Understanding Microsoft SQL Server Transaction Log Simplified

on Data Management Tools, Database Management Systems • March 25th, 2021 • Write for Hevo

The volume of data that businesses collect has grown exponentially in recent years. This is primarily due to the fact that businesses today rely on data-driven decision-making like never before. For obvious reasons, it is of paramount importance to these businesses that their data is not lost in any way. Most modern databases house features to ensure that user data is not lost. One of the most well-known and robust implementations to ensure no data loss is by Microsoft in SQL Server Transaction Logs.

This article will help you understand what Microsoft SQL Server is, what Microsoft SQL Server Transaction Logs are and how they can help you ensure that you never lose your data.

Table of Contents

Introduction to Microsoft SQL Server

Microsoft SQL Server Logo
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Microsoft SQL Server is a well-known Relational Database Management System (RDBMS). Considering it is an RDBMS, its primary purpose is to store and retrieve data as per the requirements of the users and the applications connected to it. Microsoft SQL Server provides an implementation for a wide range of functionalities including Transactional Processing, Business Intelligence, Data Analytics, etc. that are required by most businesses today. It is considered to be one of the top three leading Database Technologies along with Oracle Database and IBM DB2.

Along with support for ANSI SQL, which is the popular Structured Query Language (SQL), Microsoft SQL Server comes with its own implementation of SQL called Transact SQL (T-SQL). T-SQL supports additional capabilities that allow users to declare variables, handle Exceptions, etc. The main interface tool for Microsoft SQL Server is called SQL Server Management Studio (SSMS).

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Understanding the Key Features of Microsoft SQL Server

The key features of Microsoft SQL Server are as follows:

1) Accelerated Data Recovery

There might be situations in which a complex transaction being run on the Microsoft SQL Server fails or the Server crashes while performing an operation. In these situations, the database has to be recovered to its initial state by rolling back all incomplete transactions and operations. This process was considered to be time-consuming until Microsoft’s implementation of Accelerated Data Recovery in Microsoft SQL Server.

The process that performs database recovery in Microsoft SQL Server has been redeveloped using Transaction Logs which has led to a drastic decrease in the time taken.

Accelerated Data Recovery
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2) Encryption

Considering that databases store sensitive information for most businesses, it is essential for Database Providers to ensure that their databases are secure. Microsoft SQL Server 2016 introduced a new Encryption Technology called AlwaysEncrypted that allowed transparent Column Encryption without giving Database Administrators access to Decryption Keys.

Encryption
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3) Intelligent Query Processing

Microsoft SQL Server houses a robust in-built Query Optimizer that generates an execution plan for the queries being executed. Various improvements have been made to this Query Optimizer to ensure that the databases provide the best performance to their users. These improvements include Dynamic Memory Grants for Rowstore Tables, Batch Mode on Rowstore, Table Variable Deferred Compilation, etc.

Intelligent Query Processing
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4) Advanced Analytics

All data stored in Microsoft SQL Server can be easily leveraged to perform a comprehensive analysis as per requirements using Microsoft SQL Server Analysis Service (SSAS) which is a Data Mining and an Online Analytical Processing tool in Microsoft SQL Server.

5) Advanced Integrations

Microsoft SQL Server also allows users to perform a broad range of tasks such as Data Extraction, Transformation, Loading, Data Migration, etc. using the Microsoft SQL Server Integration Services (SSIS). SSIS can be used to extract and transform data from various sources such as Relational Databases, Flat Files, XML Files, etc. using its Graphical User Interface (GUI) without writing a single line of code.

Understanding ACID Properties

In order to understand what Microsoft SQL Server Transaction Logs are, you first need to understand what ACID properties are and why they’re important.

In the concept of RDBMS, the ACID Properties refer to four key properties of every transaction that takes place in the database. These properties are as follows:

ACID Properties
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  • Atomicity: Any changes that are being performed to the database are performed in a single operation. This means that either all changes are made or none of them are. For example, in a Banking Transaction, Atomicity ensures that if money is debited from the sender’s account, it is deposited in the recipient’s account as well.
  • Consistency: Ensures that the database stays consistent before and after a transaction has taken place. For example, in a Banking Transaction, if money is being transferred from one account to another, Consistency ensures that the total amount of money in both the accounts before and after the transaction is the same.
  • Isolation: Ensure that multiple transactions can run concurrently on a database in a serialized form. For example, in a Banking Transaction in which money is being transferred from Account ‘A’ to Account ‘B’ in one transaction, if another parallel transaction tries to check the balance of Account ‘A’ and Account ‘B’, it will see the funds being transferred in only one of the accounts and not in both or neither of them.
  • Durability: Once the transaction is complete, the changes cannot be reversed even if the system fails. For example, in the Banking Transaction, there is no way to reverse a transfer of money from one account to another.

Introduction to Microsoft SQL Server Transaction Log

A Microsoft SQL Server Transaction Log is an integral part of every SQL Server Database. The basic idea behind a Microsoft SQL Server Transaction Log is that it is used to maintain a record of every operation that is performed on the database in the order in which it was performed. This information is used in case the transaction fails or the Server crashes while an operation is being performed in order to recover the database to its original state. Hence, Microsoft SQL Server Transaction Log is considered to be the most important component for Disaster Recovery.

