What is integration testing?

Integration Testing is a type of testing aimed at verifying the interaction between different modules, components, or systems. Its goal is to identify defects related to integration, that is, how parts of the system work together.

Main objectives of integration testing:

• Verify the correctness of data transfer between modules.
• Detect interface and interaction protocol errors.
• Clarify the boundaries of responsibility between modules.
• Verify that the entire integrated system meets functional requirements.

Approaches to integration testing:

1. "Big Bang": All modules are integrated simultaneously, and then testing is conducted.
   • Advantages: Simple implementation with a small number of modules.
   • Disadvantages: Difficult to localize defects, high risk of late problem detection.
2. "Top-Down": Testing starts from the top levels of the module hierarchy. Dependent modules are either implemented or replaced with stubs.
   • Advantages: Early detection of architectural issues, better manageability of the process.
   • Disadvantages: Need to develop a large number of stubs, possible difficulties testing lower levels.
3. "Bottom-Up": Testing starts from the lower levels of the hierarchy. Higher-level modules are replaced with drivers.
   • Advantages: Early detection of defects at lower levels, ease of driver development.
   • Disadvantages: Late detection of architectural issues, upper levels are integrated late.
4. "Sandwich" or "Hybrid": Combines top-down and bottom-up approaches. The upper and lower levels are tested in parallel and then connected in the middle.

# Example pseudocode for interaction testing between two modules
# Assuming the existence of UserModule and OrderModule

def test_user_creates_order_integration():
    # Step 1: Create a test user in UserModule
    user_id = UserModule.create_user("test_user", "password")

    # Verify that the user was successfully created (can use assert)
    assert UserModule.get_user(user_id) is not None, "User was not created"

    # Step 2: Create an order for this user in OrderModule
    product_id = "product_abc"
    quantity = 2
    order_id = OrderModule.create_order(user_id, product_id, quantity)

    # Verify that the order was successfully created and linked to the user
    assert order_id is not None, "Order was not created"
    order_details = OrderModule.get_order(order_id)
    assert order_details["user_id"] == user_id, "Order is not linked to the correct user"
    assert order_details["items"][0]["product_id"] == product_id, "Incorrect product in order"
    assert order_details["items"][0]["quantity"] == quantity, "Incorrect product quantity"

    # Step 3 (optional): Check the user's state after creating the order,
    # if OrderModule affects UserModule (e.g., adds order to user's history)
    user_profile = UserModule.get_user_profile(user_id)
    assert order_id in user_profile["order_history"], "Order not added to user's history"

    # Cleanup after test (if necessary)
    # OrderModule.delete_order(order_id)
    # UserModule.delete_user(user_id)

# Call the test function
# test_user_creates_order_integration()

Integration testing is a critical link between unit testing and system testing, ensuring confidence in the correct interaction of system components.