Replication: optimistic approaches

Replication: optimistic approaches

Marc Shapirowith Yasushi Saito (HP Labs)

Cambridge Distributed Systems Group

Bases de Données Avancées -- 2002-10-23 Replication: optimistic approaches 2

Motivations for this work

Peer-to-peer, decentralised write sharing

Lessons and commonalities

Understand limitations

Different solutions: spectrum or discrete points?

Simple formal model


Optimistic replication

Replicas of shared objects on sitesWithout synchronisation:

peer-to-peer read and update!

Consistency: a posteriori, offlineMerge independent updates

Applications:high latency networksdisconnected operationcooperative work

Improves availability & performance


Example: cooperative engineering with CVS

CVS: developing shared code

Local, disconnected replica: no interference

Conflicts:Write same file = syntacticOverlap in file = violates edit semanticsDoesn’t compile, test = violates

application semantics

Both sides of a conflict are excluded

Manual repair


Example: Bayou

General-purpose databaseAny replica can update, log actions

action = { dependency check, operation, merge-procedure }

Optimistic replication:epidemic exchange logs{ roll-back, replay }*; commitdep-check: semantic check for conflict merge-proc: semantic repair


Basic vocabulary

While isolated: tentative updates

When connected, reconcile:Propagate & collect updates(Conceptually) Restart from initial stateReplay updates (if possible)

Overriding goal: consistency

1. Consistency

Study component issues of consistency


What is consistency?

Consistent with user intentsapply operationsaccording to user scenario

Consistent with data invariantsdependent actionspre- and post-conditionsconflict resolution

Replicas consistent with each otherconverge towards same values


Consistency: problem taxonomy1. Objects & updates

Internal vs. external consistency Value / value log / operation log Single master / multi-master

2. Detecting dependence vs. concurrency

3. Concurrency control

4. Laziness of concurrency control Pessimistic / advanced concurrency /

optimistic

5. Convergence


Operation-based reconciliation

Updates: concurrent, unsynchronised

Local log of actions = operation descriptionsobject identifier, method, arguments

Multi-log collects local + remote logs

Reconciliation schedule: merge multi-log & run sequentially

Scheduling issues:Include vs. excludeExecution order


Operation-based model

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Dependence vs. concurrency

Two actions are either have a dependency or commutative / concurrent

Dependent actions:do not conflictmust be scheduled in dependence order

Concurrent actionspotentially conflict

Dependence / concurrency detection is a fundental mechanism


Concurrency control

Concurrent & no conflict commute: execute both, arbitrary order

Conflict detection options

Conflict resolution options


Convergence

Liveness: sites receive same/all actions

Safety: given same actions, sites compute the same value

Stability: actions eventually not undone

2. Dependency & Concurrency

Mechanisms to detect if actions are dependent or concurrent


Scalar clocks and timestamps

Wall clock, Lamport clockTotal orderTotal order, consistent with

causal dependenceSchedule in timestamp orderCan’t detect concurrency


Happens-before

e1 precedes e2 in processe1 sends, e2 receives

e1 e2

(e1 e2) (e2 e1) e1 || e2

e1 || e2: e1 does not cause e2

e1 e2: e1 might cause e2

Partial order, consistent with causal dependence

Schedule consistent with


Syntactic vs. semantic mechanisms

Scalar timestampsno concurrency detectionvery conservative approx.

of causalityVector timestamps

detect concurrencyconservative approx. of

causality

Alternative: explicit semantic constraints


Locks as semantic constraints

Read(x) depends onprevious Write(x) in same process, orpreviously-received Write(x), whichever

is laterWrite(x) depends on

previous Read(*) in same processMore semantic information than Happens-

BeforeStep in the right direction, but still too coarse


IceCube: Primitive constraints

Declarative (“static”):MustHave: a b

if as and ab then bs(not necessarily contiguous nor in

order)Order: a b

if a, bs and ab then a before b in s(not necessarily both nor contiguous)

Within log, across logs

Imperative (dynamic): preCondition (State)


Log constraints

parcelpredecessor-

successor

alternatives

Express user intents:Predecessor/successor: a b b a

b uses effect of a; “a causes b”Parcel: a b b a

transactionAlternatives: a b b a

3. Concurrency control & scheduling

Policies for dealing with concurrent actions


Optimistic concurrency control & scheduling

Two actions are either:dependent

schedule in dependence orderconcurrent and non-conflicting or

commutative schedule in any order

concurrent and conflicting schedule in non-conflicting order or exclude one, the other, or both


Concurrency control

Concurrent & no conflict commute: execute both, arbitrary order

Conflict detection options:2 concurrent actions conflictonly if operate on same objectonly if both writeonly if violate semantic invariant

Conflict resolution options:exclude bothexclude 1st, include 2nd (or vice-versa)execute both in favorable order(rewrite and execute both)


What is a conflict?

