limiters to compare bursts, fairness, state, 429 responses, and refill behavior. Sat, 05 Sep 2026 09:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/rate-limiting-algorithms-visualized https://jakubhyl.dev/tutorials/debugging-distributed-systems-logs-metrics-traces Inspect trace waterfalls, correlated logs, span errors, latency, throughput, and retries to find where a distributed request actually failed. Sat, 05 Sep 2026 08:40:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/debugging-distributed-systems-logs-metrics-traces https://jakubhyl.dev/tutorials/push-notifications-expo-apns-fcm-app-lifecycle Trace a notification from an Expo backend through APNs or FCM into foreground, background, and terminated app states—and then into a deep link. Sat, 05 Sep 2026 08:30:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/push-notifications-expo-apns-fcm-app-lifecycle https://jakubhyl.dev/tutorials/caching-browser-cdn-api-database-visualized Send requests through memory, browser, CDN, API, and database layers while changing TTL, capacity, frequency, staleness, and revalidation. Sat, 05 Sep 2026 08:20:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/caching-browser-cdn-api-database-visualized https://jakubhyl.dev/tutorials/background-jobs-message-queues-visualized Watch producers, queues, workers, retries, duplicate deliveries, backpressure, and dead-letter handling change under real failure scenarios. Sat, 05 Sep 2026 08:10:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/background-jobs-message-queues-visualized https://jakubhyl.dev/tutorials/sql-query-planner-explain-visualized Change table size, selectivity, and available indexes to see a conceptual planner choose scans and joins—and learn to read EXPLAIN ANALYZE safely. Sat, 05 Sep 2026 08:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/sql-query-planner-explain-visualized https://jakubhyl.dev/tutorials/sql-transactions-isolation-locks-race-conditions Interleave two transactions and see lost updates, snapshots, row locks, deadlocks, and PostgreSQL serialization failures as they happen. Sat, 05 Sep 2026 08:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/sql-transactions-isolation-locks-race-conditions https://jakubhyl.dev/tutorials/server-authoritative-architecture Use trust boundaries, authorization, validation, idempotency, transactions, and server-owned rules to keep mobile games and APIs correct under tampering and retries. Sat, 05 Sep 2026 07:40:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/server-authoritative-architecture https://jakubhyl.dev/tutorials/http-request-lifecycle-visualized Follow fetch through DNS, connection security, edge infrastructure, authentication, cache, database, serialization, retries, and the final UI update. Sat, 05 Sep 2026 07:30:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/http-request-lifecycle-visualized https://jakubhyl.dev/tutorials/react-native-performance-visualized See how JavaScript, React rendering, native/UI work, lists, images, and state updates compete for a frame—and which optimizations actually help. Sat, 05 Sep 2026 07:20:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/react-native-performance-visualized https://jakubhyl.dev/tutorials/database-indexes-b-trees-visualized Explore page-oriented B-Tree indexes, splits, range scans, composite keys, selectivity, and the tradeoff between faster reads and more expensive writes. Sat, 05 Sep 2026 07:10:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/database-indexes-b-trees-visualized https://jakubhyl.dev/tutorials/javascript-event-loop-visualized Step through the call stack, runtime APIs, microtasks, tasks, and rendering opportunities—and see why asynchronous JavaScript can still freeze an app. Sat, 05 Sep 2026 07:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/javascript-event-loop-visualized https://jakubhyl.dev/blog/why-big-o-is-only-the-beginning-of-performance Complexity explains how work grows. Real performance also depends on memory, I/O, data layout, runtimes, and the workload we actually ship. Fri, 04 Sep 2026 20:55:23 GMT Jakub Hýl https://jakubhyl.dev/blog/why-big-o-is-only-the-beginning-of-performance https://jakubhyl.dev/tutorials/cpu-caches-locality-visualized See how cache lines, locality, access order, and finite capacity make two algorithms with the same Big O behave differently on real hardware. Fri, 04 Sep 2026 20:54:22 GMT Jakub Hýl https://jakubhyl.dev/tutorials/cpu-caches-locality-visualized https://jakubhyl.dev/tutorials/virtual-memory-visualized Follow a virtual address through the TLB and page table to physical memory, including page faults, protection, and the limits of the model. Fri, 04 Sep 2026 20:53:20 GMT Jakub Hýl https://jakubhyl.dev/tutorials/virtual-memory-visualized https://jakubhyl.dev/tutorials/processes-threads-concurrency-visualized See how processes isolate resources, threads share state, schedules interleave, and synchronization prevents races while creating new tradeoffs. Fri, 04 Sep 2026 20:52:17 GMT Jakub Hýl