# Sandwich Integration (Hybrid)

Sandwich Integration (also known as Hybrid Integration) is a testing strategy that combines the strengths of both Top-Down and Bottom-Up approaches. It targets the middle layer of the system while simultaneously testing the top and bottom layers.

## 1. The Concept

In large systems, pure Top-Down or Bottom-Up approaches can be inefficient. Sandwich integration attempts to reap the benefits of both by testing the system in three layers:
1.  **Top Layer:** The User Interface or main control modules.
2.  **Middle Layer:** The target logic or middleware.
3.  **Bottom Layer:** Database, file system, or low-level utilities.

## 2. How it Works

Testing happens in parallel:

*   **Top-Down:** Testing starts from the UI and moves down. **Stubs** are used to simulate the Middle layer.
*   **Bottom-Up:** Testing starts from the low-level modules and moves up. **Drivers** are used to simulate the Middle layer calling them.
*   **Convergence:** The Middle layer is integrated last, replacing the stubs and drivers.

## 3. Pros and Cons

| Pros | Cons |
| :--- | :--- |
| **Parallel Testing:** Top and Bottom layers can be tested simultaneously. | **Complexity:** Requires managing both Stubs and Drivers. |
| **Early Feedback:** UI is available early (Top-Down benefit) and critical low-level logic is verified (Bottom-Up benefit). | **Cost:** High overhead in creating test doubles. |
| **Fault Isolation:** Easier to isolate bugs in the specific layer being integrated. | |

## 4. When to use it?

*   **Large Systems:** Where separate teams work on UI, Logic, and Database.
*   **Middleware-Heavy Apps:** Systems like Operating Systems or complex APIs where the middle layer is the core.

[[programming/incremental-integration]]
[[programming/software-testing-basics]]
[[programming/software-development-life-cycle]]