https://abidalamin.dev A personal engineering knowledge base: software concepts, system design practices and AI notes, written from real engineering experience. en Tue, 06 Oct 2026 10:51:18 GMT https://abidalamin.dev/concepts/rate-limiting-algorithms https://abidalamin.dev/concepts/rate-limiting-algorithms Token bucket, leaky bucket, fixed window, sliding window log and sliding window counter — how each one decides, where each one breaks, and how to choose between them. Concepts Scalability Tue, 06 Oct 2026 00:00:00 GMT https://abidalamin.dev/concepts/file-permissions https://abidalamin.dev/concepts/file-permissions Every file on a Unix system carries an owner, a group and nine permission bits. This is what those bits mean, why 600 and 700 keep showing up, how the octal digits are built, and the rules that make permissions behave differently than most people assume. Concepts Security Sat, 29 Aug 2026 00:00:00 GMT https://abidalamin.dev/concepts/nat-port-forwarding https://abidalamin.dev/concepts/nat-port-forwarding A packet can arrive at one port number and be delivered to another. Once you know where that rewrite happens, firewall rules stop being guesswork — you allow the port the service actually receives traffic on inside the machine, not the port you typed into the client. Concepts Networking Sat, 29 Aug 2026 00:00:00 GMT https://abidalamin.dev/concepts/ssh-keys https://abidalamin.dev/concepts/ssh-keys An SSH key is a public/private keypair that proves who you are to a server without ever sending a secret over the wire — replacing passwords with a challenge you can only answer if you hold the private half. Concepts Security Sat, 29 Aug 2026 00:00:00 GMT https://abidalamin.dev/concepts/circuit-breaker https://abidalamin.dev/concepts/circuit-breaker A circuit breaker stops a service from repeatedly calling a dependency that's already failing, giving it room to recover instead of being buried under retries. Concepts Architecture Wed, 12 Aug 2026 00:00:00 GMT https://abidalamin.dev/ai/llm-evaluation https://abidalamin.dev/ai/llm-evaluation Building eval sets, choosing between deterministic checks and model graders, and catching regressions before a prompt change ships. AI Learning Evaluation Mon, 10 Aug 2026 00:00:00 GMT https://abidalamin.dev/ai/tool-calling https://abidalamin.dev/ai/tool-calling How structured tool use actually works under the hood, why it isn't function execution, and what makes a tool definition easy for a model to call correctly. AI Learning Agents Mon, 27 Jul 2026 00:00:00 GMT https://abidalamin.dev/concepts/consistent-hashing https://abidalamin.dev/concepts/consistent-hashing Consistent hashing distributes keys across nodes so that adding or removing a node only reshuffles a small fraction of keys, instead of nearly all of them. Concepts Distributed Systems Sat, 25 Jul 2026 00:00:00 GMT https://abidalamin.dev/ai/ai-agents https://abidalamin.dev/ai/ai-agents What separates an agent from a single LLM call, how the agent loop actually runs, and where autonomous systems fail in practice. AI Learning Agents Mon, 13 Jul 2026 00:00:00 GMT https://abidalamin.dev/concepts/idempotency https://abidalamin.dev/concepts/idempotency An idempotent operation produces the same result no matter how many times it's applied, which is the property that makes retrying a failed request safe in a distributed system. Concepts Distributed Systems Fri, 10 Jul 2026 00:00:00 GMT https://abidalamin.dev/ai/rag-vs-fine-tuning https://abidalamin.dev/ai/rag-vs-fine-tuning Two different mechanisms for adapting an LLM to your domain, what each one actually changes inside the system, and how to pick between them. AI Learning RAG Mon, 29 Jun 2026 00:00:00 GMT https://abidalamin.dev/concepts/outbox-pattern https://abidalamin.dev/concepts/outbox-pattern The outbox pattern makes writing to a database and publishing an event look atomic by writing both in one transaction and relaying the event asynchronously. Concepts Distributed Systems Mon, 22 Jun 2026 00:00:00 GMT https://abidalamin.dev/system-design/file-storage-system https://abidalamin.dev/system-design/file-storage-system Design a file storage and sync service like Dropbox: chunking, deduplication, metadata separation, and multi-device sync. System Design Storage Fri, 19 Jun 