Friday, January 09, 2009

Troubleshooting SqlCacheDependency in SQL Server 2008 and SQL Server 2005

"Getting immediate notification from SQL Server when data changes" is a very attractive feature, but unfortunately it's not easy to implement.
(It took me full day to identify and fix all issues...).

SQL Server Query Notification framework is quite fragile and may not work for multiple reasons.
If you get error messages -- consider youself lucky. Sometimes there will be no error messages, but notifications simply would refuse to work.

There are two major steps in troubleshooting SqlCacheDependency notifications:
Step 1: Make SqlCacheDependency clean up ASP.NET Cache item.
Step 2: Prevent SqlCacheDependency from cleaning up ASP.NET Cache item when it's inapropriate.

Both steps are hard, but Step 1 is the hardest.

I strongly recommend iterative approach: implement the easiest possible solution first, and then make it more advanced one small step at a time. Test every little step.

Business context

In this example I use SqlCacheDependency in order to get list of blocked IP addresses on my web site
From time to time I delete bad users and write their IP addresses into BlackListIpAddress table.

I can retrieve the list of recently blocked IP addresses like that:

CREATE Procedure spGetBlockedIpList
@cutDate datetime

set nocount on
select IpAddress
from BlackListIpAddress with (nolock)
where (DecisionDate > @cutDate)
group by IpAddress
having count(1) > 1

When anybody opens web page -- I check if current web page request came from that list of blocked IP addresses.
I created C# function that does that check:
public static bool IsBlackListed(string ipAddress)
bool cached;
if (GetBlockedIpAddresses(out cached).Contains(ipAddress)) return true;
return false;

Because I run IsBlackListed() on every page, I don't want to run spGetBlockedIpList without need.
So, I keep database results in ASP.NET Cache object and use SqlCacheDependency to clean up Cache object as soon as new IP address is blacklisted in BlackListIpAddress table.


public static List GetBlockedIpAddresses(out bool cached)
HttpContext context = HttpContext.Current;
List blockedIPs = (List)context.Cache["BlockedIPAddresses"];
if (blockedIPs == null)
SqlCommand cmdDependency = new SqlCommand(@"select IpAddress from dbo.BlackListIpAddress where DecisionDate > @cutDate",
SqlUtilities.AddInputParameter(cmdDependency, "@cutDate", DateTime.UtcNow.AddMinutes(-1), SqlDbType.DateTime);
SqlCacheDependency dependency = new SqlCacheDependency(cmdDependency);
SqlUtilities.ExecuteNonQuery(cmdDependency, "PostJobFreeConnectionString");
blockedIPs = LoadBlockedIPsFromDatabase();
// Cache retrieved blockedIPs in ASP.NET Cache object:
context.Cache.Insert("BlockedIPAddresses", blockedIPs, dependency);
cached = false;
cached = true;
return blockedIPs;

Note, that almost always blockedIPs will be retrieved from ASP.NET Cache.
But if Cache["BlockedIPAddresses"] is empty -- I execute two SQL queries instead of one query.

I run simple query "select IpAddress from dbo.BlackListIpAddress where DecisionDate > @cutDate" in order to make SqlCacheDependency work.
I then run more complex query LoadBlockedIPsFromDatabase() (it runs spGetBlockedIpList) in order to get data I need.
spGetBlockedIpList is too complex to work with SqlCacheDependency.

Simple query is not smart enough to give me the data I need.

When you debug your own code -- dump more complex query and use only simple one.
Remember -- first step is to make SqlCacheDependency clean up ASP.NET Cache item.
If SqlCacheDependency cleans up your ASP.NET Cache -- you are about 70% done.

You may even start with even simpler SQL query. For example: "select IpAddress from dbo.BlackListIpAddress". You would polish it later.


1) Make sure that when your Web Application start, you run SqlDependency.Start().
I do it this way:
public sealed class DenyIpAddressModule : IHttpModule
void IHttpModule.Init(HttpApplication application)
string connectionString = WebConfigurationManager.
If you forget to do that, you would get "When using SqlDependency without providing an options value, SqlDependency.Start() must be called prior to execution of a command added to the SqlDependency instance." error message.

2) Enable Service Brocker on your database.
I do it like this in SQL Server Management Studio (SSMS):
use PostJobFree;
alter database PostJobFree set ENABLE_BROKER;

The trick here is to kill all existing connections prior to altering your database.
Use these SQL commands in SSMS:
kill 52 -- or whatever SPID is

You may check your Service Broker setting like this:
use PostJobFree;
select is_broker_enabled from sys.databases where database_id=db_id()

1 - Enabled; 0 - Disabled (default).

3) Make sure that permissions in your database are not out of whack. SQL Server 2005 and SQL Server 2008 have unpleasant bug that [almost] silently kills Queue Notifications:
"You cannot run a statement or a module that includes the EXECUTE AS clause after you restore a database in SQL Server 2005"

I fixed it by running this command:
use PostJobFree
sp_changedbowner [MyServerName\dennis]

You may check current database settings by this command:
sp_helpdb PostJobFree


1) If you are still unable to make SqlCacheDependency to invalidate your ASP.NET Cache, I recommend you great article "Using and Monitoring SQL 2005 Query Notification"
Sanchan explains how to use SQL Profiler to see what's going on with query notifications.
2) Using Profiler helped me to find the following errors in SQL Profiler:
- An exception occurred while enqueueing a message in the target queue. Error: 33009, State: 2. The database owner SID recorded in the master database differs from the database owner SID recorded in database 'PostJobFree'. You should correct this situation by resetting the owner of database 'PostJobFree' using the ALTER AUTHORIZATION statement.
- Cannot execute as the database principal because the principal "dbo" does not exist, this type of principal cannot be impersonated, or you do not have permission.
That gave me an idea to run:
sp_changedbowner [MyServerName\dennis]

3) These couple of queries would let you take a look at what active Query Notification Subscriptions you currently have:
select * from sys.dm_qn_subscriptions
select * from sys.transmission_queue

4) I didn't need that step, but while troubleshooting I did it anyway.
Grant these permissions to the user account that runs your web application (it's usually either "aspnet" or "NT AUTHORITY\NETWORK SERVICE").
use PostJobFree
GRANT CREATE QUEUE TO [MyServerName\aspnet]
GRANT SELECT ON OBJECT::dbo.BlackListIpAddress TO [MyServerName\aspnet]
GRANT SELECT ON OBJECT::dbo.T TO [MyServerName\aspnet]
GRANT RECEIVE ON QueryNotificationErrorsQueue TO [MyServerName\aspnet]

Clean up Cache only when needed

I assume that at this point you are done with the hardest part ("Make SqlCacheDependency object clean up Cache").
There is still some work ahead.
For example I noticed that my SqlCacheDependency code when I was playing with it -- always invalidated ASP.NET Cache. It didn't matter if I updated underlying BlackListIpAddress table or not.
By using trial & error approach I found that the problem was caused by using inapropriate version of SQL query.
I found that:
- "group by" doesn't work no matter what.
- "top 10" doesn't work.
- "with (nolock)" hint doesn't work.
- passing @cutDate parameter to the query _does_ work.

See documentation on SELECT statements that are supported by Query Notification:

Other useful resources

1) More tips about SQL Server Query Notification:

2) Troubleshooting Query Notifications

3) If you are lucky and expect everything to go smooth - use this articles:
SqlCacheDependency using ASP.NET 2.0 and SQL Server 2005
SQL 2005 and SQL2008 Enabling Notifications. SQL Chache Dependancy Part-I

ASP.NET 2.0, ASP.NET 3.5, C#


About Me

My photo
Email me: