Types and scope of locks in Derby systems

There are several types of locks available in Derby systems, including exclusive, shared, and update locks.

• Exclusive locks
  When a statement modifies data, its transaction holds an exclusive lock on data that prevents other transactions from accessing the data.
• Shared locks
  When a statement reads data without making any modifications, its transaction obtains a shared lock on the data.
• Update locks
  When a user-defined update cursor (created with the FOR UPDATE clause or by using concurrency mode ResultSet.CONCUR_UPDATABLE) reads data, its transaction obtains an update lock on the data.
• Lock compatibility
  The following table shows the compatibility between lock types. "Yes" means that the lock types are compatible, while "No" means that they are incompatible.
• Scope of locks
  The amount of data locked by a statement can vary.
• Notes on locking
  In addition to the locks already described, foreign key lookups require briefly held shared locks on the referenced table (row or table, depending on the configuration).