Learn the Purpose of Transaction Log in SQL Server

Andrew Jackson | Published: January 18, 2019 | SQL |

Transaction Log files are an inseparable part of SQL Server database. LDF file or transaction log file records all the transactions and modifications performed in the database. This comes very handy during the system failure, as this file can provide information necessary for putting back the database into its normal state.

If you think this is the only function of LDF files, you are wrong. This post will discuss what are the purposes of Transaction Log in SQL Server. Before that, lets us learn the main features of LDF files of SQL Server.

Major Characteristics of Transaction Log

These are some of the notable attributes/ features of Transaction Log File:

  • The transaction log is a separate file of SQL database, commonly known as LDF file. The cache of this file is managed separately, and not with the data pages buffer cache. Hence, it produces a fast and simple code within the Engine of SQL Server Database.
  • There is no such rule that the format and pages of log records need to be similar to data pages format.
  • The log file implementation can be done in several files. The Log files can be set to expand automatically through applying the FILEGROWTH value for the log. This not only reduces the chance of running out of space in Log file, but also decreases administrative expense.
  • The fast technique of reusing the log files’ own space out minimum impact of the output of the transaction.

Purpose of Transaction Log in SQL Server

SQL Transaction Log files serve different purposes, such as:

  • Recovery of Individual Transaction
    Transaction Log is used for rolling back the changes done by some incomplete/broken transactions. This function is done when a ROLLBACK command is issued or some errors have been detected by the SQ Database Engine.
  • All Incomplete Transactions Recovery During Server Startup
    In case of server failure, some changes are never recorded in the data file from the buffer cache. It is also possible to have some modifications made to the data files even from the incomplete transactions. Whenever SQL Server starts, it recovers every database. Any modification, that had been recorded in the LDF file but not got written on the data files, gets rolled forward. Then all the incomplete transactions recorded in the transaction log are rolled back to ensure the retention of database integrity.
  • Roll Forward a Restored File, Filegroup, Page or Database to the Point of Failure
    If database files get damaged due to disk failure or some other hardware problems, users perform database restoration to the point of failure. The basic rule here is to restore last complete database backup, last differential database backup, and finally the next sequence of the LDF backups to the point of failure.
    As the restoration processes take place, all the modifications from the Log record are reapplied by the Database Engine in order to roll forward the complete list of transactions. After the last Log file restoration is over, the log file information is used by the Database Engine to roll back all the incomplete transactions at that point
  • Creating Replica of Transactions
    Every SQL database has an LDF file that is designated for transactional replication. Log Reader Agent keeps track of this log file and replicates the transactions marked for copying to the distribution database.
  • Providing Support for Various Disaster Recovery solutions
    Transaction Log helps in situations when different standby-server solutions are implemented:
    a. Always On availability groups: In this case, all the updates made in the primary replica (a database) are instantly reproduced in the secondary replica (separate copies of database). All the log records are sent immediately by the primary replica to the secondary replicas, by which, the incoming log records get applied to the availability group databases, by continually rolling forward.
    b. Log Shipping: In this scenario, the active LDF file of the primary database is sent to one or multiple destinations by the primary server. The Log file is restored to the local secondary database by every secondary server.
    c. Database Mirroring: Here, as soon as an update is made to the database, the principal database reproduces its copy in a separate but complete database (known as mirror database). Every log record is promptly sent by the principal server instance to the mirror server instance. Then the mirror server instance implements the incoming log records to the mirror database, and constantly rolls forward.

How to View Transaction Log Data in SQL Server

As we have read in the discussion, Transaction Logs have multifaceted purpose. But the problem is, it is not possible for the users to read and retrieve data from the LDF file manually. In order to do so, users need to take help of SQL LDF File viewer. Using this software, users will be able to open and read Transaction Log of SQL Server. This software support LDF files of all latest versions of SQL Server including 2017. The latest version of this tool is also capable of displaying LDF file records from the Live database.


Since many SQL users wonder about the purpose of Transaction Log in SQL Server, this write-up discusses the usages of LDF file. Users will be able to learn about the functions and features of SQL Transaction Log files. If anyone is interested to read Log files, they can go for SQL Log Analyzer.