Overview The default method by which SQLite implements atomic commit and rollback is a rollback journal.
I think it is better to rephrase: Why does new distributed VoltDB use a command log over write-ahead log?
Undoubtedly you are advanced enough to abstract a file system and use block storage along with some additional optimizations. Next step is to execute some command: Please note several important aspects: A command may affect many stored entities, so many blocks will get dirty Next state is a function of the current state and the command Some intermediate states can be skipped, because it is enough to have a chain of commands instead.
Finally, you need to guarantee data integrity.
Write-Ahead Logging - central concept is that State changes should be logged before any heavy update to permanent storage. Following our idea we can log incremental changes for each block.
Command Logging - central concept is to log only Command, which is used to produce the state. There are Pros and Cons for both approaches. Write-Ahead log contains all changed data, Command log will require addition processing, but fast and lightweight.
Command Logging and Recovery The key to command logging is that it logs the invocations, not the consequences, of the transactions.
Write-Ahead Logging The traditional rollback journal works by writing a copy of the original unchanged database content into a separate rollback journal file and then writing changes directly into the database file. Thus a COMMIT can happen without ever writing to the original database, which allows readers to continue operating from the original unaltered database while changes are simultaneously being committed into the WAL.
Write-Ahead Logging WAL Using WAL results in a significantly reduced number of disk writes, because only the log file needs to be flushed to disk to guarantee that a transaction is committed, rather than every data file changed by the transaction.
The log file is written sequentially, and so the cost of syncing the log is much less than the cost of flushing the data pages. This is especially true for servers handling many small transactions touching different parts of the data store. Furthermore, when the server is processing many small concurrent transactions, one fsync of the log file may suffice to commit many transactions.Hence it is always a better idea to log the details into log file before the transaction is executed.
In addition, it should be forced to update the log files first and then have to write the data into DB. i.e.; in ATM withdrawal, each stages of transactions should be logged into log files, and stored somewhere in the memory.
Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. A detailed description can be found in most (if not all) books about transaction processing.
A detailed description can be found in most (if not all) books about transaction processing. This section describes the role of the write-ahead transaction log in recording data modifications to disk.
SQL Server uses a write-ahead log (WAL), which guarantees that no data modifications are written to disk before the associated log record is written to disk.
In computer science, write-ahead logging (WAL) is a family of techniques for providing atomicity and durability (two of the ACID properties) in database systems.
The changes are first recorded in the log, which must be written to a stable storage before converted unto a disk. The command log records the transaction invocations instead of each row change as in a write-ahead log.
By recording only the invocation, the command logs are kept to a bare minimum, limiting the impact the disk I/O will have on performance. Books Online: Write-Ahead Transaction Log - Microsoft® SQL Server™ , like many relational databases, uses a write-ahead log.