Concurrency

Concurrency

• When multiple threads access the same resources, care must be taken to ensure that data is accessed in a consistent manner.
  • Example: Bank account transfer, academic record
• Atomic operations execute without interference from other operations.
  • Is a += 1 an atomic operation?
  • See java.util.concurrent.atomic package.
  • Should a transfer or a GPA update be atomic?
• The Java keyword synchronized ensures that only 1 thread may access an object at any time.
  • Bank account: getBalance, setBalance, deposit, withdraw
  • Is this too restrictive?

Conflicts

• Operations are read or write.
• Conflicts occur if the order in which operations are executed impacts the result.
• Write/write and read/write are conflicting operations.
• Read/read is OK.

Locking

• Retrieve a lock on a resource before accessing it.
• Rules:
  • if requested lock is read and (no lock held or read lock held): grant read lock
  • if requested lock is read and write lock held: wait
  • if requested lock is write and no lock held: grant write lock
  • if requested lock is write and (read lock held or write lock held): wait
• Java's wait/notify enable threads to communicate. wait blocks waiting for a lock. notify releases a lock and notifies all those waiting for the lock.

Models for XML DB Thread Safety

• Synchronize all - the most restrictive model that does not allow concurrent reads.
• Copy update - make changes to a copy of the DB and replace old with an (atomic) assignment. Keeps two copies of DB in memory during update.
• Locking