Browse Prior Art Database

Method for Reading Uncommitted Changes of Other Users

IP.com Disclosure Number: IPCOM000100636D
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 2 page(s) / 76K

Publishing Venue

IBM

Related People

Hoffman, RD: AUTHOR [+2]

Abstract

Disclosed is a way to allow users access to the uncommitted changes of other users, without unduly affecting the integrity of transactions.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method for Reading Uncommitted Changes of Other Users

       Disclosed is a way to allow users access to the
uncommitted changes of other users, without unduly affecting the
integrity of transactions.

      UNCOMMITTED READ is a new isolation level which a user may
specify when he or she wishes to read rows from tables irrespective
of any locks which may be held by other users. This kind of access
improves both performance of the accessing application and
concurrency of the overall system, at the expense of exposing the
user to certain integrity risks. Four main problems can occur:
      1.   THE UNDO PROBLEM: Process P1 reads a record, and changes
it. Process P2, operating under UNCOMMITTED READ, reads the record.
P1 then issues a ROLLBACK, which restores the record to its original
state. P2 now has bad data.
      2.   THE CHANGE PROBLEM: P1 reads a record.  P2, operating
under UNCOMMITTED READ, reads the record.  P1 then changes the record
and issues a COMMIT.  P2 now has bad data.
      3.   THE UNEXPECTED ACCESS PROBLEM: P1, operating under
REPEATABLE READ, reads a record.  P2, operating under UNCOMMITTED
READ, reads and changes the record. Now P1 has bad data, even though
REPEATABLE READ guarantees this will not happen.
      4.   THE DROP PROBLEM P2, operating under UNCOMMITTED READ,
opens a cursor on a table. P1 drops the table.  P2 is now in trouble.

      The first two problems affect only the process using
UNCOMMITTED READ and are regarded as the price to be paid. The third
problem, however, must be avoided because it invalidates the
guarantees of other isolation levels, and the fourth problem must be
avoide...