SQL 2000 Replication Architecture

SQL 2000 Replication Architecture
Replication: SQL Server 2000– Table of Contents

Understanding Replication
Database management systems are among the most important software systems driving the information age. In many Internet applications, a large number of users who are geographically dispersed may routinely query and update the same database. In this environment, the location of the data can have a significant impact on application response time and availability. A centralized approach manages only one copy of the database. The centralized approach suffers from two major drawbacks:
• Performance problems due to high server load or high communication latency for remote clients.
• Availability problems caused by server downtime or lack of connectivity. Clients in portions of the network that are temporarily disconnected from the server cannot be serviced.
These issues would be effectively answered by Replication
Replication is the process of sharing data between databases in different locations. Using replication you create copies of the Database and share the copy with different users so that they can make changes to their local copy of database and later synchronize the changes to the source database.

Replication benefits
• Users working in different geographic locations can work with their local copy of data thus allowing greater autonomy.
• Database replication can also supplement your disaster-recovery plans by duplicating the data from a local database server to a remote database server. If the primary server fails, your applications can switch to the replicated copy of the data and continue operations.
• You can automatically back up a database by keeping a replica on a different computer. Unlike traditional backup methods that prevent users from getting access to a database during backup, replication allows you to continue making changes online.
• You can replicate a database on additional network servers and reassign users to balance the loads across those servers. You can also give users who need constant access to a database their own replica, thereby reducing the total network traffic.
• Database-replication logs the selected database transactions to a set of internal replication-management tables, which can then be synchronized to the source database. Database replication is different from file replication, which essentially copies files.

SQL Server platform for Replication
Microsoft SQL server uses publishing industry model to represent the components and processes in replication architecture. Publishing industry publishes Magazines/Books; there are Distributors and Agents who carry these publications to the Subscribers. Subscriber of the magazine obtains copies of the publication and read the articles of interest to him; this is how SQL Server Replication model works. Figure 1 depicts the typical Publishing industry flow.


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Based on the above model we can identify following Entities for the SQL Server replication model.
• Publisher
• Distributor
• Agent
• Subscriber
• Articles
• Publications
• Subscriptions
Let us know more about each of these Entities:

Publisher
Publisher is a server that makes the data available for subscription to other servers. In addition to making data available for replication, publisher also identifies what data has changed at the subscriber during the synchronizing process. Depending on the type of the replication, changed data is identified at different instances. We will learn more about Replication types in the Replication.

Distributor
Distributor maintains the Distribution Database. The role of the distributor varies depending on the type of replication. Two types of Distributors are identified, remote distributor and Local distributor. Remote distributor is separate from publisher and is configured as distributor for replication. Local distributor is a server that is configured as publisher and distributor.

Agents
Agents are the processes that are responsible for copying and distributing data between Publisher and subscriber. There are different types of Agents supporting different replication types.

Subscriber
Subscriber is a server that receives and maintains the published data. Modifications to the data at subscriber can be propagated back to the publisher; in some cases Subscriber may re-publish the data to the other subscribers.

Articles
An article can be any database object, viz. Tables (Column filtered or Row filtered), Views, Indexed views, Stored Procedures, User defined functions.

Publication
Publication is collection of articles.

Subscriptions
Subscription is a request for copy of data or database objects to be replicated.

Subscription Types:
Changes to the subscriptions at the publisher can be replicated to subscribers via PUSH subscription or PULL subscription.
With Push subscription the publisher is responsible for synchronizing all the changes to the subscriber without subscriber asking for those changes.
With Pull subscription the subscriber initiates the replication instead of the publisher.

Replication Types
Microsoft SQL Server supports, following types of replication.
• Snapshot Replication
• Transactional Replication
• Merge Replication
Snapshot Replication
Snapshot replication is also known as static replication. Snapshot replications copies and distributes data and database objects exactly as they appear at current moment in time.

Characteristics of Snapshot replication
1. The changes to data at subscriber are not updated to the subscriber continuously
2. Subscribers are updated with complete modified data and not by individual transactions
3. Propagating the changes to the subscribers takes more time as it is one time process or scheduled process.
When do I use Snapshot replication?
Following are some of the scenarios where snapshot replication fits in ideally
• Data/Db objects are static or does not change frequently
• Replicate Look Up tables that do not change frequently
• Amount data to be replicated is small
• Users often work in disconnected mode, and are not always interested with latest data.
Transactional Replication
Transactional replication is also known as dynamic replication. In transactional replication, modifications to the publication at the publisher are propagated to the subscriber incrementally.

