Concurrent Access Problems

1
Concurrent Access Problems

• On Transaction Processing

2
Concurrent Access Problems

• 3 Problems
• Lost Updates
• Uncommitted Data
• Inconsistent Retrieval

3
Lost Updates

• Assume
• Transaction 1
• Update Inventory
• set PROD_QOHPROD_QOH100
• Transaction 2
• Update Inventory
• set PROD_QOHPROD_QOH-30

4
Lost Updates

• Transaction 1 3 processes
• Read PROD_QOH
• PROD_QOHPROD_QOH100
• WRITE PROD_QOH

5
Lost Updates
Table 9.2
Table 9.3
6
Uncommitted Data

• Assume
• Transaction 1
• Update Inventory
• set PROD_QOHPROD_QOH100
• Rollback
• Transaction 2
• Update Inventory
• set PROD_QOHPROD_QOH-30

7
Uncommitted Data
Table 9.4
Table 9.5
8
Inconsistent Retrievals
Table 9.6
Table 9.7