Ti Write then Tj Read. Ti Write then Tj Write. All tentative changes are made permanent. Tentaive versions of each data item are committed in transaction timestamp order. Transactions may wait for earlier transactions to complete their writes.
Tentative version becomes data item, and timestamp becomes timestamp of data item.
Used in file servers and SDD-1 database system. If cycle detected, coordinator kills off a process to break deadlock. With two phase locks, cannot release and then obtain more data items, so this cannot occur. Only situation is if a transaction is aborted while in deadlock. I just received a message with timestamp T which leads to deadlock.