- General concepts
- Replication topologies
- Replication types
- Replication agents
- Checking necessary conditions
- Step by step example
- Backup and restore strategies
In this article, I want to tell you about some general Microsoft
SQL Server replication topics: replication topologies, replication
types, and replication agents. I will also discuss Snapshot
replication: how to check necessary conditions for this type of
replication and how to backup and restore the databases in this
replication scenario. To illustrated these concepts, I'll also
be providing a step by step guide to setting up a Snapshot
Because it's only a test example, I used only one server to replicate
data: Publisher, Subscriber and Distributor databases resided
on the same machine.
Replication is the process whereby data is copied between databases
on the same server or different servers connected by LANs, WANs, or
Microsoft SQL Server replication uses a publisher, distributor and
Publisher is the server or database that sends its data to another
server or database.
Subscriber is the server or database that receives data from another
server or database.
Distributor is the server that manages the flow of data through the
replication system. This server contains the distribution database.
Publisher contains publication/publications.
Publication is a collection of one or more articles that is sent
to a subscriber server or database.
Article is the basic unit of replication and can be a table
or a subset of a table.
Subscription is the group of data that a server or database will
There are push and pull subscriptions.
Push subscription is subscription when the publishing server will
periodically push transactions out to the subscribing server or
Pull subscription is subscription when the subscribing server will
periodically connect to the distribution database and pull information.
The Distribution database is a system database, which is stored on the
Distributor and does not contain any user tables. This database is used
to store snapshot jobs and all transactions waiting to be distributed
Microsoft SQL Server supports the following replication topologies:
- Central publisher
- Central subscriber
- Central publisher with remote distributor
- Central distributor
- Publishing subscriber
This is one of the most used replication topologies. In this scenario,
one server is configured as Publisher and Distributor and another
server/servers is/are configured as Subscriber/Subscribers.
This is a common topology in data warehousing. Many servers or databases
replicate their data to a single central server in one or more databases.
Central publisher with remote distributor
In this topology, the distribution database resides on another server
than publisher. This topology is used for performance reasons when the
level of replication activity increases or the server or network
resources become constrained. It reduces Publisher loading, but it
increases overall network traffic.
This topology requires separate Microsoft SQL Server installations,
one for the Publisher and one for the Distributor.
In this topology, several publishers use only one distributor, which
resides on another server than publishers. This is one of the most
rarely used replication topologies because it has only single point
of failure (on the single server with central distributor), and if
distributor's server will fail, the entire replication scenario will
This is a dual role topology. In this topology, two servers publish
the same data. One publishing server sends data to the subscriber, and
then this subscriber publishes data to any number of other subscribers.
This is useful when a Publisher must send data to Subscribers over
a slow or expensive communications link.
Microsoft SQL Server 7.0/2000 supports the following replication types:
Snapshot replication is the simplest type of replication. With this kind
of replication, all replicated data (replica) will be copied from the
Publisher database to the Subscriber's/Subscribers' database(s) on a
periodic basis. Snapshot replication is best used as a method for
replicating data that changes infrequently and when the size of
replicated data is not very large.
With Transactional replication, SQL Server captures all changes that
were made in an article and stores INSERT, UPDATE, and DELETE
statements in the distribution database. These changes are then sent to
subscribers from the distribution database and applied in the same
order. Transactional replication is best used when the replicated
data changes frequently or when the size of replicated data is not
small and is not necessary to support autonomous changes of the replicated
data on the Publisher and on the Subscriber.
Merge replication is the most difficult replication type. It makes
possible autonomous changes to replicated data on the Publisher and
on the Subscriber. With Merge replication, SQL Server captures all
incremental data changes in the source and in the target databases,
and reconciles conflicts according to rules you configure or using
a custom resolver you create. Merge replication is best used when
you want to support autonomous changes of the replicated data on the
Publisher and on the Subscriber.
