Database

Database transactions make groups of reads and writes atomic, but atomicity alone does not answer what concurrent transactions are allowed to observe. That is the job of isolation. The practical challenge appears when correct transactions conflict. Strong isolation can intentionally abort one transaction rather than allow an invalid interleaving. Applications need to distinguish those retryable concurrency failures from ordinary errors. Isolation protects invariants, not just statements Consider two concurrent requests that reserve the last available item.

MySQL installations are commonly configured to listen only on a local interface. If a database must accept connections from another machine, you need to adjust the server’s listening address, create an appropriately scoped account, and make sure the network firewall allows only the required clients. Step 1: Update the MySQL Configuration Edit the MySQL configuration file On Ubuntu and Debian installations, a common file is: Copy sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf Change bind-address

Backups are an important part of MongoDB administration. Two standard command-line tools for logical backups and restores are mongodump and mongorestore, which work with BSON data and collection metadata. 1. Back Up a Database with mongodump Use mongodump to create a BSON backup that can later be restored. Basic syntax: Copy mongodump -d <database_name> -o <backup_directory> -d <database_name> → the database to back up. -o <backup_directory> → the directory where the dump will be written. Example:

Applications often need to represent hierarchical data such as parent categories, child categories, and deeper descendants. MongoDB’s aggregation framework includes $graphLookup, which can recursively traverse relationships within a collection. This article shows how to use $graphLookup to retrieve a category hierarchy from documents that reference their parent category. 1. Data Structure Consider the following documents in a MongoDB collection: Copy { "_id" : 1, "cat_id" : 1, "title" : "Parent Category", "parent" : null } { "_id" : 2, "cat_id" : 2, "title" : "Child Category", "parent" : 1 } { "_id" : 3, "cat_id" : 3, "title" : "Sub Child Category", "parent" : 2 } The relationships are:

Managing users and databases is a fundamental MySQL administration task. This guide shows how to create a user, create a database, and grant that user the permissions needed to work with the database. 1. Create a New MySQL User Start by creating a user with its own login credentials. Basic syntax: Copy CREATE USER 'user' IDENTIFIED BY 'password'; 'user': the username to create. 'password': the password for that user. Example: