A compact advanced-level reference covering graph/tree models, traversal, shortest paths, connectivity, and practical problem-solving patterns.
1. Core Relationship A tree is a connected acyclic graph .\ For a tree with n vertices: Edges = n 1 Exactly one simple path exists between any two vertices. Removing any edge disco…
Real admin screenshots from two live sites: who uses them, when, and on which device. Plus the two paisa that quietly break most bill-splitting apps, how a whole month settles in just 5 payments, and a 75-lesson course you can unlock with one word.
Note: This is the first post in a series about the sites I actually run in production: Bachelor Point , Team Sober , Mila Mart and Love . No toy demos here. These are real users, r…
One request, one correlation id, every step on record.** PayNexa writes structured logs with Serilog from every service. Each log goes to three places at once: the console, a rolling JSON file, and Seq, a searchable log server running in Docker. OpenTelemetry traces go to Seq as well. Every line from the first byte of a request to the last Kafka message it causes carries the same `CorrelationId`: HTTP, validation, SQL Server, MongoDB, Redis, the outbox, Kafka and calls to other services. Personal data and secrets are masked before anything is written. This document uses the **Customer API** as the worked example. Every other PayNexa service gets the same behavior from the shared building blocks with no extra code.
Contents 0. Github Open Source 1. The big picture 2. Where Serilog runs and where logs go 3. Prerequisites 4. Startup: how logging is switched on 5. The HTTP pipeline: what wraps e…
Note: Guys, For better explaining of the scenario in learning purpose and analysis, I'm using an bank name lets say Sober Bank(SB) Imagine in our Sober Bank the database is complet…
How SQL's "unknown" value quietly hides broken data that a `0` never could.
0 is a known, concrete value — it means "zero, exactly." NULL means "unknown / not applicable / missing" — it's the absence of a value. SQL treats them completely differently: | Be…
Quick reference for running Bachelor Point's local Docker stack (`docker-compose.yml`) with Docker Desktop — building the image, applying Prisma migrations, and managing the `db`, `migrate` and `app` containers.
Stack overview All commands below assume your terminal's working directory is the project root (where docker compose.yml lives). Docker Compose reads .env in that same folder autom…
solrsearch
Creating a collection Run below commands in powershell v1 — GET v2 — POST
A compact advanced-level reference covering linked-list invariants, pointer techniques, complex operations, variants, and high-value interview patterns.
1. Core Structure A linked list stores elements as nodes connected through references rather than contiguous memory. A typical singly linked node contains: Unlike arrays, random ac…
What protects the app today, what each layer actually stops, and what it cannot stop.
The honest summary You cannot tell whether sdfsfsfsdfsf@gmail.com is a real mailbox without sending it mail. Gmail publishes MX records, the address is syntactically valid, and Goo…
A green pipeline, an email that actually arrives, and six failures that taught me more than any tutorial
What Gives Me Shanti? Shanti — শান্তি. Not the absence of work. The absence of dread . The specific quiet that arrives when the Stage View turns green all the way across, and you k…
Wiring the Jenkins server at **<http://16.171.254.209:8080>** so that every push to the `develop` branch automatically builds and deploys to the k3s server at `54.251.180.71`. Written for someone who has not set up a Jenkins pipeline before. Follow the steps in order — each one depends on the one before it.
What you are building Three separate machines are involved. Keeping them straight makes the rest much easier: | Machine | Address | Role | | | | | | Jenkins | 16.171.254.209 | Runs…
How `admin@team-sober.com` sends and receives mail, for free. Without this configured **members cannot self-register**: registration creates the account and issues a password-setup token, but the link never reaches anyone.
1. How the app decides to send isMailConfigured() in src/lib/mailer.ts is the whole gate: Both SMTP HOST and SMTP FROM must be non empty. If either is missing, sendMail() returns e…
Overview When developing a Node.js application locally, you often need to: Stop any previously running Node.js processes. Set environment variables (such as passwords or secrets).…
A step-by-step guide for students new to Docker — three commands take you from source code to a running blog at localhost:3000, with an explanation of what each one does.