Microsoft SQL Server 7.0/2000 supports the following replication agents:
- Snapshot Agent
- Log Reader Agent
- Distribution Agent
- Merge Agent
The Snapshot Agent
is a replication agent that make snapshot files,
stores the snapshot on the Distributor, and records information about
the synchronization status in the distribution database. The Snapshot
Agent is used in all replication types (Snapshot, Transactional, and
Merge replications) and can be administered by using SQL Server
The Log Reader Agent is a replication agent that moves transactions
marked for replication from the transaction log on the Publisher to
the distribution database. This replication agent is not used in
The Distribution Agent is a replication agent that moves the snapshot
jobs from the distribution database to Subscribers, and moves all
transactions waiting to be distributed to Subscribers.
The Distribution Agent is used in Snapshot and Transactional
replications and can be administered by using SQL Server
The Merge Agent is a replication agent that applies initial snapshot
jobs from the publication database tables to Subscribers, and merges
incremental data changes that have occurred since the initial snapshot
was created. The Merge Agent is used only in Merge replication.
Checking Necessary Conditions
Check the following before setting up Snapshot replication:
1. The Localsystem account has no access to shares on the network as it
isn't an authenticated network account.
So, if you want to set up replication you must change the account
the MSSQLServer and SQLServerAgent services runs under to a account
with the Windows NT/Windows 2000 administrator's rights.
If your Microsoft SQL Server runs on Windows NT or Windows 2000,
you can create a Windows NT/Windows 2000 account and include it
in the local Administrators group and the Domain Users group, and set
Log in as a service permission for this account.
Windows 9x does not support Windows NT services, so if your Microsoft
SQL Server runs on Windows 9x, you do not need to create an SQL account.
2. Only members of the sysadmin server role can set up and configure
replication, so if you do not have these rights, you cannot set up
3. Don't forget to start the SQLServerAgent service (and the MSSQLServer
service, of course).
4. You should allocate adequate disk space in the snapshot folder.
5. You should allocate adequate disk space for the distribution
6. You should ensure that the server being replicated to is defined
as a remote server.
In this example, I will use only one server to replicate data:
Publisher, Subscriber and Distributor databases will be residing
on the same machine.
I will use Snapshot replication with push subscription. To set up
Snapshot replication, you can use GUI interface (from the SQL Server
Enterprise Manager), or you can run SQL Server system stored procedures.
The first way is easier and more understandable, so I will use it.
First of all, you should register the remote server to be replicated.
Because I use only one server to replicate data, I don't need to make
this step. Figure 1 shows the remote server name (the same as the
local server name, in this case).
In this example, I will replicate data from the pubs database into
Configure Publishing, Subscribers, and Distribution...
as shown in Figure 3.
This will launch the Configure Publishing and Distribution Wizard,
as shown in Figure 4.
Select the Next button to create the Distributor, as shown in Figure 5.
Here you can configure SQLServerAgent service to start automatically
when the computer is started. Check
Yes, configure the SQL Server Agent service to start automatically
and click the Next button, as shown in Figure 6.
Specify snapshot folder using a network path and click the Next button,
as shown in Figure 7.
Now you can customize the publishing and distribution settings,
or you can choose the default settings.
No, use the following default settings
and click the Next button, as shown in Figure 8.
Click the Finish button, as shown in Figure 9.
Microsoft SQL Server created the distribution database, enabled
publishing, and installed the distributor. Once completed, you should
see Figure 10.
Click OK button and see Figure 11.
As we installed CHIGRIK as Distributor, so Replication monitor
has been added to the console tree on CHIGRIK server.
Click Close button.
Now we are ready to start creating publications and articles. Select
Tools => Replication => Create and Manage Publications
as shown in Figure 12.
You will see Create and Manage Publications dialog box,
as shown in Figure 13.
Choose pubs database and click the Create Publication button.
The Create Publication wizard will be launch.
Click the Next button, as shown in Figure 14.
Choose the pubs database and click the Next button, as shown in
Select Snapshot publication and click the Next button, as shown in
Select all of the types of Subscribers that you expect to subscribe
to this publication and click the Next button, as shown in Figure 17.
Choose authors table to publish as article and click the Next button,
as shown in Figure 18.
