Transaction Priorities

Transaction Priorities

• Transactions are granted access to the processors based on their priorities.
  
• All the transactions in the real time DB are associated with a deadline, hence Earliest Deadline First (EDF) algorithm finds a greater solution to schedule those transactions. But EDF algorithm will be working efficiently if the number of transactions are moderate. But usually the DB transactions are huge in a systems and EDF may not be suitable which leads to congestion.
  
• To avoid the above solution an algorithm Adaptive Earliest Deadline (AED) algorithm which uses congestion control and also can process huge number of transactions. It uses two groups HIT and MISS group.
  
• All the transactions in the HIT group will be meeting their deadlines, if by chance any transaction does not meets its deadline, then that transaction will be aborted.
  

Operation of AED

• When a transaction arrives, the system randomly inserts it in a list of pending transactions, if the assigned transaction is in the list 1…..H (H is any parameter), then it will be put in the HIT list else MISS group.
  
• The objective is to try to meet all the deadlines of the HIT group and if any time left over, then the MISS group transaction may be executed.
  
• Transactions within the HIT group are scheduled using EDF. If a transaction is completed, it will be removed from the list.
  
• The transaction from one Group won't move to other groups.
  
• The H value can be set adaptively according to the following algorithms
  

  
   

Success (HIT) = the ratio of Number of Transactions in HIT class that meet their deadlines to Total number of transactions in the HIT class.

Success (ALL) = the ratio of Number of Transactions that meet their deadlines to Total number of transactions.

The above parameters are measured repeatedly.