SQL Server: Database Replication Interview Questions and Answers (Day-1)

What is Replication in SQL Server?

As per the msdn help,

Replication is a set of technologies for copying and distributing data and database objects from one database to another and then synchronizing between databases to maintain consistency. Using replication, you can distribute data to different locations.

Database replication is the frequent electronic copying data from a database in one computer or server to a database in another so that all users share the same level of information. The result is a distributed database in which users can access data relevant to their tasks without interfering with the work of others.

What is Publishers?

The Publisher is a server that makes data available for replication to other servers. In addition to being the server where you specify which data is to be replicated, the Publisher also detects which data has changed and maintains information about all publications at that site.

What is Distributors?

Distributor is a bridge between publisher and subscriber. Distributor gathers all the published data and holds it until it sends it across to all subscriber. So as it is a bridge who sits in between publisher and subscriber, it supports multiple publisher and subscriber concept.

What is Subscribers?

Subscribers are servers that receive replicated data and it is the end source or the final destination to which data has to be transmitted.

What is a Snapshot Agent?

The Snapshot Agent is used with all types of replication. It prepares the schema and the initial bulk copy files of published tables and other objects, stores the snapshot files, and records information about synchronization in the distribution database. The Snapshot Agent runs at the Distributor.

What is a Log Reader Agent?

The Log Reader Agent is used with transactional replication. It moves transactions marked for replication from the transaction log on the Publisher to the distribution database. Each database published using transactional replication has its own Log Reader Agent that runs on the Distributor and connects to the Publisher (the Distributor can be on the same computer as the Publisher).

What is a Distribution Agent?

The Distribution Agent is used with snapshot replication and transactional replication. It applies the initial snapshot to the Subscriber and moves transactions held in the distribution database to Subscribers. The Distribution Agent runs at either the Distributor for push subscriptions or at the Subscriber for pull subscriptions.

What is a Merge Agent?

The Merge Agent is used with merge replication. It applies the initial snapshot to the Subscriber and moves and reconciles incremental data changes that occur. Each merge subscription has its own Merge Agent that connects to both the Publisher and the Subscriber and updates both. The Merge Agent runs at either the Distributor for push subscriptions or the Subscriber for pull subscriptions.

What is a Queue Reader Agent?

The Queue Reader Agent is used with transactional replication with the queued updating option. The agent runs at the Distributor and moves changes made at the Subscriber back to the Publisher. Unlike the Distribution Agent and the Merge Agent, only one instance of the Queue Reader Agent exists to service all Publishers and publications for a given distribution database.

What are the different types of Replication available in SQL Server?

  • Transactional Replication
  • Merge Replication
  • Snapshot Replication

What is the Transactional Replication?

SQL Server Transactional replication is typically used in server-to-server scenarios that require high throughput, including improving scalability and availability, data warehousing and reporting, integrating data from multiple sites, integrating heterogeneous data, and offloading batch processing.

Transactional replication typically starts with a snapshot of the publication database objects and data. As soon as the initial snapshot is taken, subsequent data changes and schema modifications made at the Publisher are usually delivered to the Subscriber as they occur (in near real time).

The data changes are applied to the Subscriber in the same order and within the same transaction boundaries as they occurred at the Publisher; therefore, within a publication, transactional consistency is guaranteed.

In which, situation Transactional Replication is suitable?

  • You want incremental changes to be propagated to Subscribers as they occur.
  • The application requires low latency between the time changes are made at the Publisher and the changes arrive at the Subscriber.
  • The application requires access to intermediate data states. For example, if a row changes five times, transactional replication allows an application to respond to each change (such as firing a trigger), not simply the net data change to the row.
  • The Publisher has a very high volume of insert, update, and delete activity.
  • The Publisher or Subscriber is a non-SQL Server database, such as Oracle.

How Transactional Replication works?

Transactional replication is implemented by the SQL Server Snapshot Agent, Log Reader Agent, and Distribution Agent. The Snapshot Agent prepares snapshot files containing schema and data of published tables and database objects, stores the files in the snapshot folder, and records synchronization jobs in the distribution database on the Distributor.

The Log Reader Agent monitors the transaction log of each database configured for transactional replication and copies the transactions marked for replication from the transaction log into the distribution database, which acts as a reliable store-and-forward queue. The Distribution Agent copies the initial snapshot files from the snapshot folder and the transactions held in the distribution database tables to Subscribers.

Incremental changes made at the Publisher flow to Subscribers according to the schedule of the Distribution Agent, which can run continuously for minimal latency, or at scheduled intervals.

What do you know about the peer-to-peer transaction publication topology?

  • Each location has identical data and acts as both a Publisher and Subscriber.
  • The same row can be changed only at one location at a time.
  • This topology is best suited for server environments requiring high availability and read scalability.

What do you know about the Bidirectional Transactional Replication?

Bidirectional transactional replication is a specific transactional replication topology that allows two servers to exchange changes with each other: each server publishes data and then subscribes to a publication with the same data from the other server.

Is it possible to publish the execution of Stored Procedure?

If you have one or more stored procedures that execute at the Publisher and affect published tables, consider including those stored procedures in your publication as stored procedure execution articles. The definition of the procedure (the CREATE PROCEDURE statement) is replicated to the Subscriber when the subscription is initialized; when the procedure is executed at the Publisher, replication executes the corresponding procedure at the Subscriber.

The stored procedure definition at the Publisher is propagated to each Subscriber. However, you can also modify the stored procedure at the Subscriber. This is useful if you want different logic to be executed at the Publisher and Subscriber.

Anvesh Patel

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