Specify pubs_article as the publication name and click the Next button,
as shown in Figure 19.
You can specify data filters on this step, but in this example,
we don't use any data filters.
No, create the publication as specified
and click the Next button, as shown in Figure 20.
Click the Finish button to create the publication, as shown in
Now the 'pubs_article' publication was created, so click the
Close button, as shown in Figure 22.
Now you can create new subscription. Click the Push New Subscription
button, as shown in Figure 23.
This will launch the Push Subscription wizard shown in Figure 24.
Click the Next button.
Select CHIGRIK to select all subscribers in that group and click
the Next button, as shown in Figure 25.
Select pubs_copy database as the subscription database and click
the Next button, as shown in Figure 26.
Specify how frequently Distribution Agent updates the subscription
(in this example, every 1 day(s), every 20 minute(s) between 9:00:00
and 18:00:00) and click the Next button, as shown in Figure 27.
Start the Snapshot Agent to begin the initialization process immediately
and click the Next button, as shown in Figure 28.
Click the Next button, as shown in Figure 29.
Click the Finish button to subscribe with the options you specified on
the previous steps, as shown in Figure 30.
Click the Close button, as shown in Figure 31.
Click the Close button to close the Create and Manage Publication
dialog, as shown in Figure 23.
Note. On the last step we got the following message:
The subscription at Subscriber 'CHIGRIK' cannot be initialized
immediately because the snapshot for this publication is not yet
available. To initialize the subscription, start the Distribution
Agent after the snapshot is available.
To work around it, you should start the Distribution Agent after
the snapshot is available.
See Figure 32.
Backup and Restore Strategies
The backup and restore strategies differ for each replication
types. Here, I want to describe the backup and restore strategies
for the Snapshot replication.
There are four main strategies for backing up and restoring Snapshot
- Backup Publisher, master and model databases.
- Backup Publisher, Distributor, master and model databases.
- Backup Publisher, Subscriber(s), master and model databases.
- Backup Publisher, Distributor, Subscriber(s),
master and model databases.
Backing up Publisher, master and model databases is a simplest strategy.
This strategy has its own advantages and disadvantages. The advantages
are that it requires the least amount of storage resources and does not
require coordinating the backup with the backup of any other servers.
The main disadvantage of this strategy is that you may need to setup
replication from the beginning in the event of a Publisher or
Distributor failure. With this strategy, you should backup publication
database after changing existing publications or after the new
publications were added.
Backing up Publisher, Distributor, master and model databases is a more
frequently used strategy than the first one, because in this case you
don't need to reestablish replication in the event of a Publisher or
Distributor failure. The main disadvantage of this strategy is that
you need to backup Publisher and Distributor's databases simultaneously
(or as closely as possible). It also requires more computing and storage
resources than the first way.
Backing up Publisher, Subscriber(s), master and model databases reduces
the amount of time required to recover a Subscriber(s) by avoiding the
need to reinitialize the Subscriber(s) with a new snapshot, but in the
event of a Distributor failure, you should setup replication from the
Backup Publisher, Distributor, Subscriber(s), master and model databases
is the most complex backup strategy. The main advantage of this strategy
is that in the event of a Publisher, Distributor or Subscriber(s)
failure, you can quickly restore fail database without setting up
replication from the beginning. The disadvantage of this strategy is that
you need to backup Publisher and Distributor's databases simultaneously
(or as closely as possible), and this strategy requires the most
computing and storage resources.
For each of the strategies you should backup msdb and master databases on
the Publisher, Distributor and Subscriber(s). msdb database is used
by SQL Server Agent for scheduling alerts and jobs (it also contains
snapshot jobs), and master database is a main system database contains
entries for each Subscriber(s), each login account, about system
configuration settings and so on.
1. SQL Server Books Online.
2. How Snapshot Replication Works
3. Planning for Snapshot Replication
4. Enhancing Snapshot Replication Performance
5. Snapshot Replication
6. Strategies for Backing Up and Restoring Snapshot Replication
7. Replication Types
See All Articles by Columnist Alexander Chigrik