Challenge: Database Access for Client-sideApps Write Fast, Read … · 2018-01-15 · Limited...

WriteFast,ReadinthePast:CausalConsistencyforClient-sideApps

withSwiftCloud

PresentedbyMarekZawirskiInria/UPMC-LIP6,Paris

(nowatGoogle,Zürich)

MarekZawirski,NunoPreguiça,SérgioDuarte,AnnetteBieniusa,ValterBalegas,MarcShapiro

Challenge:DatabaseAccessforClient-side Apps

Zawirski etal.,WriteFast,ReadinthePast:CausalConsistencyforClient-sideApplicationswithSwiftCloud 1

App

App

App

App



App

App

App

App

API

API

Limitedboundariesofserver-sidedatabaseguarantees



App

App

App

App

� ad-hocontheclient-side

API

API

Limitedboundariesofserver-sidedatabaseguarantees



API

App

App

API

App

App

API

API

ExtendedboundarieswithSwiftCloud



API

App

App

API

App

App

API

API

ExtendedboundarieswithSwiftCloud

• Consistent,available and convergentdataaccess

• Scalability with#objectsand#clients

• Fault-tolerance

StrongerthanEventual:CausalConsistency


bob_posts.add(“don’tthinkofvisitingVancouver…”)

bob_posts.add(“…justdoit!YOLO”)

replies.add(“Alice:totallyJ”)

clientorder

read

Defaultonclient-side:eventualconsistency� anomalies






clientorder

read


read



!?!




clientorder

readread



ü

Causalconsistency:readsfromcausally-closedsnapshot



readread

clientorder







read

replies.add(“Eve:Bob�”)

replies:???

clientorder

ConvergentCausalConsistency:NoLostUpdates



replies:{“Alice:totallyJ”,“Eve:Bob�”}

High-levelconvergentobjects[CRDTs] resolveconcurrency



clientorder

read

replies.add(“Eve:Bob�”)

ConvergentCausalConsistency:NoLostUpdates


Challenge:CausalConsistencywithPartialReplicas


objB.op

objB.op

objA.op

objC.op

[PRACTI,NSDI’06]Challenge:CausalConsistencywithPartialReplicas


objB.op

objB.op

objA.op

objC.opobjAobjB

[PRACTI,NSDI’06]

currentstate



objB.op

objB.op

objA.op

objC.opobjAobjB

objBobjC

[PRACTI,NSDI’06]Challenge:CausalConsistencywithPartialReplicas


objB.op

objB.op

objA.op

objC.op

objAobjC

objAobjB

objBobjC

[PRACTI,NSDI’06]



objB.op

objB.op

objA.op

objC.opobjAobjB

objBobjC

[PRACTI,NSDI’06]

objAobjC



objB.op

objB.op

objA.op

objC.opobjAobjB

objBobjCdependencies

[PRACTI,NSDI’06]

objB.op

objA.op

objC.op

objB.op

objAobjC

dependencies



objB.op

objB.op

objA.op

objC.opobjAobjB

objBobjC

[PRACTI,NSDI’06]

objB.op

objB.op1 0 2Inherenttrade-offsinthegeneralcase:scalabilityvs.availabilityvs.fault-tolerance

objAobjC

Approach:Cloud-backedPartialReplicas


DCgeo-replication

DataCenterfullreplicas:

üProvideconsistentviewAssignsmallmetadataü4 9

DC



DCgeo-replication

4 9

DC

logcache

App


üProvideconsistentviewAssignsmallmetadataü



DCgeo-replication

4 9

DC

logcache

App

update





DCgeo-replication

4 9

DC

logcache

App

update

4 9



5



DCgeo-replication

4 9

DC

logcache

App

Clientreads:cachedfragmentofcloudversion� ownlog

ü HighavailabilityConsistencyw/read-your-writesü



read

PotentialofCloud-backedClientReplicas


Setup:DCsin3AWSEC2 regions,YCSBworkload,cache=256objects

Responsetimeofoperation[ms]

CDFfor10

00clients

PotentialofCloud-backedClientReplicas


Responsetimeofoperation[ms]

