What is Checkpoints in database recovery? How does it help in database recovery? Explain.

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The checkpoint in Database Recovery is used to declare a point before which the DBMS was in a consistent state, and all transactions were committed. During transaction execution, such checkpoints are traced. After execution, transaction log files will be created.

Upon reaching the savepoint/checkpoint, the log file is destroyed by saving its update to the database. Then a new log is created with upcoming execution operations of the transaction and it will be updated until the next checkpoint and the process continues.

Database Recovery using Checkpoint:

In the following manner, a recovery system recovers the database from this failure:

Recovery using Checkpoint

  • The recovery system reads the logs backward from the end to the last checkpoint i.e. from T4 to T1.
  • It will keep track of two lists – Undo and Redo.
  • Whenever there is a log with instructions <Tn, start>and <Tn, commit> or only <Tn, commit> then it will put that transaction in Redo List. T2 and T3 contain <Tn, Start> and <Tn, Commit> whereas T1 will have only <Tn, Commit>. Here, T1, T2, and T3 are in the redo list.
  • Whenever a log record with no instruction of commit or abort is found, that transaction is put to Undo List <Here, T4 has <Tn, Start> but no <Tn, commit> as it is an ongoing transaction. T4 will be put on the undo list.

All the transactions in the redo-list are deleted with their previous logs and then redone before saving their logs. All the transactions in the undo list are undone and their logs are deleted.

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