Databases

Data persistence is at the heart of every backend system. This domain covers relational databases (SQL, indexes, transactions), NoSQL trade-offs, connection pooling (HikariCP), and schema migration tools (Flyway and Liquibase). Understanding how to write efficient queries and manage schema evolution safely is expected at every level.

What You'll Find Here

Notes are being added. Planned topics:

Topic	Description
SQL Fundamentals	SELECT, JOIN types, GROUP BY, window functions, subqueries.
Indexes & Query Performance	B-tree indexes, composite indexes, EXPLAIN plans, index pitfalls.
Transactions & ACID	Atomicity, Consistency, Isolation (levels), Durability; deadlocks.
Connection Pooling	HikariCP configuration (maximum-pool-size, connection-timeout).
NoSQL Trade-offs	CAP theorem; when to use Redis, MongoDB, Cassandra, or Elasticsearch.
Schema Migration	Flyway vs. Liquibase; versioned migrations; repeatable scripts.

Learning Path

1. SQL Fundamentals — JOIN types and GROUP BY with aggregation are the most common SQL interview questions.
2. Indexes — understand when an index helps and when it hurts (write amplification).
3. Transactions & ACID — isolation levels (READ COMMITTED, REPEATABLE READ, SERIALIZABLE) and their phantom-read implications.
4. Connection Pooling — HikariCP is Spring Boot's default; know the key pool size configuration settings.
5. Schema Migration — Flyway is the simpler option; Liquibase supports rollbacks. Know the trade-offs.

Related Domains

• Spring Data — JPA and @Transactional are the Spring abstraction over the database layer.
• System Design — database selection (relational vs. NoSQL) is a core system design skill.
• Docker — databases in local development environments are typically run with Docker Compose.