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Concept
Transaction log
Summary
In the field of databases in computer science, a transaction log (also transaction journal, database log, binary log or audit trail) is a history of actions executed by a database management system used to guarantee ACID properties over crashes or hardware failures. Physically, a log is a 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. Additionally, all transactions that are already committed but whose changes were not yet materialized in the database are re-applied. Both are done to ensure atomicity and durability of transactions.
This term is not to be confused with other, human-readable that a database management system usually provides.
In database management systems, a journal is the record of data altered by a given process.
A database log record is made up of:
Log Sequence Number (LSN): A unique ID for a log record. With LSNs, logs can be recovered in constant time. Most LSNs are assigned in monotonically increasing order, which is useful in recovery algorithms, like ARIES.
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.
All log records include the general log attributes above, and also other attributes depending on their type (which is recorded in the Type attribute, as above).
Update Log Record notes an update (change) to the database. It includes this extra information:
PageID: A reference to the Page ID of the modified page.
Length and Offset: Length in bytes and offset of the page are usually included.
Before and After Images: Includes the value of the bytes of page before and after the page change.
Official source
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