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MAINTAINING SQL INVARIANTS IN WEAKLY CONSISTENT DATABASES

Nuno Preguiça (NOVA LINCS, FCT/Universidade NOVA de Lisboa)

Joint work with:Valter Balegas, Cheng Li (MPI, now Oracle), João Sousa, David Lopes, Sérgio Duarte, Carla Ferreira, João Leitão, Allen Clement (MPI, now Google), Viktor Vafeiadis (MPI), Rodrigo Rodrigues (now Inesc-Id/IST)

INTERNET SERVICES NOWADAYS

• Services operate on a global scale.

• An unprecedented number of people are using Internet services.

• Systems use geo-replication for low latency and high availability.

2

GEO-REPLICATION

3

20 ms

150 ms

20 ms

SYNC DC2

DC1

GEO-REPLICATION

4

DC2

DC1

GEO-REPLICATION

5

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

create table player( id varchar(20), primary key id)

create table tournament( id varchar(20), primary key id)

create table pt( p varchar(20), t varchar(20), foreign key (p) REFERENCES player (id), foreign key (t) REFERENCES tournament (id))

PT

Sonic, A

Sonic, B

GEO-REPLICATION

6

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Pacman, A

enroll(Pacman, A):insert into PT values(‘Pacman’,’A’)

PT

Sonic, A

Sonic, B

GEO-REPLICATION

7

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Pacman, A

enroll(Mario, A):insert into PT values(‘Mario’,’A’)

PT

Sonic, A

Sonic, B

Mario, A

PT

Sonic, A

Sonic, B

Pacman, A

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

8

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

removeTournament(A):delete from tournament where id = ’A’

delete from PT where t = ’A’

PT

Sonic, A

Sonic, B

Pacman, A

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

9

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

SYNC

PT

Sonic, A

Sonic, B

Pacman, A

Mario, A

PT

Sonic, A

Sonic, B

Pacman, A

Mario, A

GEO-REPLICATION

10

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

SYNC

Mario is enrolled in tournament that was

concurrently removed.Referential integrity

violation.

OUTLINE

• Context / problem• First take: Sieve• Second take: SQL IPA• Final remarks

11

RedBlue ConsistencyBuilds replicated systems that are fast and correct


Blue ops: local, fast, weakly consistent


State convergence

Invariant preservation



State convergence

Invariant preservation


Red ops: global, slow, strongly consistent

Choosing between Blue or Red

operation u

commutative?

Red

No

Ensuring state convergence


operation u

commutative?

breaks invariants?

Red Blue

No

Yes

Yes

No


Ensuring invariant

preservation


operation u

commutative?

breaks invariants?

Red Blue

No

Yes

Yes

No


Ensuring invariant

preservation

Good performance obtained if blue ops dominate op space

SIEVE

Transforming

Operation stream

Slow,Strongly consistent

Fast,Weakly consistent

Commutative shadow operations

Classifying

SIEVE

Transforming

Operation stream


SIEVE

Transforming

Operation stream


Challenges:• Making arbitrary side effects commute• Minimizing human intervention

CRDT Annotation Example

@AUSET CREATE TABLE BankAccount(id INT(11) NOT NULL,

@NUMDELTA balance INT(11) default 0,@LWW name char(60) default NULL,

PRIMARY KEY (id)) ENGINE=InnoDB

SIEVE

Transforming

Operation stream




Classifying

SIEVE




Classifying

SIEVE




Classifying

Challenge:• How to classify accurately and efficiently?

OUTLINE


29

Limitations of Sieve

• Operations that may violate the invariant need to be red/coordinated => slow– Acquiring reservation/token (Indigo/CISE)

• Static analysis of complete application(s)– Changes in applications require rerunning the

analysis process

30


• Operations that may violate the invariant need to be blue/coordinated => slow– Acquiring reservation/token (Indigo/CISE)


analysis process

31

Goal: maintain invariants while avoiding coordination

Tournament

A

B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

GEO-REPLICATION

32

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1


delete from PT where t = ’A’

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

33

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1


PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

34

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

enroll(Mario, A):insert into PT values(‘Mario’,’A’)touch tournament where id = ‘A’

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

35

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1


touch cascade PT where t=’A’

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

36

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1


touch cascade PT where t=’A’


delete cascade from PT where t = ’A’

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

37

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

SYNC

Rules add-wins:1. || => 2. || =>

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

38

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

SYNC

Rules add-wins:1. || => 2. || =>

Other invariants

• Primary key (uniqueness)– Split keyspace

• Check constraint– E.g. stock int CHECK (stock >= 0)– Solved using bounded counter (escrow)

39


• Operations that may violate the invariant need to be blue/coordinated => slow– Acquiring reservation/token (Indigo/CISE)


analysis process

40

Goal: “modify” operations in runtime.Use schema definition.

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

41

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1




create table pt( p varchar(20), t varchar(20), foreign key (p) REFERENCES player (id), foreign key (t) REFERENCES tournament (id))

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

PT

Sonic, A

Sonic, B

Mario, A

GEO-REPLICATION

42

Tournament

A

B

Player

Sonic

Pacman

Mario

Tournament

A

B

Player

Sonic

Pacman

Mario

DC2

DC1

enroll(Mario, A):insert into PT values(‘Mario’,’A’)touch tournament where id = ‘A’touch player where id = ‘Mario’



create table pt( p varchar(20), t varchar(20), AW foreign key (p) REFERENCES player (id), AW foreign key (t) REFERENCES tournament (id))

OUTLINE


43

Status

• Implementing prototype on top of Antidote database

• Runtime solution equivalent to static solution implemented in IPA

44

Impact of additional updates

0 10 20 30 40

1 2 4 8 16 32 64

Spee

d-up

Number of updated Keys

IPA/Strong

45

TOURNAMENT

46

0 50

100 150 200 250 300 350

0 50 100 150 200 250 300 350 400

Late

ncy

[ms]

Throughput [TP/s]

StrongIndigo

IPACausal

TOURNAMENT: OPERATIONS LATENCY

47

0 50

100 150 200 250 300

BeginFinish

Remove

DoMatch

EnrollDisenroll

Status

Late

ncy

[ms] Indigo

IPACausal

Final remarks

• SQL schema allows to define constraints• First approach– Coordinate on operations that may break invariants

• Second approach–Maintain invariants without coordination (or

minimizing coordination)

48

QUESTIONS?

Valter Balegas – NOVA LINCS, FCT-UNL - Putting Consistency Back Into Eventual Consistency @ Eurosys'15 49

maintaining sql invariants in weakly consistent … · sonic pacman mario tournament a b player...

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