Sometimes an application may need data stored in another database on a different instance of SQL Server. That different instance might be on the same physical machine or might be on another machine altogether. So what do you do in this situation? Your options depend on your data requirements, like how up to date does the data need to be. Also, the network/computer topology might be a factor in what you can and cannot do. To discuss all the different possible options would make this article quite lengthy so let me narrow down the scope a little bit. For the purpose of this article, I will be discussing how a linked server can be used to seamlessly provide an application access to data on a different instance of SQL Server. Note linked servers can also be used to access other non-SQL Server data sources, but that notion will be outside the scope of this article. I will only be discussing options and situations related to using linked servers to access information stored in a SQL Server database.
Basic Linked Sever Architecture
Before I get into how to setup a linked server, let me discuss the basic architecture of a linked server. A linked server is a mechanism that allows a query to be submitted on one server and then have all or part of the query redirected and processed on another SQL Server instance, and eventually have the results set sent back to the original server to be returned to the client. To better show how this works look at the following diagram:
In this diagram, a Client can connect to either an Application Server or directly to SQL Server to submit a query. If the Client or application running on the Application Server submits a query to SERVER1 that needs to retrieve data from a database housed on SERVER2, then this kind of query is known as a distributed query. Defining a linked server definition for SERVER2 on SERVER1 allows for a client or an application to submit these kinds of distributed queries. A distributed query that runs against SERVER2 from a linked server defined on SERVER1 would look something like this:
SELECT name "DBs on SERVER2" FROM SERVER2.master.sys.databases
Here I identify the object I want to reference on my linked server by using a four part naming convention. In my example, I wanted to return the names of all the databases on SERVER2. Therefore, I used a four part naming which consisted of <linked server>.<database>.<schema>. <object>, or in my case SERVER2.master.sys.databases. SERVER2 is the name of the linked server, which is defined on SERVER1.
How to Define a Linked Server
To create or list the available linked servers already defined you look under the Server Objects folder within SQL Server Management Studio (SSMS). You can also use sp_addlinkedserver system stored procedure to add a linked server, or sp_helpserver to list linked servers.
To create linked SERVER2 in my above example in SSMS, I would first expand the Server Objects folder, and then right click on the Linked Servers item. This would display the following window:
On this window, you name your new linked server and identify the type of data source your linked server will be. Remember linked servers can be defined for a number of different kinds of data sources. For the purpose of this article, I will be defining SERVER2, which is a SQL Server data source. In order to do that I will need to identify the name of the linked server and then use the Security and Server Options pages to define how I would like to authenticate to my linked server and what options will be associated with my linked server. To begin defining my linked server I enter SERVER2 in the Name field and then click on the SQL Server radio button to identify that my new linked server is a SQL Server data source. When I do that my window looks like this:
To define how clients would authenticate to SERVER2 I would click on the Security item in the upper left hand corner of this page, under the Select a page section. When I click on the Security item, the following page is displayed:
Here you have a number of different ways to identify how your clients would be authenticated to the linked server. Let me go through each one of these options.
At the top of this screen, in the right hand pane you can define login mappings. Login mapping is a way to associate a login on the local server, with a login on the remote server. There are two different ways a local login can be mapped to a remote login. The first method is to impersonate, and the second is to associate the local login with a remote login and password. The impersonate option takes the local Windows login and uses it to connect to the linked server. It does this by impersonating the local login. In order for the local server to impersonate, the login requires that delegation be setup between the local server and the linked server. A discussion on delegation is outside the scope of this article. To map a local login you would associate it with a remote login and password. The remote login needs to be a SQL Server Authenticated user on the remote server. The following screen shot shows how I have mapped some local logins to remote logins on SERVER2:
Here I have mapped three different local logins to two different remote logins. The first login mapping is for DJ\GREG, which is a Window domain authenticated user that is defined on the local server. Ive identified the mapping so DJ\GREG is to be impersonated when connecting to SERVER2. This means anytime DJ\GREG is logged onto SERVER1 and issues a linked server query to SERVER2 those request will connect and run the query on SERVER2 in the security context of DJ\GREG. The second mapping is for WEB_USER which is a SQL Server authenticated user. Ive mapped WEB_USER to the same remote login. In doing so, I had to provide the password for login WEB_USER. This password must be the password for the WEB_USER on linked server, in my case that would be SERVER2. The third login mapping demonstrates how you can map multiple local logins to a single remote login. In my example I mapped the Windows domain authenticated login DJ\LINDA to the remote login WEB_USER. Using mapped logins is a way to identify only those users from the local machine that can connect to the linked server.
