As we know blocking occurs when one SPID holds a lock on a specific resource and a second SPID attempts to acquire a conflicting lock type on the same resource. Typically, the time frame for which the first SPID locks the resource is very small. 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 may happen many times throughout the course of a day with no noticeable effect on system performance.
By using the read-committed snapshot isolation level introduced in Microsoft SQL Server 2005, we can reduce blocking and deadlocking issues caused by lock contention.Every time a row is modified by a specific transaction, the instance of the Database Engine stores a version of the previously committed image of the row in tempdb. 

As per BOL, The read-committed snapshot isolation level provides the following benefits:

  • SELECT statements do not lock data during a read operation. Read transactions do not block write transactions and vice versa.
  • Since SELECT statements do not acquire locks in most cases, the number of locks required by a transaction is reduced, which reduces the system overhead required to manage locks.
  • The possibility of blocking is significantly reduced.
  • SELECT statements can access the last committed value of the row while other transactions are updating the row without getting blocked.
  • The number of blocks and deadlocks is reduced.
  • Fewer lock escalations occur.

A quick and easy way to remember how Read-Committed Snapshot Isolation works:

  • Readers DO NOT block Writers
  • Writers DO NOT block Readers
  • Writers DO block Writers

REMEMBER: TempDB is a critical resource for the performance of Read-Committed Snapshot Isolation. RCSI puts a copy of the records that are being modified in TempDB before the transaction is committed so you need to make sure that TempDB has appropriate space and is placed on fast drives that ideally are isolated away from your user database data and log files.

The default Syntax is as

SET TRANSACTION ISOLATION LEVEL
{ READ COMMITTED | READ UNCOMMITTED | REPEATABLE READ | SERIALIZABLE }

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