How do I prevent linked server errors?
Many of us use systems where multiple SQL Servers need to talk to each other over the network. But nobody's network is perfect. Sometimes the linked server is down, sometimes the link between them is down, sometimes another user changed the login credentials. Whatever the case, you need to be prepared in the event that the server is not available.
In SQL Server 2000, you will see this:
In SQL Server 2005, you get a much more useful error message:
Most of these errors could be due to a misconfigured linked server, but they could also be due to network or hardware issues. Of course, these errors abort the entire batch, so whether or not the rest of your code requires the remote server to be active, it still terminates. In SQL Server 2000, you cannot trap for this type of error using @@ERROR, ROLLBACK, etc.
There are some workarounds, thankfully.
PING: the knee-jerk reaction
A very common suggestion to avoid this pitfall is to use PING from a command line; for example:
There are several problems with this approach:
A Better Approach
A more reliable way to determine if a SQL Server is up is to actually try to connect to it. As opposed to calling it directly, you can capture any errors by shelling out to osql instead of querying the server directly (e.g. using four-part naming). Note however that this will take 10 seconds to timeout if the server cannot be reached.
In SQL Server 2005, first, you have to enable xp_cmdshell, otherwise you will see this error:
To enable xp_cmdshell, run the following code:
A little more elaborate sample for SQL Server 2000 or SQL Server 2005 uses SQL-DMO. See usp_serverup.
In SQL Server 2005, you have some new options. You can use the new system procedure sp_testlinkedserver within the new TRY/CATCH construct:
This will still raise warning messages, but it will at least let you get out of the batch on your own terms.
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