Deferred Update and Immediate: Updates are techniques employed by Database Management Systems (DBMSs) for maintaining transaction log files of their databases. A transaction log, also referred to as a journal or redo log, is a physical file that records transactions including their Transaction IDs, timestamps, and prior and new values of any given transaction; it provides the DBMS with the information needed to track data before and after transactions occur; once committed the log can be truncated so as to clear out completed transactions from view.
What is Deferred Update?
- Deferred Update, also referred to as NO-UNDO/REDO is an approach used to recover/support transaction failures caused by the operating system, power, memory or machine problems. As soon as a transaction begins running, any updates or modifications it makes to the database by way of this transaction are not applied immediately; rather they are recorded in a log file before eventually being applied back on commit – known as re-doing.
- Rollback means any modifications recorded in a log file are ignored; as a result, no database changes occur. If a transaction fails and does not commit for one of the reasons above, records in its log file are also disregarded and restarted with another attempt; but if all transactions committed before the system crashes have taken effect after restarting they become part of its database instead.
Advantages of Deferred Update
- The deferred update can significantly boost performance by storing changes temporarily before committing them in one transaction; this reduces disk writes for faster updates and ensures all updates occur together, helping maintain data integrity.
Disadvantages of Deferred Update
- One major drawback of deferred updates is data inconsistency. When changes are made but not committed, they won’t appear for other users who access the database, leading to confusion or errors in data retrieval and errors that require correction in batch update procedures may become hard to spot and correct.
What Are Immediate Updates?
- Immediate Update, also referred to as UNDO/REDO, is another technique designed to recover/support transaction failures that occur due to errors with operating systems, power supplies, memory, or machines. As soon as a transaction runs, any updates or modifications it makes to the database are immediately written into it, with both original values as well as modified ones being recorded in its log file before changes take place to the database itself.
- On commit, all changes made to the database become permanent while records in the log file are removed from service. On rollback, old values from log file storage can be restored into the database using old values contained within that log file. Un-doing” refers to the process by which changes made through transactions to a database are undone when restarting from a crash, only committed transactions will become permanent while uncommitted ones will revert back using values found in log files.
- Advantages of Immediate Update
One key benefit of immediate updates is providing immediate visibility of changes for other users of the database, helping prevent errors and facilitating communication among them. Furthermore, since changes are committed immediately there’s no need to store them temporarily thereby saving resources and helping conserve resources.
- Disadvantages of Immediate Update
One major drawback of immediate updates is their potential to decrease system performance when used frequently, since each update requires disk writes that can decelerate performance significantly. Furthermore, any mistakes or inaccuracies found during the update could become harder to remedy once committed into the database.
Why is a difference -Deferred Update and Immediate Update?
- Although Deferred Updates and Immediate Updates both provide ways for recovering after a system failure, their respective processes vary considerably. Under different update methods, any data modifications made during a transaction are first recorded in a log file and only applied when a commit occurs. Conversely, with the immediate update method changes are instantly made available in the database and both old values and their new equivalents are recorded as logfile entries.
- Records in log files used for rollback can help restore old values when rolling back is activated; while under the deferred update method records in log files are discarded on rollback and never applied to the database; one disadvantage of the deferred update method being an increased recovery time due to system failure; while immediate update method’s frequent I/O operations while active are seen as disadvantageous.
Differences Between Deferred Update and Immediate Update
There are various differences between delayed updates and immediate updates that merit consideration.
- Time of update
One major distinction between these approaches lies in how and when updates occur, with immediate updates, changes are committed as quickly as they’re made to the database. With both approaches in play, however, immediate commitment means changes made are added immediately into production mode for use by clients or internal systems.
- Â Frequency Update
Immediate update can significantly slow down a system if multiple changes are being made quickly; by contrast, a delayed update only involves one transaction that stores and commits all changes at the same time.
- Level of Complexity
The deferred update can be more complex than the immediate update since changes must be stored temporarily and then committed at once as one transaction. Conversely, the immediate update allows changes to take effect immediately without waiting to commit them all at once.
- Data Integrity
Deferring updates is often effective at upholding data integrity as changes are committed in one transaction, helping prevent inconsistencies between various updates. On the other hand, immediate updates can sometimes cause data inconsistencies if errors arise during update processing.
- Performance
Deferring updates may improve performance compared to immediate ones since changes can be implemented as one-off transactions.
- When to Select Deferred Update and Immediate Update
Your decision between a deferred update or an immediate update depends upon your specific use case.
- Use Cases for Deferred Update
Deferred updates can be particularly useful when there are numerous changes being made to a database and maintaining data integrity is vitally important. Furthermore, deferred updates may help reduce disk writes by keeping changes separate.
- Use Cases for Immediate Update
Immediate update can be advantageous in situations in which it’s crucial that changes be immediately visible to other users of a database, as well as situations with few changes being made, since no temporary storage location needs to be created for changes made to it.
Conclusion
It is vital that anyone working with databases understand the differences between deferred update and immediate update approaches, particularly for performance enhancement and data integrity reasons; deferred update can improve performance while immediate update provides immediate visibility of changes for other users of the database; which strategy best fits with specific use cases should ultimately determine your decision between deferred update or immediate update approaches.