One of the key functions of a database management system (DBMS) is to ensure that multiple users can read and write records in the database without reading inconsistent sets of records due to in-progress changes and without overwriting each other’s changes inadvertently. Oracle and Microsoft® SQL Server™ approach this task with different locking and isolation strategies. You must consider these differences when you convert an application from Oracle to SQL Server or the resulting application may scale poorly to high numbers of users.
Oracle uses a multiversion consistency model for all SQL statements that read data, either explicitly or implicitly. In this model, data readers, by default, neither acquire locks nor wait for other locks to be released before reading rows of data. When a reader requests data that has been changed by other writers, but not yet committed, Oracle re-creates the old data. It uses its rollback segments to reconstruct a snapshot of rows for the reader as of the start of the reader’s statement or transaction.
Data writers in Oracle perform request locks on data that is updated, deleted, or inserted. These locks are held until the end of a transaction, and they prevent other users from overwriting uncommitted changes.
SQL Server, in contrast, uses shared locks to ensure that data readers only see committed data. These readers take and release shared locks as they read data. These shared locks do not affect other readers. A reader waits for a writer to commit the changes before reading a record. A reader holding shared locks also blocks a writer trying to update the same data.
Releasing locks quickly for applications that support high numbers of users is more important in SQL Server than in Oracle. Releasing locks quickly is usually a matter of keeping transactions short. If possible, a transaction should neither span multiple round-trips to the server nor wait for the user to respond. You also need to code your application to fetch data as quickly as possible because unfetched data scans can hold share locks at the server and thus block updaters.