In the present business environment, massive amounts of data are generated by most of the organizations. It is, therefore, essential that organizations have a robust Database Management System (DBMS) in place that helps users to access databases, modify and manipulate data and generate the required reports.

Types of Database Management System

There are primarily four types of DBMS – Hierarchical, Network DBMS, Relational DBMS, and Object-Oriented Relation DBMS. For this post, Relational DBMS will be considered only as it is related to SQL. Relational DBMS defines database relationships in the form of tables which are also called relations. RDMS does not bear many to many relationships and generally have pre-fixed data types that they can support like MySQL, Oracle, and Microsoft SQL Server database.

Definitions of Critical Terms

Before going to specific SQL Server replication tools, it is essential to clearly define a few critical terms.

What is SQL?

It stands for Structured Query Language and is the standard platform for dealing with Relational Database. SQL is used to search, insert, update, and delete database records. Other operations that can be carried out with SQL are optimizing and maintaining relational databases.

What is Replication?

Replication is a process that helps copy and distribute data and database objects from one database to another. Once done, it synchronizes between the databases to ensure consistency. Replication is also used to distribute data to different locations and remote and mobile users through wireless connections, the Internet, dial-up connections, and local and wide area networks. 

Then, what is the difference between Replication and the good old-fashioned mirroring? Both are intricately linked to copying data in DBMS. But while Replication copies data and database objects from one database to another database, Mirroring copies database to another location only. However, Replication and Mirroring both help to improve the performance of data in a database. 

What is SQL Server Replication

Before SQL Server replication was introduced, most applications were situated in a standalone environment. There was a single server that responded to multi-user activity from various locations. It resulted in many performance problems, availability problems, and maintenance issues. Presently, SQL Server replication tools are used to skirt these problems and maintain database copies at multiple locations.

SQL Server data replication is a highly optimized process used by organizations to increase both the performance of SQL Server databases and the availability of information across an organization. It enables businesses to integrate and load data into data warehouses, offload batch processing, copy and distribute data across multiple database servers in remote offices, and migrating data to cloud storage platforms. All these activities are regardless of whether replication is implemented using built-in MSSQL replication utilities or as a third-party one customized by a replication expert.

In cases where access to the latest information is the top priority and business efficiency is critical, firms use SQL change data capture (CDC) technology. It is a sure way by organizations to accelerate, augment, and reduce the cost of SQL data replication. SQL Server CDC ensures that data-driven enterprises can integrate data in real-time and achieve faster time-to-insight. Changes that are made individually at various locations are synchronized to the main server quickly as SQL Server replication does not distribute the entire database but only a part of the tables and views.

Fundamental Components of SQL Server Replication

Given here are some of the basic components of SQL Server replication.          

• Publisher – This is the source database and the starting point of replication. It makes data accessible for replication with publishers defining what they publish through a publication.
• Article – Articles are the actual database objects that comprise replication and include tables, views, indexes, and more. Before sending it to the subscriber, it is possible to filter an article.
• Publication – Publication is a cluster of articles. It is required since an article cannot be distributed individually. 
• Distributor – A distributor is an intermediary between the publisher and the subscriber. It publishes snapshots or transactions and then stores and forwards these publications to the subscriber. There are 6 system databases including distribution.
• Subscriber – In the SQL server, a data replicationsubscriberis the destination database where the process of replication ends. A subscriber can subscribe to different publications from multiple publishers and send data back to the publisher or publish data to other subscribers.
• Subscription – It is a request from a subscriber to receive a publication. There are two forms of subscriptions.
• Push Subscriptions – This is where the publisher is responsible for updating all the changes to the subscriber without the subscriber requesting those changes. Push subscriptions are created at the Publisher server.
• Pull Subscriptions – As different from push subscriptions, the subscriber initiates the replication instead of the publisher with subscriptions being created at the Subscription server. 

Types of SQL Server Replication Tools

There are mainly three types of SQL Server replication tools.  

• Snapshot Replication – It is a simple process whereby a “snapshot” of the data on one server is taken and the data is moved to another server or another database on the same server. After the first synchronization snapshot, replication refreshes the data in published tables over fixed pre-programmed periods. This technology is the easiest to set up and maintain but on the flip side, all data has to be copied each time a table is refreshed. In between scheduled refreshes, data on the publisher might be different from that on the subscriber. In a nutshell, snapshot replication is emptying the destination tables and importing data from the source using a DTS package.   
• Transactional Replication – It is a tool to copy data once from the publisher to the subscriber/s. These transactions are then delivered to the subscriber/s whenever they occur on the publisher. The first copy of the data is transmitted through the same process as snapshot replication. Subsequently, when database users insert, update, or delete records on the publisher, transactions are forwarded to the subscriber/s. Further, the main advantage here among other SQL server replication tools is that by making a simple configuration change, transactions can be delivered continuously. Typically, database servers on transactional publications do not modify data and use it for read-only purposes.
• Merge Replication – This technology merges data from multiple sources into a single central database. As in transactional replication, this tool also synchronizes data initially by taking a snapshot of the publisher and moving it to subscribers. But unlike transactional replication, merge replication allows changes of the same data on publishers and subscribers, even when the subscribers are not connected to the network. When they connect to the network, replication will notice and combine changes from all subscribers and change data on the publisher accordingly. This tool is useful when data on remote computers have to be modified and when subscribers are not assured of continuous connection to the network.

SQL Server replication is a robust and flexible system that synchronizes data across enterprises. Replication to SQLCE 3.5 and SQLCE 4.0 are supported by both Windows Server 2012 and Windows 8.