Two-Phase Locking Lecture #17 Database Systems Andy Pavlo 15-445/15-645 Computer Science Fall 2018 AP Carnegie Mellon Univ. 2 L AST CL ASS Conflict Serializable → Verify using either the "swapping" method or dependency graphs. → Any DBMS that says that they support "serializable" isolation does this. View Serializable → No efficient way to verify. → Andy doesn't know of any DBMS that supports this. CMU 15-445/645 (Fall 2018) 3 EXAMPLE Schedule T1 T2 BEGIN R(A) BEGIN R(A) W(A) TIME W(A) COMMIT R(A) COMMIT CMU 15-445/645 (Fall 2018) 4 OBSERVATION We need a way to guarantee that all execution schedules are correct (i.e., serializable) without knowing the entire schedule ahead of time. Solution: Use locks to protect database objects. CMU 15-445/645 (Fall 2018) 5 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN LOCK(A) Granted (T1→A) R(A) BEGIN LOCK(A) W(A) R(A) TIME UNLOCK(A) R(A) W(A) COMMIT UNLOCK(A) COMMIT CMU 15-445/645 (Fall 2018) 5 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN LOCK(A) Granted (T1→A) R(A) BEGIN LOCK(A) Denied! W(A) R(A) TIME UNLOCK(A) R(A) W(A) COMMIT UNLOCK(A) COMMIT CMU 15-445/645 (Fall 2018) 5 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN LOCK(A) Granted (T1→A) R(A) BEGIN LOCK(A) Denied! W(A) R(A) TIME UNLOCK(A) Released (T1→A) R(A) W(A) COMMIT UNLOCK(A) COMMIT CMU 15-445/645 (Fall 2018) 5 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN LOCK(A) Granted (T1→A) R(A) BEGIN LOCK(A) Denied! W(A) R(A) TIME UNLOCK(A) Released (T1→A) R(A) Granted (T2→A) W(A) COMMIT UNLOCK(A) Released (T2→A) COMMIT CMU 15-445/645 (Fall 2018) 6 TODAY'S AGENDA Lock Types Two-Phase Locking Deadlock Detection + Prevention Hierarchical Locking Isolation Levels CMU 15-445/645 (Fall 2018) 7 LOCKS VS. LATCHES Locks Latches User transactions Threads Database Contents In-Memory Data Structures Entire Transactions Critical Sections Shared, Exclusive, Update, Read, Write Intention Deadlock Detection & Resolution Avoidance Waits-for, Timeout, Aborts Coding Discipline Kept Lock Manager Protected Data Structure Source: Goetz Graefe CMU 15-445/645 (Fall 2018) 8 BASIC LOCK TYPES S-LOCK: Shared locks for reads. X-LOCK: Exclusive locks for writes. Compatibility Matrix Shared Exclusive Shared ✔ X Exclusive X X CMU 15-445/645 (Fall 2018) 9 EXECUTING WITH LOCKS Transactions request locks (or upgrades). Lock manager grants or blocks requests. Transactions release locks. Lock manager updates its internal lock-table. → It keeps track of what transactions hold what locks and what transactions are waiting to acquire any locks. CMU 15-445/645 (Fall 2018) 10 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) UNLOCK(A) BEGIN TIME X-LOCK(A) W(A) UNLOCK(A) S-LOCK(A) R(A) UNLOCK(A) COMMIT COMMIT CMU 15-445/645 (Fall 2018) 10 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) UNLOCK(A) Released (T1→A) BEGIN TIME X-LOCK(A) W(A) UNLOCK(A) S-LOCK(A) R(A) UNLOCK(A) COMMIT COMMIT CMU 15-445/645 (Fall 2018) 10 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) UNLOCK(A) Released (T1→A) BEGIN TIME X-LOCK(A) Granted (T2→A) W(A) UNLOCK(A) Released (T2→A) S-LOCK(A) R(A) UNLOCK(A) COMMIT COMMIT CMU 15-445/645 (Fall 2018) 10 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) UNLOCK(A) Released (T1→A) BEGIN TIME X-LOCK(A) Granted (T2→A) W(A) UNLOCK(A) Released (T2→A) S-LOCK(A) Granted (T →A) R(A) 1 UNLOCK(A) Released (T1→A) COMMIT COMMIT CMU 15-445/645 (Fall 2018) 10 EXECUTING WITH LOCKS Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) UNLOCK(A) Released (T1→A) BEGIN TIME X-LOCK(A) Granted (T2→A) W(A) UNLOCK(A) Released (T2→A) S-LOCK(A) Granted (T →A) R(A) 1 UNLOCK(A) Released (T1→A) COMMIT COMMIT CMU 15-445/645 (Fall 2018) 11 CONCURRENCY CONTROL PROTOCOL Two-phase locking (2PL) is a concurrency control protocol that determines whether a txn is allowed to access an object in the database on the fly. The protocol does not need to know all of the queries that a txn will execute ahead of time. CMU 15-445/645 (Fall 2018) 12 TWO-PHASE LOCKING Phase #1: Growing → Each txn requests the locks that it needs from the DBMS’s lock manager. → The lock manager grants/denies lock requests. Phase #2: Shrinking → The txn is allowed to only release locks that it previously acquired. It cannot acquire new locks. CMU 15-445/645 (Fall 2018) 13 TWO-PHASE LOCKING The txn is not allowed to acquire/upgrade locks after the growing phase finishes. Transaction Lifetime # of Locks # of Growing Phase Shrinking Phase TIME CMU 15-445/645 (Fall 2018) 14 TWO-PHASE LOCKING The txn is not allowed to acquire/upgrade locks after the growing phase finishes. 2PL Violation! Transaction Lifetime # of Locks # of Growing Phase Shrinking Phase TIME CMU 15-445/645 (Fall 2018) 15 EXECUTING WITH 2PL Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) BEGIN X-LOCK(A) TIME R(A) UNLOCK(A) COMMIT W(A) UNLOCK(A) COMMIT CMU 15-445/645 (Fall 2018) 15 EXECUTING WITH 2PL Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) BEGIN X-LOCK(A) Denied! TIME R(A) UNLOCK(A) COMMIT W(A) UNLOCK(A) COMMIT CMU 15-445/645 (Fall 2018) 15 EXECUTING WITH 2PL Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) BEGIN X-LOCK(A) Denied! TIME R(A) UNLOCK(A) Released (T1→A) COMMIT W(A) UNLOCK(A) COMMIT CMU 15-445/645 (Fall 2018) 15 EXECUTING WITH 2PL Schedule Lock Manager T1 T2 BEGIN X-LOCK(A) Granted (T1→A) R(A) W(A) BEGIN X-LOCK(A) Denied! TIME R(A) UNLOCK(A) Released (T1→A) COMMIT W(A) Granted (T2→A) UNLOCK(A) Released (T →A) COMMIT 2 CMU 15-445/645 (Fall 2018) 16 TWO-PHASE LOCKING 2PL on its own is sufficient to guarantee conflict serializability. → It generates schedules whose precedence graph is acyclic. But it is subject to cascading aborts. CMU 15-445/645 (Fall 2018) 17 2PL CASCADING ABORTS Schedule T1 T2 BEGIN BEGIN X-LOCK(A) X-LOCK(B) R(A) W(A) UNLOCK(A) TIME X-LOCK(A) R(A) W(A) R(B) ⋮ W(B) ABORT CMU 15-445/645 (Fall 2018) 17 2PL CASCADING ABORTS Schedule This is a permissible schedule in T1 T2 BEGIN BEGIN 2PL, but the DBMS has to also X-LOCK(A) abort T