Blocking is a common issues noticed in any of any relational database management system (RDBMS) which uses lock-based concurrency. In SQL Server, blocking generally occurs when one SPID holds a lock on a specific resource and a another SPID attempts to acquire a conflicting lock type on the same resource. Generally, the time frame which the first SPID holds locks on the resource is very small, so when it releases the lock, the second connection is free to acquire its own lock on the resource and continue processing. This is normal behavior and happens many a times throughout with no noticeable effect on system performance.
It is common for DBAs to notice blocking on various servers and typically the locks held for a short time does not cause any issues, nor is identified by any monitoring tool. Sometimes, blocking can get severe and cause performance issues from timeouts to slowness, etc. DBAs generally deal with blocking issues by checking on the head or lead blocker causing the blocking and try to understand why the head blocker is taking long time to complete and it is not very important, then DBAs kill the SPID. If the blocking issue is persistent then DBAs identify the culprit and tune it by adding indexes or rewriting the query.
However, sometimes there can be strange blocking issues, one of such issue is blocking caused by a SPID which is in sleeping state. In general, a SPID which finishes its task will leave any locks held, so the sleeping sessions are not expected to cause any blocking issues, however in some environments, persistent blocking issues are noticed due to SPID which is in sleeping state still holds the locks and does not release it until the connection is closed from the application of till the SPID is killed by the DBAs. If we try to understand why the blocking occurs with a session in sleeping state, there could be different reasons and some of the are mentioned below.
– Mostly this issue happens when someone uses SQL Server Management Studio(SSMS) query analyzer window, runs some bad queries and leave the session open. The queries which were run either have a explicit open transaction or an implicit transaction which holds locks and does not release the locks because no commit or rollback was issued. We need to identify such sessions, the user who is running queries from that session, what queries are run, what locks are held, from which application are the session from, hostname, etc and then DBAs can go ahead and kill the sessions. Then DBAs need to contact the respective user or management and report the issue to avoid happening in future.
– This can also happen when an application opens a connection with SQL Server, runs some query and then due to some issues, the application connection is lost either due to application crash or hang or some application problems. No signal is sent to SQL Server due to sudden crash, so SQL Server still assumes that the application may still send additional queries and keeps the session available. This is due to bad application design and needs to fix the application code.
– This can also happen if application has bad code, which opens transactions and fails to commit or rollback in case of query failures or other issues.
– Applications also uses session pooling, where is sessions are reused when required. Periodically applications need to close sessions which have not been reused after sometime.
– We can also suggest developer to use SET XACT_ABORT ON for the connection, or in any stored procedures which begin transactions and are not cleaning up following an error. In the event of a run-time error, this setting will abort any open transactions and return control to the client. It is important to note that T-SQL statements following the statement which caused the error will not be executed.
– Some older or incompatible client components may cause these kind of issues too, so need to keep them updated and use latest and compatible connectivity components.
Hope this was helpful.
This is applicable for below versions of SQL Server
SQL Server 2005
SQL Server 2008 R2
SQL Server 2012
SQL Server 2014
SQL Server 2016
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