What is test containerization with Testcontainers?

Testcontainers is an open-source library that provides lightweight, throwaway Docker container instances for integration tests. It starts real infrastructure (PostgreSQL, Redis, Kafka, Elasticsearch, etc.) as Docker containers for test duration, then stops and removes them. Example in Java: @Container PostgreSQLContainer postgres = new PostgreSQLContainer("postgres:15"); @Test void testSave() { var ds = DataSourceBuilder.create().url(postgres.getJdbcUrl()).build(); ... }. Benefits: (1) Real dependencies: same database type as production — no SQLite vs PostgreSQL behavior differences. (2) Isolation: each test run gets a fresh container. (3) No shared state: no leftover data between test runs. (4) CI/CD compatible: works in any environment with Docker. (5) No manual setup: no need to maintain a shared test database server. Available for: Java, .NET, Python, Go, JavaScript, Rust. Slower than in-memory databases — offset by running in parallel. The gold standard for database integration tests.