Upgrade

This phase is responsible for applying the upgrade plan step by step (see Upgrade a cloud).

For the control-plane, COU upgrades applications in a strict sequential manner, following a predefined order (see Upgrade groups) and halting the process should any application encounter an upgrade failure. All applications within the control-plane employ an all-in-one method, upgrading all units of an application simultaneously.

COU adopts a more cautious strategy when upgrading data-plane applications to minimize the risk of downtime, which can occur if the unit undergoing upgrade is actively handling client requests. Between applications, similar to the control-plane, upgrades are executed sequentially following the predefined order (see Upgrade groups). But for applications that support the openstack-upgrade action (such as nova-compute and its colocated services like cinder), COU handles the upgrades in a paused-single-unit fashion. This method is notably more time-intensive, so to streamline the process while still maintaining cloud stability, units are grouped based on the Juju availability zones of their respective hosting machines. COU then progresses through these availability zone groups in a sequential manner, performing upgrades on units within a single zone in parallel.

Note: ceph-osd, while being a component of the data-plane, employs the all-in-one method for upgrades. because its charm is designed to maintain service availability throughout the upgrade process.