Characteristics of Transactional Replication
• Publisher and the subscriber are always in synchronization
• Transaction boundaries are preserved, i.e. if there modification to 5 rows of data, either all the 5 modified rows are propagated to the subscriber or none
• The publisher and the subscriber should be connected always.
When do I use Transactional replication?
• Replicating Database with rollup information, Database with regional, central sales or inventory database that is updated and replicated to different sites.
• Subscribers always need the latest data for processing
Merge Replication
Merge replication provides advantageous of both Snapshot replication and Transactional replication. The initial snapshot applied to the subscribers and then SQL server tracks changes to the data at publisher and subscriber. The data is synchronized on scheduled basis or on demand. Since data modifications are made independently at publisher and subscriber, conflicts are likely to occur during synchronizing.

Characteristics of Merge Replication
• Updates to the data are made independently at more than one server
• Data is merged on scheduled basis or on demand
• Allows users to work online/offline and synchronize the publisher and subscriber on scheduled basis or on demand.
When do I use Merge replication?
• Site autonomy is very critical
• Multiple subscribers need to update the data at same time or different time and propagate the changes to the publisher
Implementing Replication
With the above basic knowledge we can now proceed to understand the implementation of replication. There are different ways by which you can implement and monitor replication based on different replication types. But in general replication has following general steps:
• Configuring replication
• Generating and applying initial snapshot
• Modifying replicated data
• Synchronizing and propagating data
Configuring replication:
• Configure the publisher and distributor. Distributor can be same server or different server
• Create publications based on data, sub sets of data and database objects
• Determine the type of replication to use, the subscriber database and location of the snapshot file
• Configure when the synchronization will occur and options that will be used with publications
• Create push and/or pull subscriptions at either the publisher or the subscriber and configure your replication schedule and options
Generating and applying initial snapshot:
SQL server 2000 creates a snapshot of data and schema and saves it in the snapshot file location. After subscription is created when the snapshot is applies is based on configured schedule when creating publication or snapshot can be applied manually
The snapshot agent is responsible for creating the snapshot file and stores it in the snapshot file location

Modifying replicated data:
Depending on the type of replication and replication options, the subscriber will be able to modify the data after the snapshot has been applied and propagate the changes back to the publisher or other subscribers.

Synchronizing and propagating data changes:
Synchronization refers to the propagation of data changes between subscriber and publisher. How the data is synchronized is dependent on the type of replication used.
• Incase of snapshot replication, snapshot file is reapplied at the subscriber
• Incase of transactional replication all data modification through Insert/Update and Delete and distributed between publisher and subscriber
• Incase of merge replication data modification at various servers are merged, conflicts if any are detected and resolved.
Data Considerations for Replication
Special consideration should be taken for some of the data types and properties during replication. These Data types and properties are:
• Identity range management
• Unique identifier and Timestamp data types
• NOT FOR REPLICATION option
Identity range management:
Value of a column marked as Identity is incremented automatically, when new rows are added to the column table. In replication where publication contains identity columns following configurations can be used to manage Identity columns

Use Auto Identity range management of SQL Server 2000
For example, you can set the Identity range of 1 to 500 for Publication ‘A’ at Publisher and 501 to 1000 for the same publication at Subscriber, with threshold of 80%
In this case newly inserted row at publisher will have Identity from 1 to 500 and a newly inserted row at subscriber will have identity from 501 to 1000
When threshold is reached 80% new identity range is used for the next inserts. In this case if the identity value reaches 400 at the Publisher and new inserts after that will use the new identity range from 1001 to 1500.
Similarly if the Subscriber threshold reaches 800, any new inserts after that will have Identity range from 1501 to 2000
The threshold value should be set carefully by evaluating the frequency of updates at the subscriber and synchronization schedule. Setting threshold to a lower value will result in many unused Identity values.
Following system-stored procedures can also be used to set Identity range explicitly.
• Sp_adjustpublisheridentityrange
• Sp_addmergearticle

Use NOT FOR REPLICATION option when defining Identity columns
Identity ranges can also be managed by defining check constraint and the NOT FOR REPLCIATION option on Identity column. When an identity column is specified as NOT FOR REPLCATION, then its range should be provided programmatically. When this option is set, SQL server retains the original values set by the replication agent but continues to increment the value of the Identity column in a normal value, i.e. without resetting the Identity value.

Step by Step approach to implementing Merge Replication
The following steps help to setup the Replication using the SQL Server Enterprise Manager. For this example I would consider the standard PUBS database.
1. Set Up right login for Replication
• To set up replication, you must use a login account that is a member of SQL Server's Process Administrators (or higher authority) server role. I used “sa” and it worked.
2. Configuring Server for Publishing, Subscribers and Distributor.
• Under SQL Server enterprise manager navigate to Tools/Replication menu, run Configure Publishing and Distribution Wizard.


