Checkpoint: Checkpoint is a mechanism where all the previous logs are removed from the system and stored permanently in a storage disk. Checkpoint declares a point before which the DBMS was in consistent state, and all the transactions were committed.
Recovery:When a system with concurrent transactions crashes and recovers, it behaves in the following manner −
- The recovery system reads the logs backwards from the end to the last checkpoint.
- It maintains two lists, an undo-list and a redo-list.
- If the recovery system sees a log with <tn, start=""> and <tn, commit=""> or just <tn, commit="">, it puts the transaction in the redo-list.
- If the recovery system sees a log with <tn, start=""> but no commit or abort log found, it puts the transaction in undo-list.
All the transactions in the undo-list are then undone and their logs are removed. All the transactions in the redo-list and their previous logs are removed and then redone before saving their logs