SQL Server locks

Explain the various types of concurrency problem. I.e. Lost or buried updates, uncommitted dependency, inconsistent analysis, phantom read.

Types of concurrency problem:-

Lost or buried updates: - When the same row is selected for updates by two or more transactions and updates the row based on the value originally selected. Here, each transaction is unaware of the other transactions. The last update overwrites updates made by the other transactions, which results in lost data.

Uncommited dependency: - here, a transaction reads data of another transaction which has not been committed yet. The value may be changed by the other transaction.

Inconsistent analysis: - here, the transaction reads the data inconsistently. This means that every time the data is read; different values are read. This is because another transaction is continuously updating the data.

Phantom Read: - Here, an insert or update is done on a row that belongs to some other transaction. Hence the transaction may read a row that may be deleted by some other transaction.

Describe optimistic and pessimistic concurrency.

Optimistic concurrency: - Assumes that a resource is likely to be available at all times. This means that resource locking is very unlikely. If a conflict occurs, the application must read the data and attempt the change again.

Pessimistic concurrency: - this locks the resources as and when required. A transaction can be assured to be completed unless a deadlock occurs.

What are the differences between lost updates and uncommitted dependencies?

In lost update, the data is lost. Here, the last update overwrites updates made by other transactions. On the other hand, in uncommitted dependency data is changed by other transactions. So, a transaction reads data of another transaction which has not been committed yet

Explain the isolation level does SQL Server support.

Isolation levels:-

• READ UNCOMMITTED: - Reads data that has been modified but not committed yet.
• READ UNCOMMITTED: - Cannot Read data that has been modified but not committed yet
• REPEATABLE READ: - cannot read data that has been modified but not yet committed by other transactions and that no other transactions can modify data that has been read by the current transaction until the current transaction completes
• SNAPSHOT: - Data read by any statement in a transaction will be a consistent version of the data that existed at the start of the transaction. Here, the transaction can only identify the data modification that was committed before the start of the transaction.
• SERIALIZABLE:- Cannot Read data that has been modified but not committed yet, until the current transaction completes, no other transaction can modify the data and no other transactions will insert new rows whose keys falls in the range of current transaction

What guidelines should be followed to help minimize deadlocks?

Guidelines to minimize deadlocks:-

• Avoid user interaction in the transactions. The transaction must not rely on any inputs from the user.
• The concurrent transactions must access data in the same order. There should be consistency in which the operations occur
• Transactions must be short and simple to avoid deadlocks. Long transactions may block other necessary activities
• A lower isolation level like read committed must be used. Using lower isolation levels reduces the possibility of holding shared locks for a longer time
• Bound connections should be used. Here, two or more connections opened by the same application can assist each other