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Physically, a log is a file listing changes to the database, stored in a stable storage format. If, after a start, the database is found in an inconsistent state or not been shut down properly, the database management system reviews the database logs for uncommitted transactions and rolls back the changes made by these transactions .
A write ahead log is an append-only auxiliary disk-resident structure used for crash and transaction recovery. The changes are first recorded in the log, which must be written to stable storage, before the changes are written to the database. [2] The main functionality of a write-ahead log can be summarized as: [3]
In the Oracle RDBMS environment, redo logs comprise files in a proprietary format which log a history of all changes made to the database. Each redo log file consists of redo records. A redo record, also called a redo entry, holds a group of change vectors, each of which describes or represents a change made to a single block in the database.
There exist several mechanisms to store and reconstruct the state of a database system that improves the performance, both in terms of space and time, compared to managing all the log files created from the beginning of the database system. These mechanisms often include incremental dumping, differential files, and checkpoints. [12]
We create log records of the form (Sequence Number, Transaction ID, Page ID, Redo, Undo, Previous Sequence Number). The Redo and Undo fields keep information about the changes this log record saves and how to undo them. The Previous Sequence Number is a reference to the previous log record that was created for this transaction.
In relational databases, the log trigger or history trigger is a mechanism for automatic recording of information about changes inserting or/and updating or/and deleting rows in a database table. It is a particular technique for change data capturing , and in data warehousing for dealing with slowly changing dimensions .
For example, appending to a file may involve three separate writes to: The file's inode, to note in the file's metadata that its size has increased. The free space map, to mark out an allocation of space for the to-be-appended data. The newly allocated space, to actually write the appended data. In a metadata-only journal, step 3 would not be ...
CDC solutions based on transaction log files have distinct advantages that include: minimal impact on the database (even more so if one uses log shipping to process the logs on a dedicated host). no need for programmatic changes to the applications that use the database. low latency in acquiring changes.