2026 00:00:00 GMT https://abidalamin.dev/concepts/WAL https://abidalamin.dev/concepts/WAL Write-Ahead Logging (WAL) is a technique used in databases to ensure data integrity and durability by recording changes to a log before applying them to the database. Concepts Databases Mon, 15 Jun 2026 00:00:00 GMT https://abidalamin.dev/ai/prompt-engineering https://abidalamin.dev/ai/prompt-engineering Structuring prompts, choosing between system and user content, and the failure modes that show up once a prompt leaves the playground. AI Learning Prompt Engineering Mon, 15 Jun 2026 00:00:00 GMT https://abidalamin.dev/concepts/cache-stampede https://abidalamin.dev/concepts/cache-stampede A cache stampede happens when a popular cached value expires and every waiting request rushes the origin at once, turning a cache miss into a self-inflicted denial of service. Concepts Caching Mon, 08 Jun 2026 00:00:00 GMT https://abidalamin.dev/ai/vector-databases https://abidalamin.dev/ai/vector-databases Why nearest-neighbor search needs its own index structure, how HNSW works, and what actually differs between vector database options. AI Learning Embeddings Tue, 02 Jun 2026 00:00:00 GMT https://abidalamin.dev/system-design/rate-limiter https://abidalamin.dev/system-design/rate-limiter Design a distributed rate limiter for an API gateway: algorithm choice, distributed counters, and protecting shared backends under load. System Design Scalability Thu, 28 May 2026 00:00:00 GMT https://abidalamin.dev/concepts/database-index https://abidalamin.dev/concepts/database-index A database index trades extra storage and slower writes for the ability to find rows without scanning the whole table, the same trade a book's index makes for its reader. Concepts Databases Mon, 25 May 2026 00:00:00 GMT https://abidalamin.dev/ai/embeddings https://abidalamin.dev/ai/embeddings How text becomes a vector, why cosine similarity works as a proxy for meaning, and what actually determines embedding quality. AI Learning Embeddings Mon, 18 May 2026 00:00:00 GMT https://abidalamin.dev/concepts/http-1-2-3 https://abidalamin.dev/concepts/http-1-2-3 Each HTTP version fixes a specific bottleneck in the one before it, moving from one request per connection to multiplexed streams to a transport that no longer blocks on packet loss. Concepts Networking Sun, 10 May 2026 00:00:00 GMT https://abidalamin.dev/system-design/chat-system https://abidalamin.dev/system-design/chat-system Design a real-time chat system like WhatsApp or Slack: connection management, message ordering, delivery guarantees, and fan-out at scale. System Design Real-time Wed, 06 May 2026 00:00:00 GMT https://abidalamin.dev/ai/rag https://abidalamin.dev/ai/rag How retrieval augmented generation grounds LLM outputs in your own data, and where the pattern breaks down in production. AI Learning RAG Tue, 05 May 2026 00:00:00 GMT https://abidalamin.dev/concepts/oauth-2 https://abidalamin.dev/concepts/oauth-2 OAuth 2.0 is an authorization framework that lets one application access resources on a user's behalf without ever seeing that user's password. Concepts Authentication Tue, 28 Apr 2026 00:00:00 GMT https://abidalamin.dev/concepts/jwt https://abidalamin.dev/concepts/jwt A JWT is a signed, self-contained token format that lets a server verify claims about a user without a database lookup, at the cost of being hard to revoke early. Concepts Authentication Wed, 15 Apr 2026 00:00:00 GMT https://abidalamin.dev/system-design/notification-system https://abidalamin.dev/system-design/notification-system Design a scalable notification system supporting email, SMS and push, with fan-out, retries, and per-channel rate limits. System Design Messaging Tue, 14 Apr 2026 00:00:00 GMT https://abidalamin.dev/system-design/url-shortener https://abidalamin.dev/system-design/url-shortener Design a URL shortening service like Bitly: encoding schemes, redirect latency, and read-heavy caching at scale. System Design Storage Fri, 10 Apr 2026 00:00:00 GMT https://abidalamin.dev/concepts/pkce https://abidalamin.dev/concepts/pkce PKCE hardens the OAuth 2.0 authorization code flow against interception attacks by binding the authorization request to the token exchange with a one-time secret. Concepts Authentication Fri, 03 Apr 2026 00:00:00 GMT