It seems like I am going to write another none SQL stuff again. No. This is a real life scenario. I am writing an ETL for my customer to load data from a Sybase server running on Unix system. The database I am accessing is a standby log-shipped database which is restored 5 minutes. The user account the ETL is using has very limited right on Sybase Server. The Sybase DBA does not have SQL Server driver on the Unix system. This means that I can’t put log restore as part of my ETL and Sybase DBA does not have connectivity to SQL Server side. We need to coordinate those 2 processes that when one is running the other must not run.
Locking and Blocking
Update with UPDLOCK
Do you think it’s a good idea to add UPDLOCK hint to the table which will be modified in an update query? Many people will say NO because it seems to be redundant. But the fact is that sometime it removes deadlocks in your application.
Locking Behavior – Foreign Keys
With Snapshot (read committed snapshot and snapshot isolation level) enabled, when data reading on a table takes place, if any records are being modified at the same time, the data will be read from the rows in version store rather than wait until data modification complete. Such behavior lets prgrammers think that reader process will NEVER ever be blocked by writer process. In most of the scenario, this is correct, however, when reader process is indirectly issued and running against a table, the isolation level for that reader process is always read committed lock isolation regardless the transacitoin isolation level of the parent statement.
Sharing Global Temp Tables
Global temp tables can be accessed by one or more sessions. This important feature of SQL Server becomes more important than it was in my programming live since more applications I worked with and am currently working on have parallel processing involved heavily. Very often, amount of data generated from one sessioin are shared to many concurrent sessions. Global temp tables come and play. Creating a global temp table is simple.
create table ##temp(i int)
It will be released when
- It’s explicitly removed by any sessions
- The creator session closed and there is no other sessions referencing this session.
sp_rename Causes Lock Leaking
One of my friends finds this issue in the systems he is managing. In my locking and blocking series, I mentioned that when a record is being modified it always acquires IX lock on the object, IX lock on the page, and X lock on the record. In rare case, this may also hold IX lock on another object. This is cased by sp_rename.
Locking and Blocking (12) – Counting Semaphore
Counting semaphores restricts number of concurrent sessions from accessing the same resource. It’s useful when code blocks perform well when number of concurrent users is under a certain number. For instance, SQL Server merge replication implements semaphores to limit number of subscribers to access publication at the same time.
Locking and Blocking (11) – Process Control and Customized Locking Hierarchies
Building lock based process control is to avoid conflicting processes from running at same time. A very common question DBAs undergo is that Task A can be launched when Task B(s) are not running. Many times I heard that “I will have to disable jobs, wait until they are finished, start another job, wait until it finishes, and then enable the jobs were disabled previously.”. The cycle of this process some time is just very few minutes but mostly it’s half to few hours which is occasionally scheduled in the midnight. The work like this is no longer to be fun to DBAs. It would be nice if something can be implemented in the process which it can detect, wait, and/or safely stop or interrupt running business operations, start its own process after all business processes stopped, and then resume the business processes automatically…This is possible in SQL Server. SQL Server application locks can even bring more value for such matters.
Locking and Blocking (10) – Binary Semaphore
Semaphore is an abstract for controlling access by multiple threads (sessions) to a common resource. Binary semaphore, it can be called Mutex, or Lock/Unlock, allows only one thread access a resource at a time. Back old days, in C++ programming, critical section object is used for this purpose. In C#, keyword lock(obj){code block} and Monitor class implements binary semaphores. Once a object is locked by one thread, other threads who need to acquire the lock on the object will have to wait until the lock on the object is released by thread previously held. SQL Server is an multi-threading application. It uses semaphores internally to control the access to the record, for instance, while you are modify a record, an exclusive lock is placed on the record and released when modification finishes. This is a binary semaphore. In T-SQL, you can implement binary semaphore by locking row(s) and application locks.
Locking and Blocking (9) – Live Locks and Deadlocks
Deadlock can happen at latch level, memory, MARS, parallel query, etc. In this post, I am only going to talk about deadlocks at lock level. Resource A and B, when Session 1 locks Resource A and tries to get a lock on Resource B which has a incompatible lock held by Session2, Session 1 will wait until Session 2 releases locks on Resource B. This is called live lock. A live lock will be eventually resolved when the blocking session releases the lock or requesting lock gets timeout. If Session 2 requests a incompatible lock on Resource A at this moment, deadlock happens, because Session 1 will be waiting for the released lock on Resource B and Session 2 will be waiting for the released lock on resource A. Both sessions are deadly locked and blocked. I am going to give you few deadlock scenarios and solutions below.
Locking and Blocking (8) – Lock Hints
Lock hints are used to customize the locking behavor of SQL Server from 3 main perspectives, granularity, mode, and duration. Available hints are HoldLock, NoLock, NoWait, PagLock, ReadCommtted, ReadCommittedLock, ReadPast, ReadUncommitted, RepeatableRead, RowLock, Serializable, TabLock, TabLockX, UpdLock, and XLock. A full understanding of them is important before using them. Let’s assume you are running your … Read more