In addition to mapping logins, you can also identify how logins that are not defined in the mappings would connect to the linked server. There are four different options that can be used. These four options are the different radio buttons in the screen shot above.
The first option Not be made is fairly obvious. When you select this option, any users not identified in the login mappings will not be able to connect to the linked server. The second method Be made without using a security context is to be used for connecting to data sources that do not require any authentication, like a text file. If you select this option to connect to a linked server then this has the same effect as selecting the Not be made option. The third option Be made using Logins current security context means you want the linked server request to use the Windows account of the login to connect to the linked server. In order for this option to work, your SQL Server machine will need to be able to impersonate a local account. This option is a simple way to identify that all Windows accounts can use a linked server, without mapping each login. However, remember this requires delegation to be set up. The last option Be made with this security context is a way to say everyone using this linked server will connect with a single remote login and password to the linked server. The remote login needs to be a SQL Server Authenticated login.
When setting up a linked server the last thing to consider is defining the Server Options. This can be done by clicking on the Server Options under the Select a page menu. When I do that, the following screen will be displayed:
On this screen, there are a number of different options. The first option, Collation Compatible, is used to identify whether or not the linked server has the same collation as the local server. You should only set this to True if you know the local collation is the same as the linked server. The next option Data Access is used to control whether you want to allow data to be accessed on the linked server. When this option is set to True, the linked server can be used to access data on the remote SQL Server instance. When this option is set to False then access to the remote server will be denied. This option is a useful way of disabling a linked server temporarily. The next option Rpc is used to allow remote procedures calls from the linked server. Whereas, the option after that Rpc Out is used to allow remote procedure calls to the linked server. The Use Remote Collation option when set to True means that the collation setting of remote columns will be used, but when this option is set to False the collation settings for the local server will be used. The Collation Name option is to specify the collation setting of the linked server. When specifying a collation name it must be a collation that SQL Server supports. The Connection Timeout is used to specify the maximum length of time the local server should wait to obtain a connection to the linked server SQL Server instance. If 0 (zero) is specified for this option then the server option remote login timeout is used. By default the server option default is 20 seconds for the remote login timeout. The Query Timeout option is used to specify the length of time a linked server process will be allowed to run before it times out. When this option is set to 0 (zero) then the server remote query timeout is used. The remote query timeout value defaults to 600 (10 minutes).
On my SERVER2 linked server, the only option I need to change is Rcp Out. I need to change this so I can run stored procedures that reside on SERVER2. Therefore, to do this I would have to change the Rcp Out option to true like so:
Once you have specified a linked server, the security associated with the new linked server, and the server options you are ready to save your new linked server definition. This is done by clicking on the OK button at the bottom of the New Linked Server window.
TSQL Examples for Using Linked Servers
Above I defined a linked server named SERVER2. As stated earlier, in order to reference objects on SERVER2 I would need to use a four part naming convention. Below are some examples of how to referencing objects on SERVER2.
Here is how I would retrieve information in the Product table in the AdventureWorks databases stored on my linked server:
SELECT * FROM SERVER2.AdventureWorks.Production.Product
All you have to do here is put the linked server name followed by a period before the fully qualified table name.
If you wanted to execute a stored procedure on a linked server, you would do something like the following:
EXECUTE SERVER2.AdventureWorks.dbo.uspGetBillofMaterials 718,'2000-06-26'
Here I have executed the uspGetBillofMaterials stored procedure on SERVER2.
Linked Servers allow you to submit a TSQL statement on one SQL Server instance, which retrieves data from a different SQL Server instances. In fact, linked server can be used to join data from multiple SQL Server instances using a single TSQL statement. When you have databases on multiple SQL Server instances, you might find it useful to use linked servers in your application to retrieve data from more than one instance. By using a linked server your application will only need to connect to one SQL Server instance to retrieve data from multiple SQL Server instances. On that single SQL Server instance, you would define linked servers so your application could retrieve data from the databases that reside on a different SQL Server instance. Next time you are considering how to handle retrieving data from multiple instances of SQL Server from a single connection or single TSQL statement you might consider looking into using a linked server.