All Relational Database Management Systems (RDBMS) are expected to be ACID compliant and Microsoft SQL Server Transaction Logs is how SQL Server ensures all its databases are ACID compliant.

A Microsoft SQL Server Transaction Log is internally split into smaller parts called Virtual Log Files (VLFs). All logs are written to these smaller VLFs. Once one VLF becomes full, new logs are written to the next available VLF in the Microsoft SQL Server Transaction Log file. The Microsoft SQL Server Transaction Log can be seen as a circular file. This means that when the logging reaches the end of the file i.e. when the log file is full, the logging process starts again at the beginning of the file from the first VLF.

An observation can be made that if the logging takes place in a circular file, it means that old logs at some point will get overwritten by new logs which means the old logs will have to be deleted. The old logs can be deleted if all of the following conditions are met:

  • The corresponding transaction for which the log is about to be overwritten is complete.
  • All changes made by the corresponding transaction have been written to disk by a checkpoint.
  • The log is not required for backup.
  • The log is not required for performing any operation that accesses the log. For example, Database Mirroring or Replication.

An example of a Microsoft SQL Server Transaction Log file is as follows:

Microsoft SQL Server Transaction Log file
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A Log Sequence Number (LSN) is used to uniquely identify each transaction in the log. The Logical Log represents the active part of any Transaction Log and MinLSN can be seen as the starting point of the oldest active transaction in the log.

The various operations supported by Microsoft SQL Server Transaction Logs are as follows:

  • Recovery of Individual Transactions.
  • Recovery of Incomplete Transactions upon Startup.
  • Perform Database Replication.
  • Support High Availability and Data Recovery Solutions such as Database Mirroring, Log Shipping, etc.

Understanding Microsoft SQL Server Recovery Models

Each Microsoft SQL Server database includes a property called the Recovery Model that helps determine how transactions are logged, whether the Microsoft SQL Server Transaction Logs can be backed up or not, and the type of restore options available. 

A Microsoft SQL Server Database can have any of the following Recovery Models:

1) Simple Recovery Model

Backups for Microsoft SQL Server Transaction Logs are not supported in this model. This means that you do not have to handle the overhead associated with maintaining Transaction Log backups. The Log Space is automatically reclaimed in this model and hence, there is no need for users to manage the Transaction Log Space. 

There are however certain disadvantages associated with this model. The biggest disadvantage is that it can only restore the database to the point of its last backup. Hence, all transactions that were performed after the most recent backup are lost. The Simple Recovery Model is, thus, considered to be suitable only for Development and Test databases, or in Application databases if data loss is acceptable.

2) Full Recovery Model

In this Recovery Model, all transactions are fully logged into the Microsoft SQL Server Transaction Log file. The Log Sequence is unbroken and is preserved for any possible Database Recovery Operations. Transaction Log files can be backed up in the model and hence, it provides full recovery to any point. Therefore, this model is considered to be less risky than the Simple Recovery Model. This Recovery Model should be used for supporting mission-critical applications to ensure that there is minimal or no data loss.

3) Bulk Logged Recovery Model

This is a special configuration option that is quite similar to the Full Recovery Model. The only difference between this model and the Full Recovery Model is that this model allows some operations to be minimally logged. The technique used by Microsoft SQL Server Transaction Log files to log bulk operation is called Minimal Logging. This model should be used when the database is subjected to bulk operations on a regular basis and the growth of the Microsoft SQL Server Transaction Log file has to be prevented.

The Recovery Model for a database can be changed simply using the ALTER DATABASE command. The query to perform the operation is as follows:

ALTER DATABASE {db_name} SET MODEL {model_name};

{db_name} in the above query has to be replaced with the name of the database for which the Recovery Model is being set and {model_name} has to be replaced with SIMPLE, FULL, or BULK_LOGGED based on requirements.

The Recovery Model can also be changed using Microsoft SQL Server Management Studio (SSMS) by opening the SQL Instance in the Object Explorer, selecting the required database, and changing the Recovery Model in the properties.

Changing Recovery Model in SSMS
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Maintaining Microsoft SQL Server Transaction Log Files

The maintenance of Microsoft SQL Server Transaction Log is an important task to ensure overall maintenance of the database. The Transaction Log Space has to be monitored regularly. If the database has a lot of users, this monitoring has to be done on daily basis.

Information on the Transaction Log Space can be monitored using the following query: 

DBCC SQLPERF (LOGSPACE)
GO
Maintaining Log Space
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Backups of the Microsoft SQL Server Transaction Log file should be made regularly. The Transaction Log file should be cleared every time a backup is made to ensure that no Auto Growth Operations take place.

Backup of the Transaction Log of a database can be made using the following command:

BACKUP LOG {db_name}
TO DISK = “D:{db_name}.trn’
GO

{db_name} in the above query has to be replaced with the name of the required database.

Conclusion

This article provided you with an in-depth understanding of what Microsoft SQL Server Transaction Logs are, why they’re important, what are the various Recovery Models that are used for the databases, and how you can maintain the Transaction Log for your Microsoft SQL Server database.

Most businesses today use multiple platforms to carry out their day-to-day operations. As a result, all their data is spread across the databases of these platforms. If a business wishes to perform a common analysis of their data, they would first have to integrate the data from all these databases and store it in a centralized location. Building an in-house data integration solution would be a complex task that would require a high volume of resources. Businesses can instead use existing data integration platforms like Hevo.

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