As software systems evolve over time, it is common for changes to be made to the data schema of objects. Handling schema evolution is crucial for smooth serialization and deserialization processes, especially in C++ where efficiency and performance are paramount. In this blog post, we will explore some techniques for effectively handling schema evolution in C++ object serialization and deserialization.

1. Versioning of Serialized Objects

One of the fundamental techniques for handling schema evolution is to introduce versioning of serialized objects. By including a version field in the serialized data, we can distinguish between different versions of the object’s schema. When deserializing, the program can then adapt its behavior based on the version of the serialized object.

It is common to use a fixed-size field to store the version number in the serialized data. This allows for efficient extraction and comparison of versions during deserialization. The version number can be incremented every time a change is made to the schema, providing a clear indication of the compatibility between different versions.

struct SerializedData {
    uint32_t version;
    // Other serialized fields...
};

2. Handling Schema Changes

When a schema change occurs, there are three main scenarios to consider:

2.1. Adding Fields

When new fields are added to the object schema, older versions of the serialized data may not include these fields. To handle this scenario, we need to provide default values for the newly added fields during deserialization. This ensures that the newly deserialized objects have all the necessary fields initialized properly.

struct SerializedDataV1 {
    uint32_t version;
    // Fields up to version 1...
};

struct SerializedDataV2 {
    uint32_t version;
    // Fields up to version 1...
    
    // Newly added fields in version 2.
    int newField = 0;
};

void Deserialize(SerializedData& data) {
    if (data.version == 1) {
        // Deserialize for version 1.
    } else if (data.version == 2) {
        // Deserialize for version 2.
    }
    // Handle future versions...
}

2.2. Removing Fields

When fields are removed from the object schema, the corresponding data in the serialized objects becomes obsolete. To handle this scenario, we can simply ignore the fields during deserialization if they are not present in the serialized data. This allows older versions of the serialized objects to be successfully deserialized without the obsolete fields.

2.3. Modifying Fields

Modifying fields in the object schema can introduce compatibility issues between different versions. In this case, it is necessary to carefully handle the conversion of data between the different formats. The program should be able to detect the old format and convert the data to the new format during deserialization.

This may involve performing type conversions, adjusting data layouts, or applying additional logic to transform the data. By carefully designing the conversion process, we can ensure that the serialized objects can be successfully deserialized, maintaining backward compatibility.

Conclusion

Handling schema evolution is a crucial aspect of object serialization and deserialization in C++. By incorporating versioning, handling field additions, removals, and modifications, we can effectively manage changes to object schemas over time. These techniques ensure that serialized objects can be seamlessly deserialized across different versions, providing a robust data interchange mechanism for evolving software systems.

#serialization #deserialization