Lecture
The journal structure is usually a private matter of a specific implementation. We mark only the most common properties.
A log is usually a purely sequential file with variable size records that can be viewed in direct or reverse order. Exchanges are performed in standard chunks (pages) using the memory buffer. In well-organized systems, the structure (and even more so, the meaning) of journal entries is known only to the components of the DBMS responsible for logging and recovery. Since the contents of the log are critical when restoring a database after failures, there are special requirements for maintaining a log file for reliability. In particular, they usually strive to maintain two identical copies of the journal on different external memory devices.
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