The scheduling service consists of a Resource module and a Version module. It mainly implements resource allocation and instance version management.
The Resource module allocates and integrates the underlying RDS resources. For example, when you create an instance through the RDS console or API, the Resource module will determine which physical server is best suited to carry traffic. This module also allocates and integrates the underlying resources required for the inter-zone migration of RDS instances. After lengthy instance creation, deletion, and migration operations, the Resource module calculates the degree of resource fragmentation in a zone and initiates resource integration regularly to improve the service carrying capacity of the zone.
The Version module is applicable to version upgrades of RDS instances. For example:
MySQL major version upgrade: Upgrading MySQL 5.1 to MySQL 5.5, MySQL 5.5 to MySQL 5.6, and so on.
MySQL minor version upgrade: Fixing a bug in the MySQL source code.