Concurrent Access

Last updated May 26, 2023 Edit

• Compsci
• Theory

• Concurrent access is concerned with how we deal with multiple changes to the database happening at the same time.
  • This usually hapens because multiple client processes are making changes.
    • Amazon order processing. Multiple clients are placing orders at the same time. These orders need to update the same piece of information (such as current stock level).

• Each record that needs to be updated is locked by the updating process.

  • A lock means that only that process can change the locked process

• Lock all records needed for the transaction before any changes are made.

  • Downside: you may need to lock hundreds of records before you can proceed

• Lock each record as it is updated but always process records in the same order.

• Disadvantages

  • Deadlocks
    • Process A locks record 1
    • Process B locks record 2
    • Process A tried to lock record 2 but cannot because it is locked by Process B. Process B tried to lock record 1 but cannot because it is locked by Process A. Both processes wait for the record to become unlocked, causing the program to hang.
      • If a process crashes, then when should records be unlocked? Will they be unlocked?

• Each record in the system has a timestamp that indicates the last time it was updated.
• All changes are similarly timestamped to show when they were issued.
• Rules to determine if the transaction can proceed:
  • If a change arrives that has a timestamp before the current record timestamp, it is discarded.
  • If a read is requested that has a timestamp before the current record timestamp, it is discarded.
• Disadvantages
  • Important changes might be lost
  • Clocks must be effectively synchronised

• Only allow the DBMS to process one transaction at a time.
• Each transaction is processed in the order they arrive.
• Disadvantages
  • Performance
    • If lots of transactions occur in a short time period then a long queue can build up and it may be some time before the changes are made.

• A minor tweak to serialisation.
• Each update is given a priority.
  • Stock updates should happen before stock queries for example.
• The system can also be made aware of invalid sequences of updates and programmed to avoid these.
  • For example, a stock check might be given a high priority. Other stock updates that have a lower priority but that happened before the stock stock check may exist and be blocked.
• Disadvantages
  • Performance

Databases