https://jakubhyl.dev/tutorials/processes-threads-concurrency-visualized https://jakubhyl.dev/tutorials/arrays-linked-lists-stacks-queues-visualized How the organization of linear data makes indexing, insertion, removal, and traversal cheap or expensive. Fri, 04 Sep 2026 15:01:10 GMT Jakub Hýl https://jakubhyl.dev/tutorials/arrays-linked-lists-stacks-queues-visualized https://jakubhyl.dev/tutorials/dynamic-programming-visualized From repeated recursion to carefully defined states, memoization, tabulation, and the tradeoffs behind dynamic programming. Fri, 04 Sep 2026 15:01:10 GMT Jakub Hýl https://jakubhyl.dev/tutorials/dynamic-programming-visualized https://jakubhyl.dev/tutorials/heaps-priority-queues-visualized Why a binary heap keeps the next priority at the root, maps naturally to an array, and supports efficient scheduling. Fri, 04 Sep 2026 15:01:10 GMT Jakub Hýl https://jakubhyl.dev/tutorials/heaps-priority-queues-visualized https://jakubhyl.dev/tutorials/recursion-call-stack-visualized Recursion explained as ordinary function calls, pending stack frames, base cases, and a controlled return journey. Fri, 04 Sep 2026 15:01:10 GMT Jakub Hýl https://jakubhyl.dev/tutorials/recursion-call-stack-visualized https://jakubhyl.dev/tutorials/stack-vs-heap-how-memory-works A careful conceptual model of call frames, dynamic allocation, ownership, lifetime, and what managed runtimes can change. Fri, 04 Sep 2026 15:01:10 GMT Jakub Hýl https://jakubhyl.dev/tutorials/stack-vs-heap-how-memory-works https://jakubhyl.dev/tutorials/trees-binary-search-trees-visualized Tree vocabulary, binary-search-tree invariants, deletion cases, traversals, and why balance determines performance. Fri, 04 Sep 2026 15:01:10 GMT Jakub Hýl https://jakubhyl.dev/tutorials/trees-binary-search-trees-visualized https://jakubhyl.dev/tutorials/designing-retry-safe-apis-idempotency Retries are normal. Idempotency makes important API operations safe to retry without pretending distributed systems can guarantee magic exactly-once execution. Fri, 04 Sep 2026 12:46:50 GMT Jakub Hýl https://jakubhyl.dev/tutorials/designing-retry-safe-apis-idempotency https://jakubhyl.dev/tutorials/secure-account-deletion-expo-supabase Account deletion in an Expo app is a controlled workflow: confirm intent, authenticate the caller, clean up data and providers, then remove the identity safely. Fri, 04 Sep 2026 12:46:50 GMT Jakub Hýl https://jakubhyl.dev/tutorials/secure-account-deletion-expo-supabase https://jakubhyl.dev/tutorials/testing-states-that-break-mobile-apps A production testing matrix for React Native apps: installation, authentication, offline behavior, purchases, lifecycle changes, and data migrations. Fri, 04 Sep 2026 12:46:50 GMT Jakub Hýl https://jakubhyl.dev/tutorials/testing-states-that-break-mobile-apps https://jakubhyl.dev/blog/features-i-decide-not-to-build Scope control is product engineering: every feature adds states, maintenance, support, and decisions that must remain understandable long after the first implementation. Fri, 04 Sep 2026 12:46:50 GMT Jakub Hýl https://jakubhyl.dev/blog/features-i-decide-not-to-build https://jakubhyl.dev/blog/mobile-forensics-building-products Lessons from C++, XML/XQuery, and investigating iOS and Android application artifacts that still shape how I approach reliability, compatibility, privacy, and product engineering. Fri, 04 Sep 2026 12:46:50 GMT Jakub Hýl https://jakubhyl.dev/blog/mobile-forensics-building-products https://jakubhyl.dev/blog/why-some-apps-should-not-have-accounts Accounts, cloud sync, analytics, and databases can be valuable product infrastructure. They should exist because the product needs them, not because they make an architecture look sophisticated. Fri, 04 Sep 2026 12:46:50 GMT Jakub Hýl https://jakubhyl.dev/blog/why-some-apps-should-not-have-accounts https://jakubhyl.dev/tutorials/apple-google-sign-in-expo-supabase A production-minded approach to social sign-in in Expo: native Apple credentials, OAuth redirects, Supabase sessions, secure storage, and the identity edge cases that affect real users. Fri, 04 Sep 2026 08:50:18 GMT Jakub Hýl https://jakubhyl.dev/tutorials/apple-google-sign-in-expo-supabase https://jakubhyl.dev/tutorials/building-offline-first-react-native-app A practical architecture for React Native apps that remain useful without a connection: local source of truth, migrations, sync queues, conflicts, and honest failure handling. Fri, 04 Sep 2026 08:50:18 GMT Jakub Hýl https://jakubhyl.dev/tutorials/building-offline-first-react-native-app https://jakubhyl.dev/blog/how-i-use-ai-without-letting-it-own-the-architecture AI helps me move faster through implementation, research, and review. It does not replace the responsibility to understand the system, validate the result, or make product decisions. Fri, 04 Sep 2026 08:50:18 GMT Jakub Hýl https://jakubhyl.dev/blog/how-i-use-ai-without-letting-it-own-the-architecture https://jakubhyl.dev/blog/what-i-learned-shipping-apps-at-university University gave me the fundamentals. Shipping Spheriz, PassportPlate, 100 Reps, Ritual