Introduction

Maintaining data model integrity is vital for seamless backend functionality. In the dynamic mobile app landscape, catering to users on older versions poses a unique challenge.

Unlike web clients, which can easily be updated for all users, mobile apps often operate on fixed versions. This requires effective backend backward compatibility management, ensuring smooth interaction for both web and mobile clients.

In this post, we'll check out some strategies and techniques to guarantee that expected data object fields remain unchanged, supported by robust unit tests and thorough documentation.

While our examples will be based on Kotlin and refer to "classes" (a fundamental concept in object-oriented programming), please note that the principles discussed are applicable and adaptable to various programming paradigms and languages.

Understanding Backend Backward Compatibility

In the area of backend software, "Backward Compatibility" refers to a backend service or API's ability to work with older clients or those expecting an older API model. It ensures that the backend's structure remains consistent, enabling smooth operation for both current and older versions of the mobile app.

Example of a Change in the Backend: Field Name Modification

Suppose we have a User data class in the backend:

data class User(
    val id: String,
    val name: String, 
    val email: String
)

The mobile app is using the getUsers API to retrieve a list of all users, with the expected user fields as mentioned in the data class: id, name, and email.

Now, we've decided that we want to change the name to fullName:

data class User(
    val id: String, 
    val fullName: String, 
    val email: String
)

In this scenario, the mobile app is unaware of the backend change. If a user with an older version of the app attempts to fetch user data from the updated backend, the app will expect the name field, resulting in an undefined or missing value. This can result in errors or even unexpected behavior further down the line.

Make backend developers notice

1. Documentation and Comments

Comprehensive documentation and comments within the codebase are essential to educate developers about the significance of not altering existing data object fields.

Example:

/**
 * Data class representing a user.
 * Will be read by the mobile app, which means we MUST KEEP all the fields for backward computability.
 *
 * @property id Unique identifier for the user. **Do not remove or change.**
 * @property name Name of the user. **Do not remove or change.**
 * @property email Email of the user. **Do not remove or change.**
 */
data class User(
    val id: String, 
    val name: String, 
    val email: String
)

2. Unit Tests for Data Object Structures

Writing unit tests that validate the structure of the expected data objects helps ensure that any changes to these structures are intentional and do not break the app.

Example:

class UserTests {

   companion object {
        val MUST_HAVE_FIELDS = setOf("id", "name", "email")
    }

    @Test
    fun testUserBackwardCompatibility() {
        val actualFields = User::class.java.declaredFields.map { it.name }.toSet()
        assertEquals(expectedFields, actualFields)
    }
}

3. Versioned Endpoints with Deprecation Notices

When introducing changes to the data model, version the endpoints and include deprecation notices to signal upcoming modifications.

Example:

@Deprecated("This endpoint will be updated in the next version. Use /api/v2/users instead.")
@GetMapping("/api/v1/users")
fun getUsersV1(): List<User> {
    // Old version of the API logic
}

Conclusion

Securing the integrity of expected data object models in the backend is vital to ensure backward compatibility and a seamless user experience. Utilize unit tests, thorough documentation, and versioning strategies to caution developers about modifying existing fields. By doing so, you'll minimize the risk of unintentional changes and foster a robust, future-proof mobile app ecosystem.