when T aborts. X-LOCK(B) 2 1 R(A) → Any information about T1 cannot W(A) be "leaked" to the outside world. UNLOCK(A) TIME X-LOCK(A) R(A) W(A) This is all wasted work! R(B) ⋮ W(B) ABORT CMU 15-445/645 (Fall 2018) 18 2PL OBSERVATIONS There are potential schedules that are serializable but would not be allowed by 2PL. → Locking limits concurrency. May still have "dirty reads". → Solution: Strict 2PL May lead to deadlocks. → Solution: Detection or Prevention CMU 15-445/645 (Fall 2018) 19 STRICT TWO-PHASE LOCKING The txn is not allowed to acquire/upgrade locks after the growing phase finishes. Allows only conflict serializable schedules, but it is often stronger than needed for some apps. Release all locks at end of txn. # of Locks # of Growing Phase Shrinking Phase TIME CMU 15-445/645 (Fall 2018) 20 STRICT TWO-PHASE LOCKING A schedule is strict if a value written by a txn is not read or overwritten by other txns until that txn finishes. Advantages: → Does not incur cascading aborts. → Aborted txns can be undone by just restoring original values of modified tuples. CMU 15-445/645 (Fall 2018) 21 EXAMPLES T1 – Move $100 from Andy’s account (A) to his bookie’s account (B). T2 – Compute the total amount in all accounts and return it to the application. T1 T2 BEGIN BEGIN A=A-100 ECHO A+B B=B+100 COMMIT COMMIT CMU 15-445/645 (Fall 2018) 22 NON-2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 W(A) UNLOCK(A) R(A) UNLOCK(A) TIME S-LOCK(B) X-LOCK(B) R(B) UNLOCK(B) R(B) ECHO A+B B=B+100 COMMIT W(B) UNLOCK(B) COMMIT CMU 15-445/645 (Fall 2018) 22 NON-2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 W(A) UNLOCK(A) R(A) UNLOCK(A) TIME S-LOCK(B) X-LOCK(B) R(B) UNLOCK(B) R(B) ECHO A+B B=B+100 COMMIT W(B) UNLOCK(B) COMMIT CMU 15-445/645 (Fall 2018) 22 NON-2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 W(A) UNLOCK(A) R(A) UNLOCK(A) TIME S-LOCK(B) X-LOCK(B) R(B) UNLOCK(B) R(B) ECHO A+B B=B+100 COMMIT W(B) UNLOCK(B) COMMIT CMU 15-445/645 (Fall 2018) 22 NON-2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 T Output W(A) 2 UNLOCK(A) R(A) A+B=1100 UNLOCK(A) TIME S-LOCK(B) X-LOCK(B) R(B) UNLOCK(B) R(B) ECHO A+B B=B+100 COMMIT W(B) UNLOCK(B) COMMIT CMU 15-445/645 (Fall 2018) 23 2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 W(A) X-LOCK(B) UNLOCK(A) R(A) S-LOCK(B) TIME R(B) B=B+100 W(B) UNLOCK(B) R(B) COMMIT UNLOCK(A) UNLOCK(B) ECHO A+B COMMIT CMU 15-445/645 (Fall 2018) 23 2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 W(A) X-LOCK(B) UNLOCK(A) R(A) S-LOCK(B) TIME R(B) B=B+100 W(B) UNLOCK(B) R(B) COMMIT UNLOCK(A) UNLOCK(B) ECHO A+B COMMIT CMU 15-445/645 (Fall 2018) 23 2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 T Output W(A) 2 X-LOCK(B) UNLOCK(A) R(A) A+B=2000 S-LOCK(B) TIME R(B) B=B+100 W(B) UNLOCK(B) R(B) COMMIT UNLOCK(A) UNLOCK(B) ECHO A+B COMMIT CMU 15-445/645 (Fall 2018) 24 STRICT 2PL EXAMPLE Schedule Initial Database State T 1 T2 A=1000, B=1000 BEGIN BEGIN X-LOCK(A) R(A) S-LOCK(A) A=A-100 W(A) X-LOCK(B) R(B) B=B+100 W(B) TIME