Home > Articles > Data > SQL Server

  • Print
  • + Share This
Like this article? We recommend

Replication needs to be planned carefully. Setting things up is easy, but there is no magic UNDO button that will reverse all your actions. Therefore, be sure to test your plan thoroughly before implementing a replication solution. The following sections discuss some of the planning steps necessary for transactional replication.

Defining Server Roles

The first step is defining server roles using the publisher/subscriber/distributor model discussed earlier. Identifying publishers and subscribers is relatively simple: Publisher is the server in which data changes occur; subscriber is where changes need to be delivered.

Be careful when defining the distributor, however. It is often tempting to use the publishing server as a distributor because it can translate into saving the cost of purchasing a separate server. However, if you're implementing replication to reduce the load on the server or to separate read/write activity, you should strongly consider using a separate server dedicated to distributing replicated transactions. Bear in mind that every single transaction that occurs on your server must be recorded in the distribution database prior to being delivered to the subscribers. Some shops opt for using one of the subscriber servers as the distributor, which can work if the subscriber is not heavily taxed with the existing load. Using the subscriber as the distributor is also common in test and quality assurance environments.

Determining What Data Needs to be Replicated

You might now be thinking: "Oh, that's easy...just replicate all database objects, right?" Not so fast! If you have a single database server with 20 users and a 120MB database, you can safely publish all data and subscribe to it from another server. But in the real world, you should replicate only data that you must have on the subscribers. SQL Server replication is a mature technology that can move many millions of transactions each day given sufficient hardware resources.

But I'd hesitate putting my production servers to such a test. To accomplish the best possible performance without bringing your servers to their knees, consider carefully which tables, views, stored procedures and user-defined functions you must replicate.

Replication Agents

Transactional replication involves three agents: snapshot, log reader, and distribution. The snapshot agent takes a snapshot of records on the publisher and copies the data out to the snapshot folder. The snapshot agent also generates scripts for database schema and includes CREATE TABLE and CREATE INDEX scripts.

The snapshot agent doesn't have to run continuously for transactional replication. If you get your replication working properly the first time, you might never have to run the snapshot agent again after the initial synchronization. However, if you do have problems with the subscriber servers missing data, the snapshot agent is there to help.

The log reader agent reads the transaction log on the published databases. This agent moves transactions that are marked for replication to the distribution database. The distribution agent delivers transactions from the distribution database to the subscribers. Log reader and distribution agents have to run continuously (or at scheduled intervals) to keep replication working.

In addition to snapshot, log reader, and distribution agents, replication also uses a few other jobs (agents) to keep things organized. The history cleanup agent, for example, is used to delete transactions that have already been delivered from the distribution database. Indeed, if this agent did not work, the distribution database would grow very large.

Agent Profile Settings

Replication agents are implemented as SQL Server jobs that call executable files with certain parameters. You should be aware that clicking through the replication wizards configures agents to run with default parameters. If you need to tweak agent parameters for troubleshooting or for performance reasons, you'll have to modify the replication agent's profile. (I'll discuss replication agents' parameters in the next article.)

Security Considerations

Replication agents must have appropriate security permissions to read data on the publisher, move transactions to the distributor and apply the data and transactions to the subscribers. You can allow replication agents to run using security credentials of SQL Server Agent service; alternatively, you can define a login that has a more limited set of permissions. Security is not a joking matter: Allow your replication agents too much freedom, and a hacker can destroy your data on publishers as well as subscribers. On the other hand, not granting sufficient permissions to the agents prevents replication from working properly.

Identity Columns

Identity columns present a special challenge for some replication scenarios. For example, consider what happens if you have multiple publishers sending transactions to a single subscriber. You want to split the user base among two servers, but many tables in your database have identity columns. How do you prevent identity values from colliding on two servers? You must manage the identity ranges on your servers somehow to avoid duplicate identity values on the subscriber. One solution is to allow only positive IDENTITY values on Server A and only negative IDENTITY values on Server B.


Collation is a combination of case-sensitivity and sort order used for character data. Earlier releases of SQL Server limited defining of collation to a particular server or database. With SQL Server 2000, you can define collation for each character column. Such freedom comes with the price of an administrative nightmare, however, if you have numerous character columns using different collations.

As far as replication is concerned, published and subscribed tables must have the same collation for corresponding columns. Having different collations on the publisher and subscriber might result in failure to replicate transactions. Furthermore, SELECT statements executed against the same table on the publisher and subscriber might return different data if tables have different collation.

Triggers and Constraints

Yet another consideration is whether triggers and constraints should be enforced on the subscribing servers. Triggers are often used to maintain data integrity by executing INSERT, UPDATE, or DELETE statements when a corresponding data modification occurs in related tables. Such statements might already be replicated from the publisher, however; therefore, if you're not careful, you might end up running the same transaction twice—once through replicating the transaction from the publisher and then through a trigger that resides on the subscriber. On the other hand, if you allow modifying data on subscribers, you might want to enforce data integrity through triggers. Constraints check the existence of the related record prior to allowing the modification of a row in a table. However, depending on your replication scheme, the related table(s) might not be replicated.

The saving grace with triggers and constraints is the NOT FOR REPLICATION option, which allows SQL Server to disregard triggers and constraints for replicated transactions (but check them for all other transactions). If your subscriber databases are used for read-only purposes, you might wish to drop triggers and constraints from subscribing databases altogether.


TEXT, NTEXT, and IMAGE are special data types for holding a large number of characters or images stored in binary format. Such data types are typically modified using the WRITETEXT and UPDATETEXT commands, which are not logged. You can also modify such data using INSERT/UPDATE statements, which are logged. Be sure to examine your application carefully to ensure that your text and image data changes are being logged. In addition, be aware that although you can replicate columns with the TEXT data type, SQL Server has a limit of characters that will be replicated for such data.

Immediate Updating and Queued Updating

When you want to use updatable subscribers, you need to consider whether transactions need to be delivered back to the publisher immediately or whether some delay is acceptable. Typically, you should opt for queued updating if continuous network connectivity is not guaranteed.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020