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• Select your server as Publisher, Distributor and Subscriber server


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• Specify the location where snapshots from publishers that use this distributor will be stored.


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• Configure your server with default settings or apply custom settings, in this example I would use the default settings.
With the above settings you have enabled your server as distributor server, and to create Publishers and Subscribers. This process will also create a New Distribution Database under the selected server.


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3. Creating Publication
• Right click on PUBS database click on New/Publication…which will run the Publication wizard.
• Select the PUBS database as Publication Database


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• Chose replication type as Merge replication.


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• Specify what types of Subscribers will subscribe to this Publication, In this example I chose, SQL Server 2000 as Subscriber types
• Chose the ‘Object type’ you want to Publish, I in this example I will chose to publish all type of objects.


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• Go to all the table article properties (Click on the ellipses) and select the Identity Range tab and Check Automatic identity range manipulation check box and set Publisher and Subscriber range to desired value, in this example I will set the Identity range value as 1500. This is tedious process, hence it is necessary to automate this process, I will use the following system stored procedures to set the Identity range explicitly.
o Sp_adjustpublisheridentityrange
o Sp_addmergearticle
Following is the source code of the stored procedure, which automates the Identity range setting process.
The Stored procedure takes ‘PublisherName’ and ‘IdentityRange’ as the parameter and internally uses ‘SP_addmergearticle’ system stored procedures to set the Identity range of all the Published Table articles.


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• Specify the Publication name.

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• Accept other settings and create publication.

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With the above steps we have Published the data in Database ‘Pubs’ and also set publication properties.

4. Create Snap Shot
• Navigate to < Server >/Replication/Publications/properties…


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• Select the status Tab


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• Click on “Run Agent Now” button this will create the snap shot files
5. Adding Subscribers
• Create a new Database, Just a new SQL Server Database. In this example I have created a New Database by name PubsSubscriber
• Right click on < Server >/Replication/Publications/Pubs:< Publication >
• Click “Push New Subscription” to run the New Subscription wizard


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• Choose the newly created database as Subscriber Database


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• Set Merge Agent schedule


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• Choose Initialize both schema and data option and check Start the Merge agent to initialize the subscription immediately.


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Note: Since my Subscriber Database was new database I have chosen to initialize both schema and Data.
Navigate through the wizard, accepting further options and click on finish to complete the wizard steps.
With the above steps we created a new subscriber database and setup the handshake between publisher and Subscriber.
Now you can distribute the subscriber Database to its users who can ‘play’ with their own copy of data, and synchronize the data as per the rules of business.

6. Synchronize
• To synchronize, Right click on the < Server >/Replication/Publications/Pubs:< Publication > And click “Start Synchronization”

7. General Replication Performance Tips
Filtering Published Data
Replication can be tuned to yield better performance, and one of the ways to achieve this is filtering the data before publishing, in other words, “Give subscribers what they want to see and play with”.
Replication facilitates this by splitting the vertical/Horizontal. By distributing partitions of data to different Subscribers, you can:
• Minimize the amount of data sent over the network.
• Reduce the amount of storage space required at the Subscriber.
• Customize publications and applications based on individual Subscriber requirements.
• Reduce conflicts because the different data partitions can be sent to different Subscribers.
In addition following are some of the tips, which will help to yield better performance
• When running SQL Server replication on a dedicated server, consider setting the minimum memory amount for SQL Server to use from the default value of 0 to a value closer to what SQL Server normally uses.
• Don’t publish more data than you need. Try to use Row filter and Column filter options wherever possible as explained above.
• For best performance, avoid replicating columns in your publications that include TEXT, NTEXT or IMAGE data types.
• Avoid creating triggers on tables that contain subscribed data
• Applications that are updated frequently are not good candidates for database replication.
• Distribute the workload into more than one SQL server using replication.
• Plan for the type of replication to be used before the database design, because the type of replication used will to certain extent guide your database design

8. Conclusion
In a nutshell, replication is the capability to reliably duplicate data from a source database to one or more destination databases. SQL Server 2000 gives you the power for replication design, implementation, monitoring, and administration. This gives you the functionality and flexibility needed for distributing copy of data and maintaining data consistency among the distributed. You can automatically distribute data from one SQL Server to many different SQL Servers through ODBC (Open Database Connectivity) or OLE DB. SQL Server replication provides update replication capabilities such as Immediate Updating Subscribers and merges replication. With all the new enhancements to SQL Server replication, the number of possible applications and business scenarios is mind-boggling.

Source By:
Mahesh M Kodli
Mahesh.kodli@wipro.com

Pub:
Trimurari Singh
trimurarisingh@gmail.com