MVCC and Distributed Txns (Spanner) 2P & CC = strict serialization Provides semantics as if only one transaction was running on DB at time, in serial order + Real-time guarantees CS 518: Advanced Computer Systems ecture 6 Michael Freedman 2P: Pessimistically get all the locks first CC: ptimistically create copies, but then recheck all read + written items before commit 2 Multi-version concurrency control Multi-version concurrency control Maintain multiple versions of objects, each with own timestamp. Allocate correct version to reads. Prior example of MVCC: Generalize use of multiple versions of objects 3 4 1
Multi-version concurrency control Maintain multiple versions of objects, each with own timestamp. Allocate correct version to reads. nlike 2P/CC, reads never rejected ccasionally run garbage collection to clean up MVCC Intuition Split transaction into read set and write set All reads execute as if one snapshot All writes execute as if one later snapshot Yields snapshot isolation < serializability 5 6 Serializability vs. Snapshot isolation Intuition: Bag of marbles: ½ white, ½ black Transactions: T1: Change all white marbles to black marbles T2: Change all black marbles to white marbles Serializability (2P, CC) T1 T2 or T2 T1 In either case, bag is either A white or A black Timestamps in MVCC Transactions are assigned timestamps, which may get assigned to objects those s read/write Every object version V has both read and write TS ReadTS: argest timestamp of that reads V ritets: Timestamp of that wrote V Snapshot isolation (MVCC) T1 T2 or T2 T1 or T1 T2 Bag is A white, A black, or ½ white ½ black 7 8 2
Executing transaction T in MVCC Find version of object to read: # Determine the last version written before read snapshot time Find V s.t. max { ritets( V ) ritets( V ) <= TS(T) } ReadTS( V ) = max(ts(t), ReadTS( V )) Return V to T Perform write of object or abort if conflicting: Find V s.t. max { ritets( V ) ritets( V ) <= TS(T) } # Abort if another T exists and has read after T If ReadTS( V ) > TS(T) Abort and roll-back T Else Create new version Set ReadTS( ) = ritets( ) = TS(T) 9 write() by TS=3 10 R(1) = 3 write() by TS=5 11 R(1) = 3 write() by (2) = 5 R(2) = 5 Find v such that max ritets(v) <= () Þ v = 1 has (rite) <= 4 If ReadTS(1) > 4, abort Þ 3 > 4: false therwise, write object 12 3
R(1) = 3 (3) = 4 R(3) = 4 (2) = 5 R(2) = 5 Find v such that max ritets(v) <= () Þ v = 1 has (rite) <= 4 If ReadTS(1) > 4, abort Þ 3 > 4: false therwise, write object 13 R(1) = 35 BEGIN Transaction tmp = READ() RITE (, tmp + 1) END Transaction Find v such that max ritets(v) <= () Þ v = 1 has (rite) <= 5 Set R(1) = max(5, R(1)) = 5 14 R(1) = 53 BEGIN Transaction tmp = READ() RITE (, tmp + 1) END Transaction (2) = 5 R(2) = 5 Find v such that max ritets(v) <= () Þ v = 1 has (rite) <= 5 If ReadTS(1) > 5, abort Þ 5 > 5: false therwise, write object 15 R(1) = 35 write() by (2) = 5 R(2) = 5 Find v such that max ritets(v) <= () Þ v = 1 has (rite) <= 4 If ReadTS(1) > 4, abort Þ 5 > 4: true 16 4
Distributed Transactions R(1) = 53 BEGIN Transaction tmp = READ() RITE (P, tmp + 1) END Transaction (2) = 5 R(2) = 5 Find v such that max ritets(v) <= () Þ v = 1 has (rite) <= 4 Set R(1) = max(4, R(1)) = 5 Then write on P succeeds as well 17 18 Consider partitioned data over servers Consider partitioned data over servers P Q R R P Q R R hy not just use 2P? How do you get serializability? Grab locks over entire read and write set n single machine, single CMMIT op in the A Perform writes Release locks (at commit time) 19 In distributed setting, assign global timestamp to (at sometime after lock acquisition and before commit) Centralized manager Distributed consensus on timestamp (not all ops) 20 5
