The Memory Structures Layer
A transaction lock is acquired when a transaction initiates its first change and is held until the transaction does a COMMIT or ROLLBACK. It is used mainly as a queuing mechanism so that other sessions can wait for the transaction to complete.
The test mapped to this layer reports the following:
the number and duration of transaction locks held by each instance of the cluster;
the health of the memory and redo buffer structures;
the root blocker processes etc
Figure 1 : The tests mapped to the Memory Structures layer