Goal Run this blog on your own machine inside a Docker container, without installing Node.js. Everything the project needs — Node 20, the dependencies, the compiled code — is built…
Part 1 — The Roles: Who Does What People often blur these three together. They are separate concerns: | Piece | What it actually is | Responsibility | | CI/CD | A practice / method…
Learn how to simplify your Git Bash prompt by displaying only the current folder instead of the full directory path.
🎯 Goal By the end of this guide, your Git Bash prompt will display only the current folder name instead of the entire directory path, making your terminal cleaner and easier to re…
The Lifecycle at a Glance The loop matters more than the sequence. Mature teams run all eight phases continuously rather than as a one time waterfall. PHASE 1 — Discovery and Produ…
Part 1 — Beginner 1.1 What Problem Does Docker Solve? "It works on my machine" happens because the application depends on things outside the code: OS version Runtime version (.NET…
One of the biggest deployment challenges is making sure the application behaves consistently across all environments while allowing environment-specific settings (like connection strings and API keys).
1. Keep Configuration Outside the Code (12 Factor App) Never hardcode environment specific values. Bad Good Store values in configuration files, environment variables, or a secret…
ORM (Object-Relational Mapping) is a technique that enables developers to interact with a relational database using programming language objects instead of writing raw SQL queries. It maps database tables to classes, rows to objects, and columns to properties, simplifying data access and improving productivity, maintainability, and code readability. In ASP.NET Core, **Entity Framework Core (EF Core)** is the most commonly used ORM.
What is ORM? ORM (Object Relational Mapping) is a technique that allows developers to work with a database using C objects instead of writing raw SQL queries. It maps: 1. Database…
Image layers accumulate quietly on a 6.6 GB volume
A single pipeline run moved disk usage from 52.8% to 60.6% on a 6.6 GB root volume — roughly 500 MB per build. Every run writes a fresh set of image layers, and nothing removes the…
Test permissions as the user that will run the command
Group membership does not apply to an already-running process.
Jenkins runs as its own system user. The permission that matters is the jenkins user's, not yours. Test it the way Jenkins will: If that fails with a socket permission error: The r…
Delivered and acted upon are different claims
HTTP 200 means the request was accepted, not that anything happened.
GitHub showed green checkmarks for every webhook delivery — the initial ping and both pushes. Jenkins was receiving them and returning HTTP 200. No build ever started. Why Green in…
Build duration is a diagnostic signal
Sub-second failures are almost always configuration, not code.
Three builds in a row failed almost instantly. That duration is the clue. A real run clones a repository, installs dependencies and builds an image — it takes minutes. Half a secon…
Declarative pipelines compile before they run
Pipeline syntax errors do not fail a stage — they fail before stages exist.
My Jenkinsfile called emailtext instead of emailext , and was missing the commas between arguments. That was not an email bug. A declarative pipeline is compiled as Groovy before a…
How a nested git repository silently published an empty project
A green push is not proof your code shipped.
My project folder was its own git repository, nested inside another one. When I ran git add automation from the parent, git did not add the files. It recorded a gitlink — a pointer…
The network failed after the charge but before the response — now what
A client calls POST /payments . The charge succeeds. The response times out on the way back. The client has no idea whether it worked. If it retries, you might charge twice. If it…
One query for the list, then one more per row, forever
You fetch 50 posts. Then, for each one, you read its author. That is 1 + 50 = 51 queries where 2 would do. It looks harmless because each individual query is fast. The problem is t…
Layer caching is ordering-sensitive, and most Dockerfiles get the order wrong
Docker caches each instruction as a layer. When one changes, that layer and every layer after it is rebuilt. So the order of your Dockerfile decides your build time. The common mis…
Turning O(n·k) into O(n) by not throwing away work you already did
Most array problems have an obvious brute force answer: check every window, take the best one. It works, and it is O(n·k). Consecutive windows overlap almost completely. Only one e…