Outline

1
Outline

• Introduction
• Background
• Distributed DBMS Architecture
• Distributed Database Design
• Distributed Query Processing
• Distributed Transaction Management
• Transaction Concepts and Models
• Distributed Concurrency Control
• Distributed Reliability
• Building Distributed Database Systems (RAID)
• Mobile Database Systems
• Privacy, Trust, and Authentication
• Peer to Peer Systems

2
2PC Recovery Protocols Additional Cases (see
book)

• Arise due to non-atomicity of log and message
  send actions
• Coordinator site fails after writing
  begin_commit log and before sending prepare
  command
• treat it as a failure in WAIT state send
  prepare command
• Participant site fails after writing ready
  record in log but before vote-commit is sent
• treat it as failure in READY state
• alternatively, can send vote-commit upon
  recovery
• Participant site fails after writing abort
  record in log but before vote-abort is sent
• no need to do anything upon recovery

3
2PC Recovery Protocols Additional Case (see book)

• Coordinator site fails after logging its final
  decision record but before sending its decision
  to the participants
• coordinator treats it as a failure in COMMIT or
  ABORT state
• participants treat it as timeout in the READY
  state
• Participant site fails after writing abort or
  commit record in log but before acknowledgement
  is sent
• participant treats it as failure in COMMIT or
  ABORT state
• coordinator will handle it by timeout in COMMIT
  or ABORT state

4
Problem With 2PC

• Blocking
• Ready implies that the participant waits for
  the coordinator
• If coordinator fails, site is blocked until
  recovery
• Blocking reduces availability
• Independent recovery is not possible
• However, it is known that
• Independent recovery protocols exist only for
  single site failures no independent recovery
  protocol exists which is resilient to
  multiple-site failures.
• So we search for these protocols 3PC

5
Three-Phase Commit

• 3PC is non-blocking.
• A commit protocols is non-blocking iff
• it is synchronous within one state transition,
  and
• its state transition diagram contains
• no state which is adjacent to both a commit and
  an abort state, and
• no non-committable state which is adjacent to a
  commit state
• Adjacent possible to go from one stat to another
  with a single state transition
• Committable all sites have voted to commit a
  transaction
• e.g. COMMIT state

6
State Transitions in 3PC
Coordinator
Participants
INITIAL
INITIAL
Prepare

Commit command

Vote-commit
Prepare
Prepare

Vote-abort
WAIT
READY
Global-abort

Prepared-to-commit

Vote-commit
Vote-abort

Ack
Global-abort
Ready-to-commit
Prepare-to-commit
PRE- COMMIT
PRE- COMMIT
ABORT
ABORT
Ready-to-commit
Global commit
Global commit
Ack
COMMIT
COMMIT
7
Communication Structure (see book)
P
P
P
P
P
P
C
C
C
C
P
P
P
P
P
P
pre-commit/
ack
commit/abort
ready?
yes/no
pre-abort?
yes/no
Phase 1
Phase 2
Phase 3
8
Formalism for Commit Protocols
9
Formalism for Commit Protocols
Properties
1. 2.
V
V

• Protocols are non-deterministic
• Sites make local decisions.
• Messages can arrive in any order.

10
Global State Definition

• Global state vector containing the states of the
  local protocols.
• Outstanding messages in the network
• A global state transition occurs whenever a local
  state transition occurs at a participating site.
• Exactly one global transition occurs for each
  local transition.

11
Global Sate Graph

• Global state is inconsistent if its state vector
  contains both a commit and abort state.

12

• Two states are potentially concurrent if there
  exists a reachable global state that contains
  both local states.
• Concurrency set of s is set of all local states
  that are potentially concurrent with it. C(s)
• C(w1) q2, a2 , w2
• The sender set for s,
• S(s) t/t sends message m m ? M
• where M be the set of messages that are received
  by s.
• t is a local state.

13
States of Various States in the Commit Protocol

• Global state inconsistent if it contains both
• local commit state
• local abort state
• Final state if
• All local states are final
• Terminal state if
• there exists an immediately reachable successor
  state
• ? deadlock
• Committable state (local) if
• all sites have voted yes on committing the
  transaction
• Otherwise, non-committable

14
An Example when Only a Single Site Remains
Operational

• This site can safely abort the transaction if and
  only if the concurrency set for its local state
  does not contain a commit state
• This site can safely commit only if
• Its local state must be committable
• And the concurrency set for its state must not
  contain an abort state.
• A blocking situation arises when
• The concurrency set for the local state contains
  both a commit and an abort state
• Or the site is in a noncommittable state and
  the concurrency set for that state contains a
  commit state
• The site can not commit because it can not infer
  that all sites have voted yes on committing
• It can not abort because another site may have
  committed the transaction before crashing.
• There observations imply the simple but power
  result in the next slide

15
Fundamental Non-blocking Theorem

• Definition protocol is synchronous within one
  state transition if one site never leads another
  site by more than one state transition.
• Theorem Fundamental non-blocking A protocol is
  non-blocking iff
• There exists no local state s
• C(s) A (abort) and C (commit)
• And there exists no non-committable state s
• C(s) C (commit)
• Lemma A protocol that is synchronous within one
  state transition is non-blocking iff
• No local state is adjacent to both a commit an
  abort state
• No non-committable state is adjacent to a commit
  state

16
Three-Phase Commit Protocol