Tree Based Protocol 3

• Only exclusive locks are allowed
• The first lock by Ti may be on any data item. Subsequently a data item Q can be locked by Ti only if the parent of Q is currently locked by Ti
• Data items may be unlocked at any time.
• A data item that has been locked and unlocked by Ti cannot subsequently be relocked by Ti
• The tree protocol ensures conflict serializability as well as freedom from deadlock( does not ensure recoverability).
• Unlocking may occur earlier in the tree locking protocol than in the two phase locking protocol

+ (plus) points

1. Shorter waiting times, and increases in concurrency.
2. Protocol is deadlock free, no rollbacks are required.
3. Freedom from deadlocks

Drawbacks

1. Protocol does not guarantee recoverability or cascade freedom ( need to introduce commit dependiencies to ensure recoverability)


Tree Based Protocol

2. In tree-locking protocol, a transaction that needs to access data item A and J in the database graph, must lock not only A and J, but also data items B,D,H ,this additional locking results in increased locking overhead and the possibility of additional waiting time and potential decrease in concurrency.