Catch Bugs Early with Testcontainers: Shift-Left Testing Made Easy

Catch bugs early by running integration tests with real services using Testcontainers, improving reliability, speed, and confidence in your code.

May 15, 2025

Abhimanyu Saharan

Modern software development demands high velocity, rapid iteration, and uncompromising reliability. Elite engineering teams, as highlighted by DORA metrics, outperform their peers by integrating best practices such as shift-left testing. Among the tools enabling this paradigm shift is , a library that empowers developers to run integration tests using real dependencies in disposable Docker containers.

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Metric	Traditional Workflow	Shift-Left with Testcontainers	Improvement
Lead Time for Changes (LTC)	1–2 hours (delayed feedback)	< 20 minutes (local feedback)	>65% faster
Deployment Frequency (DF)	3–4 times/day	10+ times/day	2x more
Change Failure Rate (CFR)	High (bugs escape)	Lower (bugs caught early)	15x cheaper fixes
Mean Time to Recovery (MTTR)	Hours to days	Minutes	DORA elite standard
Infrastructure Cost	High (shared envs, CI time)	Low (local, ephemeral)	Significant savings

@pytest.fixture(scope="module", autouse=True)
def setup(request):
    postgres = PostgresContainer("postgres:16-alpine")
    postgres.start()

    os.environ["DB_HOST"] = postgres.get_container_host_ip()
    os.environ["DB_PORT"] = postgres.get_exposed_port(5432)
    os.environ["DB_USERNAME"] = postgres.username
    os.environ["DB_PASSWORD"] = postgres.password
    os.environ["DB_NAME"] = postgres.dbname

    customers.create_table()

    def teardown():
        postgres.stop()

    request.addfinalizer(teardown)

def test_get_all_customers():
    customers.create_customer("Alice", "[email protected]")
    customers.create_customer("Bob", "[email protected]")
    assert len(customers.get_all_customers()) == 2

async registerUser(email: string, username: string): Promise<User> {
    const existingUser = await this.userRepository.findOne({ where: { email } });
    if (existingUser) {
        throw new Error("Email already exists");
    }
    // proceed to register user
}

beforeAll(async () => {
    container = await new PostgreSqlContainer("postgres:16").start();
    dataSource = new DataSource({
        type: "postgres",
        host: container.getHost(),
        port: container.getMappedPort(5432),
        username: container.getUsername(),
        password: container.getPassword(),
        database: container.getDatabase(),
        synchronize: true,
        entities: [User],
    });
    await dataSource.initialize();
    userService = new UserRegistrationService(dataSource.getRepository(User));
});

test('should prevent registration with same email in different case', async () => {
  await userService.registerUser({ email: '[email protected]', password: 'pw' });
  await expect(
    userService.registerUser({ email: '[email protected]', password: 'pw' })
  ).rejects.toThrow('Email already exists');
});