Strawman: Consensus per group? P Q R R Spanner: Google s Globally- Distributed Database R S SDI 2012 Single amport clock, consensus per group? inearizability composes! But doesn t solve concurrent, non-overlapping problem 21 22 Google s Setting Scale-out vs. fault tolerance Dozens of zones (datacenters) Per zone, 100-1000s of servers Per server, 100-1000 partitions (tablets) Every tablet replicated for fault-tolerance (e.g., 5x) 23 P P P Q QQ Every tablet replicated via Paxos (with leader election) So every operation within transactions across tablets actually a replicated operation within Paxos RSM Paxos groups can stretch across datacenters! (CPS took same approach within datacenter) 24 6
TrueTime Disruptive idea: Do clocks really need to be arbitrarily unsynchronized? Can you engineer some max divergence? Global wall-clock time with bounded uncertainty earliest TT.now() 2*ε latest time Consider event e now which invoked tt = TT.new(): Guarantee: tt.earliest <= t abs (e now ) <= tt.latest 25 26 Timestamps and TrueTime Commit ait and Replication T Pick s > TT.now().latest s Commit wait Release locks ait until TT.now().earliest > s T Start consensus Pick s Achieve consensus Notify followers Release locks Commit wait done average ε average ε 27 28 7
Client-driven transactions Client: 1. Issues reads to leader of each tablet group, which acquires read locks and returns most recent data 2. ocally performs writes 3. Chooses coordinator from set of leaders, initiates commit 4. Sends commit message to each leader, include identify of coordinator and buffered writes 5. aits for commit from coordinator 29 Commit ait and 2-Phase Commit n commit msg from client, leaders acquire local write locks If non-coordinator: Choose prepare ts > previous local timestamps og prepare record through Paxos Notify coordinator of prepare timestamp If coordinator: ait until hear from other participants Choose commit timestamp >= prepare ts, > local ts ogs commit record through Paxos ait commit-wait period Sends commit timestamp to replicas, other leaders, client All apply at commit timestamp and release locks 30 Commit ait and 2-Phase Commit Commit ait and 2-Phase Commit T C T P2 T P1 Compute s p for each Start logging T C T P2 T P1 Compute s p for each Done logging Prepared Send s p 1. Client issues reads to leader of each tablet group, which acquires read locks and returns most recent data 31 2. ocally performs writes 3. Chooses coordinator from set of leaders, initiates commit 4. Sends commit msg to each leader, incl. identity of coordinator 32 8
Commit ait and 2-Phase Commit Start logging Done logging Release locks T C T P2 T P1 Compute s p for each Compute overall s c 5. Client waits for commit from coordinator Committed Notify participants s c Release locks Release locks Prepared Send s p Commit wait done Example Remove X Risky post P from friend list T C T 2 s p = 6 s c = 8 s = 15 T P Remove myself from X s friend list s p = 8 Time <8 My friends My posts X s friends [X] [me] 8 [] [] s c = 8 15 [P] 33 34 Read-only optimizations Given global timestamp, can implement read-only transactions lock-free (snapshot isolation) Step 1: Choose timestamp s read = TT.now.latest() Step 2: Snapshot read (at s read ) to each tablet Can be served by any up-to-date replica Disruptive idea: Do clocks really need to be arbitrarily unsynchronized? Can you engineer some max divergence? 35 36 9
TrueTime Architecture TrueTime implementation GPS GPS GPS now = reference now + local-clock offset ε = reference ε + worst-case local-clock drift GPS Atomic-clock GPS = 1ms + 200 μs/sec Client +6ms ε Datacenter 1 Datacenter 2 Datacenter n 0sec 30sec 60sec 90sec time Compute reference [earliest, latest] = now ± ε hat about faulty clocks? 37 Bad CPs 6x more likely in 1 year of empirical data 38 Known unknowns > unknown unknowns Monday lecture Caching Rethink algorithms to reason about uncertainty Project Proposals due Monday night, 11:59pm 39 40 10