Editing Two-phase commit protocol (section)

====Tree two-phase commit protocol====
The [[Tree (data structure)|Tree]] 2PC protocol<ref name="weikum2001" /> (also called Nested 2PC, or Recursive 2PC) is a common variant of 2PC in a [[computer network]], which better utilizes the underlying communication infrastructure. The participants in a distributed transaction are typically invoked in an order which defines a tree structure, the invocation tree, where the participants are the nodes and the edges are the invocations (communication links). The same tree is commonly utilized to complete the transaction by a 2PC protocol, but also another communication tree can be utilized for this, in principle. In a tree 2PC the coordinator is considered the root ("top") of a communication tree (inverted tree), while the participants are the other nodes. The coordinator can be the node that originated the transaction (invoked recursively (transitively) the other participants), but also another node in the same tree can take the coordinator role instead. 2PC messages from the coordinator are propagated "down" the tree, while messages to the coordinator are "collected" by a participant from all the participants below it, before it sends the appropriate message "up" the tree (except an abort message, which is propagated "up" immediately upon receiving it or if the current participant initiates the abort).

The Dynamic two-phase commit (Dynamic two-phase commitment, D2PC) protocol<ref name="weikum2001" /><ref name="raz1995">[[Yoav Raz]] (1995): [[doi:10.1007/3-540-58907-4_14|"The Dynamic Two Phase Commitment (D2PC) protocol "]],''Database Theory — ICDT '95'', ''Lecture Notes in Computer Science'', Volume 893/1995, pp. 162-176, Springer, {{ISBN|978-3-540-58907-5}}</ref> is a variant of Tree 2PC with no predetermined coordinator. It subsumes several optimizations that have been proposed earlier. Agreement messages (Yes votes) start to propagate from all the leaves, each leaf when completing its tasks on behalf of the transaction (becoming ready). An intermediate (non leaf) node sends ready when an agreement message to the last (single) neighboring node from which agreement message has not yet been received. The coordinator is determined dynamically by racing agreement messages over the transaction tree, at the place where they collide. They collide either at a transaction tree node, to be the coordinator, or on a tree edge. In the latter case one of the two edge's nodes is elected as a coordinator (any node). D2PC is time optimal (among all the instances of a specific transaction tree, and any specific Tree 2PC protocol implementation; all instances have the same tree; each instance has a different node as coordinator): By choosing an optimal coordinator D2PC commits both the coordinator and each participant in minimum possible time, allowing the earliest possible release of locked resources in each transaction participant (tree node).