Articles

Explanations, step-by-step guides, cheat sheets and fixes. Pick an area, or tell us what you need.

309 entries

0235 Fix: MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017 Your app or tool tried to reach MongoDB on port 27017 and nothing accepted the connection. Usually mongod is not running, crashed… Intermediate · Fixes an error · Linux, macOS, Windows Article Administration · Basics 15 Oct 2026
0233 Turn on MongoDB authentication and create least-privilege users A fresh MongoDB install accepts connections without a password. Create a user administrator, enable authorization, then give each application and backup job… Intermediate · Step by step · Linux Article Administration · Security & access 15 Oct 2026
0232 mongosh cheat sheet: CRUD, indexes and explain() The mongosh commands you use every day: switching databases, inserting, querying, updating and deleting documents, creating indexes and checking a query plan… Beginner · Cheat sheet · Any system Article Basics 15 Oct 2026
0231 MongoDB documents and collections compared with SQL tables MongoDB stores data as JSON-like documents grouped in collections instead of rows in tables. This explains the model, the _id field, embedding… Beginner · Explains an idea · Any system Article Basics · Choosing a database 15 Oct 2026
0227 Fix: ORA-01017: invalid username/password; logon denied Oracle rejected the login. Check the password (case-sensitive), that you connect to the right PDB service, and whether the account is locked… Intermediate · Fixes an error · Any system Article Administration · Security & access 15 Oct 2026
0226 Fix: ORA-12541: TNS:no listener The client reached the host but nothing was listening on that port. Start or fix the Oracle listener, or correct the host… Intermediate · Fixes an error · Linux, Windows Article Administration · Concepts & tools 15 Oct 2026
0225 Oracle Data Pump: export and import schemas with expdp and impdp Data Pump moves schemas, tables or whole databases between Oracle databases as dump files. Set up a directory object, export with expdp… Intermediate · Step by step · Linux, Windows Article Administration · Backup with RMAN 15 Oct 2026
0222 Connecting to Oracle: SQL*Plus, SQLcl, EZConnect, tnsnames.ora and the listener Clients reach an Oracle database through the listener, using an Easy Connect string or a tnsnames.ora alias. Here are the formats, the… Intermediate · Cheat sheet · Linux, macOS, Windows Article Administration · Concepts & tools 15 Oct 2026
0221 Oracle Database basics: instance vs database, CDB and PDB An Oracle instance is memory and processes; the database is the files on disk. Since 21c every database is a container database… Beginner · Explains an idea · Linux, Windows Article Concepts & tools 15 Oct 2026
0220 Fix: ERROR 1045 (28000): Access denied for user ‘appuser’@’localhost’ (using password: YES) The server rejected the user, host and password combination. Usually it is a wrong password, a host part that does not match,… Beginner · Fixes an error · Linux Article Administration · Security & access 15 Oct 2026
0218 Setting up MySQL source-replica replication with GTIDs Step by step: configure a MySQL 8.4 source and replica with GTIDs, create the replication account, copy the data, connect the replica… Advanced · Step by step · Linux Article Administration · Replication 15 Oct 2026
0217 MySQL binary logs and point-in-time recovery The binary log records every change after your last backup. Restore the backup, then replay the binary log up to just before… Advanced · Explains an idea · Linux Article Backup & restore · Replication 15 Oct 2026
0216 MySQL vs MariaDB: what actually differs MariaDB started as a fork of MySQL and still speaks the same client protocol, but GTIDs, JSON storage, authentication and tooling have… Intermediate · Explains an idea · Linux Article Choosing a database · MySQL & MariaDB 15 Oct 2026
0215 mysql command-line client cheat sheet The connection options, built-in commands and everyday SQL you need in the mysql (and mariadb) command-line client, on one page. Beginner · Cheat sheet · Any system Article Administration · Queries 15 Oct 2026
0214 MySQL users and privileges: CREATE USER, GRANT and authentication plugins A MySQL account is a user name plus the host it connects from. You create it with CREATE USER, give it rights… Beginner · Step by step · Linux Article Administration · Security & access 15 Oct 2026
0211 Fix: psql connection to server on socket /var/run/postgresql/.s.PGSQL.5432 failed psql tried the local Unix socket and found nothing there. The server is not running, listens on another port, or uses a… Beginner · Fixes an error · Linux Article Install & connect 15 Oct 2026
0210 Fix: FATAL: no pg_hba.conf entry for host The server received the connection but no pg_hba.conf rule matches this client address, user, database and SSL mode. Add or correct a… Intermediate · Fixes an error · Linux Article Install & connect · Security & access 15 Oct 2026
0209 Fix: FATAL: password authentication failed for user PostgreSQL reached the server and found a matching pg_hba.conf rule, but the password check failed. Usually a wrong password, a role without… Intermediate · Fixes an error · Any system Article Roles & administration · Security & access 15 Oct 2026
0205 Set up PostgreSQL streaming replication with a hot standby Step-by-step: prepare a primary, clone it with pg_basebackup, start a read-only standby that streams WAL, check it works, and understand promotion and… Advanced · Step by step · Linux Article Replication & high availability 15 Oct 2026
0204 VACUUM, autovacuum and table bloat in PostgreSQL Updates and deletes leave dead row versions behind. VACUUM makes that space reusable, keeps statistics fresh and prevents transaction ID wraparound; autovacuum… Intermediate · Explains an idea · Linux Article Performance & tuning · Roles & administration 15 Oct 2026