CDFfor10

00clients

Objectsinthecache� immediate,consistentresponse

Setup:DCsin3AWSEC2 regions,YCSBworkload,cache=256objects

ChallengefortheCloudApproach:SafeDCFailover


cache

log



cache

log

riskyread



cache

log

riskyread

operationswithriskydependencies



cache

log

riskyread

operationswithriskydependencies

newDCinincompatiblestate

SupportingFailoverbyConservativeReads


cacheK=2

log

Foreignupdates:readversionreplicatedinK>1DCsOwnwrites:readfromthelog,recovertoanewDC



cacheK=2

log


conservativeread



cacheK=2

log

ü


newDCincompatiblestate

Experiment:InjectionofShortDCDisconnection


respon

setim

e[m

s]



remotereads

fastconservativereads

respon

setim

e[m

s]



remotereads


remoteops:smoothfailover

…unaffected…

respon

setim

e[m

s]



Trade-offcontrolledbyK:stalenessvs.availability• StalenessnegligibleinmostK=2setups,<1%reads• Incherry-pickedunfavorablesetup,1.0–2.5%reads

remotereads


remoteops:smoothfailover

…unaffected…

respon

setim

e[m

s]ChallengefortheCloudApproach:ProtocolRetries


inc() 44

x=0 x=0x=0

x=1

time

x.inc()(x=1)

ChallengefortheCloudApproach:ProtocolRetries


inc() 44

inc() 3

x=0 x=0x=0

x=1

x=1

x.inc()(x=1)



inc() 44

inc() 33

x=0 x=0x=0

x=1

x=1

x.inc()(x=1)



inc() 44

inc() 33

inc() 3inc()4geo-replication

x.inc()(x=1)

x=0 x=0x=0

x=1

x=1

x=2



duplicatedelivery!

inc() 44

inc() 33

inc() 3inc()4geo-replication

x.inc()(x=1)

x=0 x=0x=0

x=1

x=1

x=2

SafeRetrieswithDecoupledMetadata


geo-replication

inc() 44

inc() 33

3inc()4

1

1

4

1~

1 ~

Solution:client-assignedtimestampsforsafety+1..NDCtimestampsforefficientsummary

ü

safe:≤1delivery

3

x=0 x=0x=0

x.inc()(x=1) x=1

x=1

x=1

1



geo-replication

inc() 44

inc() 33

3inc()4

1

1

4

1~

1 ~


ü

safe:≤1delivery

3

x=0 x=0x=0

x.inc()(x=1) x=1

x=1

x=1

1



geo-replication

inc() 44

inc() 33

3inc()4

1

1

4

1~

1 ~


Extension:logpruningindependentofclientavailability

ü

safe:≤1delivery

3

x=0 x=0x=0

x.inc()(x=1) x=1

x=1

x=1

1

#clientreplicas

Experiment:SizeofMetadataonClient-DCLink


1

10

100

1K

metadata/up

date[B

]

SwiftCloud’s decoupledmetadata:const size

Setup:3DCs,YCSBBuniformworkload

Client-assignedvectors[PRACTI,NSDI’06]:unboundedoverhead

Summary


SwiftCloud providesclient-side apps:

• Consistent,available and convergentobjectdatabase

• Scalability:fullreplicasatDCbackpartialatclient� smallcausalitymetadata(<15B/update)

• Fastfailoverthankstoconservativereads(<1%stale)

• Saferetryofinterruptedtransferandsafelogpruningthankstodecoupledmetadata

Researchprototypeatgithub.com/SyncFree/SwiftCloud

SwiftCloud comparedto“LazyReplication”


• Assumeclient-sideapplicationlogic• Describecausalconsistencysupport• Supportcommunicationwithmultipleservers• Usedecoupledmetadata

• DB=RDTobjects+globaltransactions

• Supportspartialclientreplicas =>fastreadsandread-your-writes

• K-stability-driventrade-off• GCindependentofclients

• MonolithicDB

• Noclient-sidereplicas

• Stabilitydiscussion• Physical-clock-drivenGC• Moreconsistencychoices



cache

log

inc 3

inc 4

duplicatedelivery!

inc 4



cache

log

inc 4

inc 3

1

14 ü

safe:≤1delivery




cache

log

inc 4

inc 3

1

14 ü

safe:≤1delivery




cache

log

inc 4

inc 3

1

14 ü

safe:≤1delivery


Extension:logpruningindependentofclientavailability

Challenge: Database Access for Client-sideApps Write Fast, Read … · 2018-01-15 · Limited...

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Transcript of Challenge: Database Access for Client-sideApps Write Fast, Read … · 2018-01-15 · Limited...