Oracle Parallel Processing

Lock Modes

An OPS database may have various types of users. Some users (we’ll call them readers) may be reading the data, and other users (we’ll call them writers) may be modifying the data. Depending on the type of access required, instance locks are acquired in one of the following modes. Note that the null mode has the lowest level of access rights and that the exclusive mode has the highest level of access rights.

Null mode (NULL)

Locks generally are created in null mode and then converted to other modes as required. A lock in null mode indicates no access rights. Multiple instances can hold a lock in null mode.

Subshared (SS)

An SS lock allows an unprotected read on an object. When an instance is holding an SS lock on an object, it is allowed to read that object. However, other instances may be able to read as well as modify the object. Multiple instances can hold a lock in SS mode.

Shared exclusive (SX)

An SX lock allows an unprotected write on an object. When an instance is holding an SX lock on an object, it is allowed to modify (write) that object. However, other instances also may be able to read and modify (write) the same object. Multiple instances can hold a lock in SX mode.

Shared mode (S)

Shared locks are needed for read operations. Multiple instances can own an instance lock in shared mode and simultaneously can read the associated block(s). Writes are not allowed on a block when it is locked in shared mode.

Subshared exclusive (SSX)

When an instance is holding an SSX lock ...