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-abort Vote-commit 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. < Q, I, 0, , V, A, C > Finite set of states Messages addressed to the site Messages sent by the site Initial state Abort states Commit states : Q Q Q Q V Q Q 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. q1 q2 xact req w1 q2 start xact w1 w2 w1 a2 yes no a1 w2 a1 a2 c1 w2 abort commit a1 a2 c1 c2 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) = {V2, 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. Site I (co-ordinator) Site i (i = 2,3,…n) (slave) q1 qi request xact2 xact4 xacti noi … xacti yesi … w1 … aborti yes2 yesn prepare2 preparen … wi no2| |non abort2 abortn ai … … preparei acki a1 p1 pi … ack2 ackn commit2 commitn commiti … c1 ci Three-Phase Commit Protocol