Oracle Row Locks and Row Level Locking

 How does row level locking work in Oracle?
Ans:  When a transaction begins it opens an exclusive lock on a resource.  The name of the lock depends on the transaction ID for the freshly started transaction.  This resource then, if requested by another transaction, will serve as a wait point until the transaction holding the lock performs either a commit or a rollback. 

For example, assume we have two transactions, each requesting the same resource.  The first transaction accesses the resource and generates a lock.  The second transaction will detect that the resource it is trying to access is locked by the first transaction, and would then request a lock in exclusive mode, on the resource.  When the first transaction commits or performs a rollback, it will release the exclusive lock on the resource.  When the lock is released, the second transaction gains an exclusive lock and proceeds.

This is a step by step look at the row locking mechanism:

1. Transaction begins either implicitly via any DML, or explicitly via  set transaction.
2. A slot is allocated in the rollback segment header.  This slot is allocated either via a round robin selection process, or specified by the set transaction use rollback segment ...  statement.
3. Blocks to be changed are identifired using the execution plan, utilizing whatever access paths are specified there.
4. The transaction looks for an empty slot on the Interested Transaction List (ITL).  If there are none available, one will be allocated from free space in the block if it is available.  If that fails, the transaction will wait on a TX enqueue in mode 4.  When the ITL slot has been allocated, it will point to the rollback segment header that was previously reserved by the transaction.
5. When the ITL slot is allocated in the block, specific rows are marked as locked in the row directory.  The lock byte will be set to point to the ITL slot held by the transaction.  This is the mechanism that acutally maintains the row-level locks in Oracle.  Once the lock byte is set, the row can be operated on.  If changes to the block are made, the before images are recorded in the rollback segment reserved by the transaction.
6. When a commit or rollback is performed, the lock is released.  When a commit occurs, in order to release the lock, the rollback segment header slot must be marked as committed.  Cleanup of the block itself is usually delayed until a later time.