Setup Merge replication: step by step guide
- 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's topics: replication topologies, replication
types, replication agents; and about Merge replication: how to
check necessary conditions for this replication type, how to backup
and restore databases participated in this replication scenario, and
how to setting up Merge replication step by step.
Because it's only test example, I used only one server to replicate
data: Publisher, Subscriber and Distributor databases were 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 receive.
There are push and pull subscriptions.
Push subscription is subscription when the publishing server will
periodically push transactions out to the subscribing server or database.
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 to Subscribers.
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(s) is/are configured as Subscriber(s).
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, distribution database resides on another server
than publisher. This topology uses 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 unused replication topologies, because it has only single point
of failure (on the single server with central distributor), and if
distributor's server will fail, entire replication scenario will be
This is a dual role topology. In this topology, two servers publish
the same data. One publishing server sends data to subscriber, and
then this subscriber publish data to any number of other subscribers.
This is useful when a Publisher should 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 a simplest type of replication. With this kind
of replication, all replicated data (replica) will be copied from the
Publisher database to Subscriber(s) database 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 the articles and stores INSERT, UPDATE, and DELETE
statements in the distribution database. This changes 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 the replicated
data on the Publisher and on the Subscriber.
Merge replication is a most complicated replication type. It allows
making 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 the replicated data on the
Publisher and on the Subscriber.
Microsoft SQL Server 7.0/2000 supports the following replication agents:
The Snapshot Agent is a replication agent that makes 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
- Snapshot Agent
- Log Reader Agent
- Distribution Agent
- Merge Agent
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 Merge 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 setting 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 Windows NT/Windows 2000 account and include it
into local Administrators group, into Domain Users group, and set
Log in as a service permission for this account.
Because Windows 9x does not support Windows NT services, so if
your Microsoft SQL Server runs on Windows 9x, you do not need to
create SQL account.
2. Only members of the sysadmin server role can setting up and configure
replication, so if you have not these rights, you cannot setting up replication.
3. Don't forgive to start SQLServerAgent service (and MSSQLServer
service, of course).
4. You should allocate adequate disk space for the distribution database.
5. You should allocate adequate disk space for the publisher
and subscriber's databases.
SQL Server 7.0/2000 uses uniqueidentifier column to identify each
row during the merge replication, so if your table does not have
a uniqueidentifier column with the ROWGUIDCOL property, Microsoft
SQL Server 7.0/2000 will add this column to the table. So, you need
the additional disk space to store your data.
6. You cannot replicate only one table with foreign key constraints,
you should include all referenced tables in the publication.
7. You should ensure the server that is 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 resided
on the same machine.
I will use Merge replication with push subscription.
To setting up Merge 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 much easy and much understandably,
so I will use it.
First of all, you should register the new remote server in which
will be replicated. Because, I use only one server to replicate
data, I don't need to make this step. Figure 1 shows 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.
Click 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. Check
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\SQL2000 as Distributor, so Replication monitor
has been added to the console tree on CHIGRIK\SQL2000 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 Figure 15.
Select Merge publication and click the Next button, as shown in Figure 16.
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.
Check Publish All checkbox to publish all tables from the pubs database
and click the Next button, as shown in Figure 18.
Click the Next button, as shown in Figure 19, to add uniqueidentifier
columns in all tables in the pubs database and set NOT FOR REPLICATION
option to the IDENTITY column of the jobs table.
Specify pubs_article as the publication name and click the Next button,
as shown in Figure 20.
You can specify data filters on this step, but in this example,
we don't use any data filters. Check
No, create the publication as specified
and click the Next button, as shown in Figure 21.
Click the Finish button to create the publication, as shown in Figure 22.
Now the 'pubs_article' publication was created, so click the
Close button, as shown in Figure 23.
Now you can create new subscription. Click the Push New Subscription
button, as shown in Figure 24.
This will launch the Push Subscription wizard shown in Figure 25.
Click the Next button.
Click on the SQL Server Group to select all subscribers in the
CHIGRIK and CHIGRIK\SQL2000 groups and click the Next button,
as shown in Figure 26.
Select pubs_copy database as the subscription database and click
the Next button, as shown in Figure 27.
Specify how frequently Distribution Agent will update the subscription
(in this example, every 1 day(s), every 30 minute(s) between 9:00:00
and 18:00:00) and click the Next button, as shown in Figure 28.
Check Start the Merge Agent to initialize the subscription immediately
and click the Next button, as shown in Figure 29.
Now you can set the subscription priority, which helps decide the
winner of conflicting data changes. Check
Use the Publisher as a proxy for the Subscriber when resolving conflicts
and click the Next button, as shown in Figure 30.
Click the Next button, as shown in Figure 31.
Click the Finish button to subscribe with the options you specified on
the previous steps, as shown in Figure 32.
Click the Close button, as shown in Figure 33.
Click the Close button to close the Create and Manage Publication
dialog, as shown in Figure 24.
The last step of the setting up Merge replication should be creation
a current script of your replication settings. It can be useful to
help recover your replication settings in the event of server failure.
Generate SQL Script...
as shown in Figure 34.
Backup and restore strategies
The backup and restore strategies are differ for each replication
types. Here, I want to describe the backup and restore strategies
for the Merge replication.
Because Merge replication is more complicated than Snapshot or
Transactional replication, and usually used when you want to
update data on both publisher and subscribers, so you should
plan backup and restore strategies more detail and attentive.
There are four main strategies for backing up and restoring Merge replication:
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, or whenever changes are made to a replicated
objects schema (for example, adding or dropping a column).
- 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, Distributor, master and model databases is a
rarely used strategy, than the first one, because in most cases, it
is not necessary to restore a distribution database when restoring
a publication database backup for merge publications. It is because
the distribution database does not store any data used in change
tracking and it does not provide temporary storage of merge replication
changes. The main disadvantage of this strategy is that you should
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 is
used when some changes were made on Subscriber(s) and you need these
changes to be synchronized with the publication database.
Backup Publisher, Distributor, Subscriber(s), master and model databases
is a 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 should backup Publisher and Distributor's databases simultaneously
(or as closely as possible), and this strategy requires most computing
and storage resources.
For each strategy, 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, and master database
is a main system database contains entries for each Subscriber(s), each
login account, about system configuration settings and so on.
Note. It is strongly recommended that you make and keep a current script
of your replication settings. It can be useful to help recover your
replication settings in the event of server failure.
1. SQL Server Books Online.
2. How Merge Replication Works
3. Planning for Merge Replication
4. Enhancing Merge Replication Performance
5. Merge Replication
6. Strategies for Backing Up and Restoring Merge Replication
7. Replication Types