The ArangoDB journal, also known as the write-ahead log (WAL), serves as a crucial component of the database's durability and fault tolerance mechanisms. Its primary purpose is to ensure data consistency, durability, and crash recovery by recording all modifications to the database before they are applied to the main data files.
Here's a breakdown of the key purposes of the ArangoDB journal :Durability and ACID Compliance :
* The journal ensures durability, one of the ACID (Atomicity, Consistency, Isolation, Durability) properties of database transactions. All changes to the database are first written to the journal before being applied to the main data files, guaranteeing that committed transactions are not lost in the event of a crash or failure.
Atomicity and Crash Recovery :
* The journal facilitates atomicity by recording transactions as atomic units in the journal before they are applied to the database. In the event of a crash or unexpected shutdown, ArangoDB can recover the database to a consistent state by replaying the journal and restoring the database to its state before the crash.
Write-Ahead Logging (WAL) :
* ArangoDB employs a write-ahead logging strategy, where all modifications to the database are first written to the journal before being applied to the main data files.
* Write-ahead logging ensures that changes are durable and recoverable, even in the event of system crashes, power failures, or other unexpected interruptions.
Transaction Logging :
* The journal records transactional changes, including inserts, updates, and deletes, as well as changes to indexes and metadata.
* By logging transactions, the journal enables atomicity and consistency of database operations, ensuring that transactions are either fully committed or fully rolled back in case of failure.
Incremental Backup Support :
* The journal can be used to support incremental backups by capturing changes to the database since the last backup.
* Incremental backups leverage the journal to identify and record only the changes made to the database since the last backup, reducing backup time and storage space requirements.
Asynchronous Durability :
* ArangoDB allows administrators to configure the journal for asynchronous durability, where transactions are acknowledged as committed once they are written to the journal, without waiting for them to be applied to the main data files.
* Asynchronous durability improves write performance by reducing the latency of commit operations while still ensuring durability through journaling.
