Ics 214B: Transaction Processing and Distributed Data Management Lecture 12: Three-Phase Commits (3PC)

ICS 214B: Transaction Processing and Distributed Data Management

• Lecture 12: Three-Phase Commits (3PC)

• Professor Chen Li

2PC is blocking

• Sample scenario:

• Coord P2

• W

• P1 P3

• W

• P4 W

Case I:

• Case I:

• P1  “W”; coordinator sent commits

• P1  “C”

• Case II:

• P1  NO; P1  A

•  P2, P3, P4 (surviving participants)

• cannot safely abort or commit transaction

Complexity analysis

• Count number of messages, rounds

• N participants

• Centralized 2PC

• Ignore DONE messages
• If there are no failures:
• 3 rounds
• 3N messages

Variants of 2PC

• Linear

• Coord

• Hierarchical

Variants of 2PC

• Distributed

• Nodes broadcast all messages
• Every node knows when to commit

Exercise

• Compare 2PC variants in terms of

• Number of rounds
• Number of messages

Cooperative Termination Protocol

• Bad case

• Participant P recovers from failure
• Has prepared record for transaction T
• No commit or abort record for T
• Coordinator is down

• Participant P is blocked until coordinator recovers

Cooperative termination protocol

• But perhaps some other participant can help?

• Requires participants “know” each other!

Cooperative Termination Protocol

• Participant P sends a DECISION-REQUEST message to other participants

• Alive participants respond with COMMIT, ABORT, or UNCERTAIN

• If any participant replies with a decision (COMMIT or ABORT), P acts on decision

• And sends decision to UNCERTAIN participants

Cooperative Termination Protocol

• When P receives a DECISION-REQUEST

• If it knows decision, responds with COMMIT or ABORT
• If it has not prepared transaction, responds ABORT
• If it is prepared but does not know decision, responds UNCERTAIN

Cooperative Termination

• Sample scenario:

• Coord P1

• C

• P2

• W

• P3 W

Cooperative Termination

• Sample scenario:

• Coord P1

• W

• P2

• W

• P3 A

Cooperative Termination

• Sample scenario:

• Coord P1

• W

• P2

• W

• P3 W

Is there a non-blocking protocol?

• Theorem:

• If communications failure or total site failures (i.e., all sites are down simultaneously) are possible, then every atomic protocol may cause processes to become blocked.

Next…

• Three-phase commit (3PC)

• Nonblocking if reliable network (no communications failure) and no total site failures
• Handling communications failures

Three-Phase Commit

• Sample scenario:

• Coord P1

• W

• P2

• W

• P3 W

3PC Principle

• If ANY operational site is in the “uncertain” state, NO site (operational or failed) could have decided to commit

• Reminder: Assume reliable network

Process categories

• Three categories

• Operational
• Process has been up since start of 3PC
• Failed
• Process has halted since start of 3PC, or is recovering
• Recovered
• Process that failed and has completed recovery

Termination Protocol

Termination Protocol

• Elect new coordinator

• Use Election Protocol (coming soon…)

• New coordinator sends STATE-REQUEST to participants

• Makes decision using termination rules

• Communicates to participants

Termination Protocol

• Sample scenario:

• Coord P1

• W

• P2

• W

• P3 W

Termination Protocol

• Sample scenario:

• Coord P1

• W

• P2

• W

• P3 PC

Note: 3PC unsafe with communication failures!

• Note: 3PC unsafe with communication failures!