Two-Phase-Commit

What are we talking about here?

We want to commit a transaction in several nodes so it will be committed in all of the nodes or in none.

Assumptions

  1. One nodes is designed as the coordinator.
  2. Each node has persistent storage.
  3. Each node can eventually communicate with any other node.
  4. Each node will recover from a crash eventually i.e., fail-recovery model.

Phase-1

  1. Coordinator sends COMMIT_REQUEST to all other nodes.
  2. A node that receive COMMIT_REQUEST write it to its a transaction log, hold the required commit resources and send AGREED message to the coordinator. If node can not commit, then it will send ABORT message to the coordinator
  3. If the coordinator did not received response from a node, it will re-send the node another COMMIT_REQUEST message or after sometime it will sent ABORT message to all the nodes.

Phase-2

  1. if the coordinator received AGREED from all nodes, it will send a COMMIT message to all nodes.
  2. If the coordinator received ABORT from any of the nodes, it will send an ABORT message to all nodes.
  3. If the coordinator did not received response from a node, it will re-send the node another COMMIT message.
  4. A node the receive COMMIT message will commit and sent a response COMMITTED.
  5. The coordinator will complete a commit after it received COMMITTED message from all the nodes.
  6. A node that receive an ABORT message will roll back the transaction.

Disadvantages

  1. This protocol is blocking i.e., a node will hold commit related resources until coordinator will receive COMMITTED message from all nodes.
  2. We assume that all nodes will recover after a crash – but this is not always the case and in case the coordinator will crash after nodes approved a commit and before he sent them COMMIT message – then nodes will wait for manager and hold resources forever.

Reference

  1. http://courses.cs.vt.edu/~cs5204/fall00/distributedDBMS/duckett/tpcp.html
  2. http://the-paper-trail.org/blog/consensus-protocols-two-phase-commit/
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