0203 Physical backups and point-in-time recovery with pg_basebackup and WAL archiving A base backup plus a continuous archive of WAL files lets you restore a PostgreSQL cluster to any moment after the backup,… Advanced · Explains an idea · Linux Article Backup & restore · Replication & high availability 15 Oct 2026
0202 Back up and restore PostgreSQL with pg_dump, pg_restore and pg_dumpall Make logical backups of one database with pg_dump, of roles and the whole cluster with pg_dumpall, and restore them with pg_restore or… Beginner · Step by step · Linux Article Backup & restore 15 Oct 2026
0201 pg_hba.conf and PostgreSQL authentication methods pg_hba.conf decides who may connect from where and how they must prove who they are. Rules are checked top to bottom and… Intermediate · Explains an idea · Linux Article Install & connect · Security & access 15 Oct 2026
0200 PostgreSQL roles, privileges and default privileges Create login and group roles, grant access to databases, schemas and tables, and use ALTER DEFAULT PRIVILEGES so tables created later get… Intermediate · Step by step · Linux Article Roles & administration · Security & access 15 Oct 2026
0199 psql meta-commands cheat sheet The backslash commands you use every day in psql: connecting, listing databases, tables and roles, describing objects, timing queries, expanded output and… Beginner · Cheat sheet · Any system Article Install & connect · Roles & administration 15 Oct 2026
0198 Install PostgreSQL and connect with psql on Debian, Ubuntu and RHEL Install the PostgreSQL server from your distribution or the official PGDG repositories, start it, and open your first psql session as the… Beginner · Step by step · Linux Article Install & connect 15 Oct 2026
0193 Linux logs reference: journalctl, journald storage, logrotate and centralized logging Read and filter systemd journal logs, keep their disk use in check, rotate classic log files with logrotate, and understand when to… Intermediate · Cheat sheet · Linux Article Linux administration · Logs 15 Oct 2026
0192 Grafana basics: connect Prometheus, build a dashboard and set up an alert Connect Grafana to Prometheus, build a dashboard for your servers with PromQL panels and a host selector, and create an alert rule… Intermediate · Step by step · Linux Article Prometheus & Grafana 15 Oct 2026
0191 Prometheus concepts: pull model, exporters, node_exporter and PromQL basics Prometheus scrapes numeric metrics from your systems over HTTP and stores them as time series. Learn the pull model, metric types, node_exporter,… Intermediate · Explains an idea · Linux Article Prometheus & Grafana 15 Oct 2026
0190 Fix: Terraform Error acquiring the state lock Another run holds the lock on your Terraform state, or a crashed run left it behind. Find out who holds it, wait… Advanced · Fixes an error · Any system Article Terraform 15 Oct 2026
0189 Terraform remote state: backends, locking and migrating local state Move Terraform state from a local file into a shared backend such as S3 with locking, so a team can work safely.… Advanced · Step by step · Any system Article Terraform 15 Oct 2026
0187 Terraform concepts: providers, state, plan and apply, modules, and OpenTofu Terraform describes infrastructure in code, compares it with what exists, and changes only the difference. Learn providers, resources, state, the plan/apply loop,… Beginner · Explains an idea · Any system Article Terraform 15 Oct 2026
0186 Fix: Ansible UNREACHABLE! Failed to connect to the host via ssh Ansible could not open an SSH session to the host, so no task ran. The text after the colon tells you why:… Intermediate · Fixes an error · Linux, macOS Article Ansible · SSH & servers 15 Oct 2026
0185 Ansible ad-hoc commands cheat sheet One-line ansible commands for quick jobs across many servers: ping hosts, run a command, copy files, manage packages, users and services, gather… Intermediate · Cheat sheet · Linux, macOS Article Ansible 15 Oct 2026
0184 Ansible Vault: keep passwords and keys encrypted in your repo Ansible Vault encrypts variable files or single values so secrets can live in Git next to your playbooks. Create, edit and rekey… Advanced · Step by step · Linux, macOS Article Ansible 15 Oct 2026
0181 Ansible concepts: inventory, modules, playbooks, idempotency and roles Ansible configures many servers over SSH from one control machine. Learn the core pieces: inventory, modules, tasks, playbooks, roles, and why runs… Beginner · Explains an idea · Linux, macOS Article Ansible 15 Oct 2026
0177 Users, groups and sudo on Debian, Ubuntu and RHEL How Linux accounts and groups work, how to create users, and how sudo gives admin rights through the sudo group on Debian/Ubuntu… Beginner · Explains an idea · Linux Article Users, sudo & hardening 15 Oct 2026
0176 apt vs dnf: package commands and automatic security updates The everyday package commands on Debian/Ubuntu (apt) and RHEL/Rocky/Alma (dnf) side by side, plus how to turn on automatic security updates on… Beginner · Cheat sheet · Linux Article Packages & updates 15 Oct 2026
0175 Open ports with firewalld and ufw firewalld (RHEL, Rocky, AlmaLinux) and ufw (Ubuntu) are friendly front ends to the kernel firewall. Allow SSH first, then open only the… Intermediate · Step by step · Linux Article Networking & firewalls 15 Oct 2026
0174 Network and DNS troubleshooting commands: ip, ss, dig, resolvectl A layer-by-layer cheat sheet for "the server cannot reach X": interfaces and routes with ip, listening ports with ss, and name resolution… Beginner · Cheat sheet · Linux Article Networking & firewalls 15 Oct 2026