Distributed databases handle schema changes through various strategies, depending on the specific database system and its design. Generally, changes to the schema can be made in a way that minimizes downtime and avoids inconsistencies across different nodes. Common approaches include online schema migrations, versioning systems, and techniques that allow backward compatibility.
One common method is online schema migration, which allows developers to modify the schema while the database is still operational. For example, adding a new column can be done without locking the entire table. The database may create a temporary copy of the table with the new structure, populate it with data, and then switch over to this updated version. This ensures that applications can continue to read and write data without interruption during the migration process. Tools like Liquibase or Flyway can help automate and manage these changes.
Another approach involves schema versioning, which helps maintain compatibility between different application versions and their corresponding schemas. For instance, the system may support multiple schema versions simultaneously, allowing older clients to interact with the database while newer clients take advantage of the latest schema changes. This method is particularly useful in distributed environments, where different nodes might have temporarily different schema versions. By carefully managing schema migrations and ensuring backward compatibility, distributed databases can effectively accommodate changes while maintaining data integrity and system performance.