SQL Server :: How Snapshot Replication Works

By default, all three types of replication use a snapshot to initialize Subscribers. The SQL Server Snapshot Agent always generates the snapshot files, but the agent that delivers the files differs depending on the type of replication being used. Snapshot replication and transnational replication use the Distribution Agent to deliver the files, whereas merge replication uses the SQL Server Merge Agent. The Snapshot Agent runs at the Distributor. The Distribution Agent and Merge Agent run at the Distributor for push subscriptions, or at Subscribers for pull subscriptions. 

Snapshots can be generated and applied either immediately after the subscription is created or according to a schedule set at the time the publication is created. The Snapshot Agent prepares snapshot files containing the schema and data of published tables and database objects, stores the files in the snapshot folder for the Publisher, and records tracking information in the distribution database on the Distributor. You specify a default snapshot folder when you configure a Distributor, but you can specify an alternate location for a publication instead of or in addition to the default. 

The following illustration shows the principal components of snapshot replication.

Snapshot Agent

For merge replication, a snapshot is generated every time the Snapshot Agent runs. For transactional replication, snapshot generation depends on the setting of the publication property immediate_sync. If the property is set to TRUE (the default when using the New Publication Wizard), a snapshot is generated every time the Snapshot Agent runs, and it can be applied to a Subscriber at any time. If the property is set to FALSE (the default when using sp_addpublication), the snapshot is generated only if a new subscription has been added since the last Snapshot Agent run; Subscribers must wait for the Snapshot Agent to complete before they can synchronize.

The Snapshot Agent performs the following steps:

1. Establishes a connection from the Distributor to the Publisher, and then takes locks on published tables if necessary:
   • For merge publications, the Snapshot Agent does not take any locks.
   • For transactional publications, by default the Snapshot Agent take locks only during the initial phase of snapshot generation.
   • For snapshot publications, locks are held during the entire snapshot generation process.
2. Writes a copy of the table schema for each article to a .sch file. If other database objects are published, such as indexes, constraints, stored procedures, views, user-defined functions, and so on, additional script files are generated.
3. Copies the data from the published table at the Publisher and writes the data to the snapshot folder. The snapshot is generated as a set of bulk copy program (BCP) files.
4. For snapshot and transactional publications, the Snapshot Agent appends rows to the MSrepl_commands and MSrepl_transactions tables in the distribution database. The entries in the MSrepl_commands table are commands indicating the location of .sch and .bcp files, any other snapshot files, and references to any pre- or post-snapshot scripts. The entries in the MSrepl_transactions table are commands relevant to synchronizing the Subscriber.
5. Releases any locks on published tables.

During snapshot generation, you cannot make schema changes on published tables. After the snapshot files are generated, you can view them in the snapshot folder using Windows Explorer.

Distribution Agent and Merge Agent

For snapshot publications, each time the Distribution Agent runs for the publication, it moves a new snapshot to each Subscriber that has not yet been synchronized, has been marked for reinitialization, or includes new articles.

For snapshot and transactional replication, the Distribution Agent performs the following steps:

1. Establishes a connection to the Distributor.
2. Examines the MSrepl_commands and MSrepl_transactions tables in the distribution database on the Distributor. The agent reads the location of the snapshot files from the first table and Subscriber synchronization commands from both tables.
3. Applies the schema and commands to the subscription database.

For an unfiltered merge replication publication, the Merge Agent performs the following steps:

1. Establishes a connection to the Publisher.
2. Examines the sysmergeschemachange table on the Publisher and determines whether there is a new snapshot that should be applied at the Subscriber.
3. If a new snapshot is available, the Merge Agent applies to the subscription database the snapshot files from the location specified in sysmergeschemachange.