SQL Server’s Replication requires many components to replicate data from one location to another. The below image is a high-level overview of the pieces involved in a replication setup.
The components used for replication setup include a Publisher and its publication database. The publication database contains a publication that may include a number of articles. The setup also includes a Distributor and its distribution database as well as a Subscriber and its subscription database, which contains the subscription. And using replication agents data replicates as per defined architecture.
The replication components details are as below.
For each SQL Server object that should be replicated, an article needs to be defined. Each article corresponds to a single SQL Server object like tables, views, stored procedures and functions (For a complete list of objects that can be replicated, check out the topic, Publishing Data and Database Objects in SQL Server Books Online.) An article’s properties determine whether that article contains the entire object or a filtered subset of its parts. For example, an article can be configured to contain only some of the columns of a table. With some restrictions, multiple articles can be created on a single object.
A publication is a collection of articles grouped together as one unit. Every article is defined to be part of exactly one publication. But in few cases we can also define different articles on the same object in separate publications. A publication supports several configurable options that apply to all its articles. Perhaps the most important option is the one that lets you define which type of replication to use.
A database that contains objects designated as articles is called a publication database, when we set up a publication on a database, SQL Server modifies the inner workings of that database and creates several replication-related objects. A publication database is also protected against being dropped. A publication can contain articles from a single publication database only.
The Publisher is a database instance that makes data available to other locations through replication. The Publisher can have one or more publications, each defining a logically related set of objects and data to replicate.
Each Publisher is linked to a single Distributor. The Distributor is a SQL Server instance that identifies changes to the articles on each of its Publishers. Depending on the replication setup, the Distributor might also be responsible for notifying the Subscribers that have subscribed to a publication that an article has changed. The information about these changes is stored in the distribution database until all Subscribers have been notified or the retention period has expired. The Distributor can be configured on a SQL Server instance separate from the Publisher, but often the same instance takes the role of the Publisher and the Distributor.
Each Distributor has at least one distribution database. The distribution database contains a number of objects that store replication metadata as well as replicated data. A Distributor can hold more than one distribution database , However, all publications defined on a single Publisher must use the same distribution database.
Each SQL Server instance that subscribes to a publication is called a Subscriber. The Subscriber receives changes to a published article through that publication. A Subscriber does not necessarily play an active role in the replication process. Depending on the settings selected during replication setup, it might receive the data passively.
A subscription is the counterpart of the publication. Each subscription creates a link, or contract, between one publication and one Subscriber. There are two types of subscriptions: push subscriptions and pull subscriptions. In a push subscription, the Distributor directly updates the data in the Subscriber database. In a pull subscription, the Subscriber asks the Distributor regularly if any new changes are available, and then updates the data in the subscription database itself.
A database that is the target of a replication subscription is called a subscription database. As in the case of the publication database, SQL Server modifies the subscription database during the first initialization. The most obvious change is the addition of a few replication-related objects. However, unlike publication databases, SQL Server doesn’t prevent a subscription database from being dropped.
The replication processes are executed by a set of replication agents. Each agent is an independent Windows executable responsible for one piece of the process of moving the data. In a default installation of replication, each agent is executed by its own SQL Server Agent job. Most of those agents usually run on the Distributor, although some can run on the Subscriber. The Publisher houses replication agents only when the Publisher and Distributor are the same instance. Instead of relying on the SQL Server Agent, you can execute any replication agent manually or by some other scheduling means. However, in most cases, these approaches provide little advantage and often make troubleshooting more complex.
The details of each replication agent types as
In all replication topologies, the Snapshot Agent provides the data required to perform the initial synchronization of the publication database with the subscription database. Transactional replication and merge replication use other agents to keep the data in sync afterwards. For both topologies, replication will use the Snapshot Agent again (after the initial synchronization) only when you request a fresh resynchronization. Snapshot replication, on the other hand, uses the Snapshot Agent exclusively to replicate data. It works by copying all the data every time from the publication database to the subscription database.
Log Reader Agent
The Log Reader Agent reads the transaction log of the publication database. If it finds changes to the published objects, it records those changes to the distribution database. Only transactional replication uses the Log Reader Agent.
The Distribution Agent applies the changes recorded in the distribution database to the subscription database. As with the Log Reader Agent, only transactional replication uses the Distribution Agent.
The Merge Agent synchronizes changes between the publication database and the subscription database. It is able to handle changes in both the publication database and the subscription database and can sync those changes bi-directionally. A set of triggers in both databases support this process. Only merge replication uses the Merge Agent.
Queue Reader Agent
The Queue Reader Agent is used for bi-directional transactional replication.
Happy readying …
Sources: Fundamentals of SQL Server 2012 Replication and SQL Server Book Online