Rollback

Change rollback refers to the reversible operation of smoothly restoring to the nearest correct version when there is an error in the service, configuration, or data. The rollback scope should be consistent with the scope before the change. Any online change must have a rollback plan. If there are probabilistic risks or system or business anomalies caused by unknown risks, there must be measures to restore to the state before the change as soon as possible.

There are two typical rollback modes based on the extraction of rollback models and attribute deconstruction according to the way the change object is restored to the state before the change: backward mode and forward mode.

1. Backward mode: Also called the rollback mode, it mainly refers to the rollback method that rolls back the change object from the current state to the state before the change. Before the change execution, the online service is in state A. After the change execution, the online service is in state B. When the change needs to be rolled back, the online service will be restored to the state before the change, A. This can be expressed with the state machine expression: rollback: A->B->A.

2. Forward mode: Also called the forward mode, it mainly refers to the rollback method that changes the change object from the current state to a new state, which is the rollback method with the value of the state before the change. Before the change execution, the online service is in state A. After the change execution, the online service is in state B. When the change needs to be rolled forward, the online service will advance to state A', which is the same as state A in terms of content. This can be expressed with the state machine expression: roll-forward: A->B-A'.

Five Elements of Rollback

To achieve a successful rollback of a change, the following five basic elements are required: change object, rollback mode, effective scope of the change, state value of the change object before the change, and state value of the change object after the change.

• Change object: Refers to the atomic resource on which the change execution operates, such as application packages, configuration items, etc.

• Rollback mode: Refers to the rollback mode adopted to restore the change object to the state before the change, either in rollback mode or forward mode.

• Effective scope of the change: Refers to the scope of the change that has taken effect during the gray period of the change.

• State value of the change object before the change: Refers to the stable state atomic description of the change object before the change.

• State value of the change object after the change: Refers to the stable state atomic description of the change object after the change.