Grove, and DevGuess taught me what happens after the code first works. Fri, 04 Sep 2026 08:50:18 GMT Jakub Hýl https://jakubhyl.dev/blog/what-i-learned-shipping-apps-at-university https://jakubhyl.dev/tutorials/revenuecat-expo-subscriptions How to add in-app purchases to an Expo app with RevenueCat: entitlements over product IDs, why restore is mandatory, and why the client is the wrong place to trust a purchase. Fri, 04 Sep 2026 08:26:19 GMT Jakub Hýl https://jakubhyl.dev/tutorials/revenuecat-expo-subscriptions https://jakubhyl.dev/tutorials/secure-api-cloudflare-workers-zod-rate-limiting Validating every input at the edge, rate limiting without a database, and using signed state so a client can hold data it cannot forge. Fri, 04 Sep 2026 08:26:19 GMT Jakub Hýl https://jakubhyl.dev/tutorials/secure-api-cloudflare-workers-zod-rate-limiting https://jakubhyl.dev/tutorials/ship-expo-app-to-app-store-with-eas The full path from a working Expo project to an approved App Store release: build profiles, credentials, versioning, TestFlight, and the review rejections that are easiest to avoid. Fri, 04 Sep 2026 08:26:19 GMT Jakub Hýl https://jakubhyl.dev/tutorials/ship-expo-app-to-app-store-with-eas https://jakubhyl.dev/tutorials/understanding-big-o-notation What Big O actually measures, how each growth class behaves as input grows, and when complexity decides your architecture versus when it is the wrong thing to optimize. Fri, 04 Sep 2026 08:26:19 GMT Jakub Hýl https://jakubhyl.dev/tutorials/understanding-big-o-notation https://jakubhyl.dev/blog/building-spheriz More than two years ago, Spheriz was a rough C++ experiment built around one question: what happens if a falling-block puzzle stops being rectangular? This is how that prototype became a full 2D and 3D mobile game. Fri, 04 Sep 2026 06:41:04 GMT Jakub Hýl https://jakubhyl.dev/blog/building-spheriz https://jakubhyl.dev/blog/building-passportplate The real story behind PassportPlate: a tired kitchen conversation, a half-joking suggestion to spin a wheel, and how that became an offline recipe app. Sun, 19 Jul 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/blog/building-passportplate https://jakubhyl.dev/blog/building-100-reps-workout-tracker How I built a focused workout tracker around daily reps, animated progress, premium unlocks, and the challenge of shipping a polished mobile app. Fri, 03 Jul 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/blog/building-100-reps-workout-tracker https://jakubhyl.dev/blog/building-ritual-grove How a rough AI-assisted prototype became a real mobile habit tracker built with React Native and Expo. Wed, 01 Jul 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/blog/building-ritual-grove https://jakubhyl.dev/tutorials/binary-search-visualized A step-by-step guide to binary search, including the core invariant, common implementation bugs, and why halving the search interval changes performance completely. Sat, 28 Feb 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/binary-search-visualized https://jakubhyl.dev/blog/how-i-built-devguess The story behind DevGuess: from Wordle-inspired curiosity to a developer-first guessing game built around real tech metadata. Thu, 12 Feb 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/blog/how-i-built-devguess https://jakubhyl.dev/tutorials/dijkstra-topological-sort-visualized Visualize shortest paths with Dijkstra's algorithm and understand dependency resolution with Topological Sort (Kahn's algorithm). Sun, 01 Feb 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/dijkstra-topological-sort-visualized https://jakubhyl.dev/tutorials/graph-traversal-algorithms-visualized A visual guide to exploring graphs using Breadth-First Search (BFS) and Depth-First Search (DFS), and understanding how traversal strategy affects performance and behavior. Sun, 01 Feb 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/graph-traversal-algorithms-visualized https://jakubhyl.dev/blog/building-this-portfolio Notes on the design decisions and technical choices behind this portfolio site. Sun, 01 Feb 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/blog/building-this-portfolio https://jakubhyl.dev/tutorials/hash-tables-visualized A visual guide to how hash tables really work: hashing, collisions, probing, resizing, and why performance can collapse if you ignore the details. Sun, 25 Jan 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/hash-tables-visualized https://jakubhyl.dev/tutorials/advanced-sorting-algorithms Explore non-comparison sorting algorithms and selection algorithms including QuickSelect, Counting Sort, and Lexicographic Counting Sort. Sun, 18 Jan 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/advanced-sorting-algorithms https://jakubhyl.dev/tutorials/understanding-sorting-algorithms A visual guide to understanding common sorting algorithms including Bubble Sort, Selection Sort, Insertion Sort, Quick Sort, and Merge Sort. Sun, 11 Jan 2026 00:00:00 GMT Jakub Hýl https://jakubhyl.dev/tutorials/understanding-sorting-algorithms