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Avoiding Blocking Issues in ASP.NET Session State Databases
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by Greg Low
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Introduction

As soon as ASP.NET systems grow to the point that more than a single web server needs to be deployed, decisions need to be taken about where the session state will be held. By default, ASP.NET session state lives in the memory of the server hosting the application. The problem with this is that a user who connects to one of our web servers will not have the same session if their next request is sent to another one of our web servers, yet this is exactly what we want to achieve to improve the robustness of our systems.

There are a number of hardware-based solutions that involve concepts like "sticky sessions" where the hardware that performs the routing of requests from users remembers which server the user last spoke to and directs all requests from the same user to that same server. The downside of this is that if the server in question stops working, the user's session will also be lost.

To avoid this, it is common to move the session state off the individual web servers. Figure 1 shows one option. It involves setting up a single server as the holder of all session state. This can be done by implementing an operating system service that communicates with the web servers and which holds the session state from all the web servers in its own memory.

Figure 1: Session State Service

The downside of such a service is that it will not survive a reboot of the server holding the state; it is limited to the memory of the server and it is a single point of failure for the entire web site.

To make the session state storage more persistent, a SQL Server database can be introduced. It would address the first two issues but it is only when we then also apply fault tolerance to the database server, that we are able to address all three issues. The most common way to do this is to introduce a SQL Server cluster for storing the session state as shown in Figure 2.

Figure 2: Clustered Session State Database

This is precisely what is provided by the ASP.NET session state database. While there is some performance impact of having to retrieve the session state from another server and more particularly from a database rather than from memory, the tradeoff is usually considered worthwhile, given the robustness of the solution.

In general I have found this works quite well. However, recently, I have been doing some consulting and performance tuning work at a busy site that was having difficulties specifically with the performance of their session state database. At this site, session state was quite large, often between 500kb and 3Mb in size. This had been working acceptably until they started to have over around 4000 concurrent sessions.

Periodically, commands that were accessing the session state database were experiencing timeouts. My first step was to use a SQL Profiler trace to find the types of commands that were timing out. This showed that the commands most commonly taking a long time were UPDATE commands. When I tested them individually, each UPDATE statement took less than a second. The command timeouts were set at the default value of thirty seconds. This then means only one of two things. Either the system is so buried in work that it just can't get to do the work in time (and that usually means disk performance) or users were being blocked by other users.

Conceptually, this troubled me as a session state database is unlike most other databases when you consider concurrency. It would be very uncommon for more than one connection to be accessing the same row in the session state database at the same time i.e., only one web server would be reading or writing a given session row at any point in time. SQL Server has implemented row level locking since version 7, and so a user updating one row should not normally be blocking another user as they would not be working with the same row.

As this was a SQL Server 2000 server, my first stop to locate the blocking was the Current Activity window in the Management node of SQL Server Enterprise Manager, as shown in figure 3. (In this example, I have created a small number of blocked processes to make the view clear.)

Figure 3: Locks per Process ID in Enterprise Manager

In this example "spid 55" is blocking "spid 56" and "spid 57". To find out what "spid 55" is doing, you can click on the Process Info node. This is also under the Current Activity node as shown in Figure 4.

Figure 4: Process Info in Enterprise Manager

 

This then lets us see the command being executed along with details of who is executing it. In the case of the blocking at the user site, the culprit was a seemingly innocuous background task set up as part of the ASP.NET session state database setup script.

Because web users might simply walk away or browse away from your site, there is a need to clean up expired sessions in the database. However, it is possible when reading these rows to ignore any that have expired; your database could quickly become full of them. To avoid this, a background job has been set up to remove expired sessions every five minutes. The job simply executed a procedure called DeleteExpiredSessions, as shown in Listing 1.

Listing 1: Standard DeleteExpiredSessions Code

CREATE PROCEDURE DeleteExpiredSessions
AS
  DECLARE @now DATETIME
  SET @now = GETUTCDATE()
 
  DELETE ASPState..ASPStateTempSessions
  WHERE Expires < @now
 
  RETURN 0
GO

While this seems very tame, if the session state rows are large and the number of sessions is high, this command can cause substantial blocking of other commands. This is truly ironic because there is no need to ever lock more than a single row in this table while removing expired rows. SQL Server on the other hand, needs to make sure that a single DELETE statement either updates all the target rows or none at all. So, it locks them as it goes, in case it needs to undo the entire statement. With most purposes for which a DELETE is used, this makes sense, but for removing expired rows, this introduces unnecessary blocking. By replacing the statement above with the code in Listing 2, the unnecessary blocking is completely removed.

Listing 2: Replacement Expired Session Deletion Code

CREATE PROCEDURE dbo.DeleteExpiredSessions
AS
  DECLARE @now datetime
  SET @now = GETUTCDATE()
 
  CREATE TABLE #ExpiredSessions 
  ( SessionID nvarchar(88) NOT NULL 
      PRIMARY KEY
  )
 
  INSERT #ExpiredSessions (SessionID)
  SELECT SessionID
  FROM [ASPState_2_0].dbo.ASPStateTempSessions
  WHERE Expires < @now
 
  DECLARE SessionCursor CURSOR LOCAL FORWARD_ONLY READ_ONLY
  FOR SELECT SessionID FROM #ExpiredSessions ORDER BY CHECKSUM(NEWID())
 
  DECLARE @SessionID nvarchar(88)
 
  OPEN SessionCursor
  FETCH NEXT FROM SessionCursor INTO @SessionID
  WHILE @@FETCH_STATUS = 0 BEGIN
    DELETE FROM [ASPState_2_0].dbo.ASPStateTempSessions
      WHERE SessionID = @SessionID
    FETCH NEXT FROM SessionCursor INTO @SessionID
  END
  CLOSE SessionCursor
  DEALLOCATE SessionCursor
 
  DROP TABLE #ExpiredSessions
  RETURN 0
GO

While I am not a fan of using CURSORs in typical SQL Server code, their use in this situation is ideal. The procedure works by obtaining a list of rows to be deleted and then stepping through the list, deleting them one at a time.

I then found that on the large system, this procedure could run for over three of the five minutes in its schedule. I decided to make the procedure work well if more than one copy of it was running at once. In the code in Figure 6, this is done by accessing the rows in CHECKSUM(NEWID()) order i.e., random order. This way, if more than one copy of the procedure is running at once, it is highly likely that the second copy would be deleting different rows than the first copy. If they both tried to delete the same row, no error would occur. One would manage to delete it and the other would not receive any error anyway.

Conclusion

By keeping two copies of the procedure scheduled to run at five minute intervals, the expired sessions at the site were being quickly cleaned up and the blocking caused by the procedure was completely eliminated.

 

Dr Greg Low

http://msmvps.com/greglow

Greg is a Senior Consultant at Readify in Australia and heads up their SQL Server practice. He is a Microsoft Regional Director and a Microsoft SQL Server MVP.



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