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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]
Oracle uses that current group to write the redo log entries. When the group is full, a log switch occurs, making another group the current one. Each log switch causes checkpoint, however, the converse is not true: a checkpoint does not cause a redo log switch. One can also manually cause a redo-log switch using the ALTER SYSTEM SWITCH LOGFILE ...
For each record we undo the changes (using the information in the Undo field) and write a compensation log record to the log file. If we encounter a Begin Transaction record we write an End Log record for that transaction. The compensation log records make it possible to recover during a crash that occurs during the recovery phase.
Thanks to their atomicity property, the transactions can be considered the unit of work in the recovery process that guarantees durability while exploiting the log. In particular, the logging mechanism is called write-ahead log (WAL) and allows durability by buffering changes to the disk before they are synchronized from the main memory. In ...
Prev LSN: A link to their last log record. This implies database logs are constructed in linked list form. Transaction ID number: A reference to the database transaction generating the log record. Type: Describes the type of database log record. Information about the actual changes that triggered the log record to be written.
Databases and other data stores which treat the integrity of data as paramount often include the ability to handle transactions to maintain the integrity of data. A single transaction consists of one or more independent units of work, each reading and/or writing information to a database or other data store.
In computer science, particularly the field of databases, the Thomas write rule is a rule in timestamp-based concurrency control. It can be summarized as ignore outdated writes . It states that, if a more recent transaction has already written the value of an object, then a less recent transaction does not need to perform its write since the ...
The snowflake schema is a variation of the star schema, featuring normalization of dimension tables. In computing, a snowflake schema or snowflake model is a logical arrangement of tables in a multidimensional database such that the entity relationship diagram resembles a snowflake shape.