1 site executes code + pre/post-conditionsPre/post-conditions often unknownDependency between successive actions

Schedule execution must satisfy pre/post-conditionsViolation conflict

pre(x0) post(x0, f(x0))

x1:= f(x0)

pre(x0) post(x’1, g(x0))

x’1:= g(x0)

pre(x1) post(x1, g(x1))

x2:= g(x1)


Thomas’ Write Rule

Pre- / post-conditions unknownScalar clocks

no concurrency detectimplicit concurrency controlschedule in clock ordera later action excludes earlier ones

Lost updates

Delete ambiguity: “tombstone” state


Value-based Version Vector concurrency control

Pre- / post-conditions unknown

Independent objectsactions to different objects commuteVV = per-object vector timestampany concurrent writes to object conflict

Resolution:ManualValues: “Resolver” per data type


Bayou scheduling

Disjoint databases; 1 primary / database

Transaction: single database

Action = { dependency check, operation, merge-procedure }

Optimistic replication:epidemic exchange logs{ roll-back, replay }*; commit

Conflict dependency check fails

merge-procedure


Bayou dependency checks

Write-write conflicts: on replay check that data unchanged

Read-write conflicts: check input datacan detect concurrent updatessemantic: only relevant changes

Application-specific checksbank account balance > £100fine grain


IceCube: Object constraints

Shared data type advertises static semanticsmutually exclusive a b b a

best order (e.g. bank: credits before debits) a b

Only between concurrent actions

Also: dynamic constraints

commutebestorder

mutuallyexclusive


IceCube scheduling

Insight: conflict: choice of which action to excludemaximise value


IceCube execution model

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log constraints

log constraintsobjectconstraints

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dynamic constraints


Search vs. syntactic order

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Performance of IceCube heuristics

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4. Convergence

Can a peer-to-peer system converge?

Hard in the general case

Formalise to understand limitations, trade-offs


Convergence

Liveness: sites receive same/all operations epidemic multicastquickly

Safety: sites compute the same valueequivalent schedules

Stability: actions eventually not undonestable schedulesUsers, external world dependencyGarbage collection


Schedule soundness & equivalence

s sound:Closed for MustHave

as ab bsConsistent with Order

(a,b s ab) a before b in sEquivalence: s t

s, t soundas atordering is irrelevant!


Stability

Peer-to-peer, indefinite tentative update + advisory reconciliation OK

But stability needed:Users, external world depend on itGarbage collect multilog

Stable: eventually decisions not changedcommitted: definitely included in all

schedulesaborted: definitely excluded


Correctness of stability

Actions known to be stable at site i:stablei = committedi abortedi

Live: action a, site i: a stablei

Safe: site i, schedule si:

si sound committedi si site i,k: committedi abortedk =

Safety invariant: strong, global!


Maintaining disjointness

site i,k: committedi abortedk = Different possibilities

Unilateral abortTWR, Holliday 2000

Unilateral commitDeterministic abort / commit rule

TWR Primary (only one) site decides

Bayou, CVSConsensus before deciding

Deno, Holliday 2000-2002


Maintaining soundness site i, schedule si:

si sound committedi si

When aborting a, also abort actions that MustHave a

When committing a, also abort uncommitted actions that are ‘Order’ed before a

Maintain both soundness and disjointness.Peer-to-peer commitment is hard!


Stability with TWR

Independent objects

Independent writes (no MustHave nor Order)

All sites take same decision:Given two writes to same object, abort

the earlierWhether concurrent or notWrite stable when seen by all sites

Disjointness: committedi =

Soundness: no MustHave (no transactions)


Stability in Bayou

Databases:DisjointIndependent: no multi-DB transaction1 primary / database

Log constraints: transactions, time order

Disjointness: Only 1 site decides about a: the primary for the database that a updates

Soundness: whole transaction commits or aborts


Holliday’s pre-commit protocol

Log constraints: multi-object transactionshappens-before order

Read transactions commit locally

Read-Write transactions: consensus to commitconvert locks to intentionspre-commit, votecommit if quorum ‘yes’abort if anti-quorum ‘no’ or conflict with

committed


Trade-offs

Deterministic rulefast, inflexible

Partition + primarysingle point of failureno MustHave across partition boundaries

Consensusslowscalabilityimpossibility of consensus in asynchronous

systems with failure

5. Conclusions


Need for OR not going away

“Network technology improving: keep everything consistent pessimistically.”

True, but:Constant latency; unavailable bandwidthMobile access unbounded latencyIncreasing numbers of replicas

“Conflicts are rare.”

True, but:Do occurVery high cost


OR pros & cons

Peer-to-peer read/write sharing

OR accepts more updates:Performance despite latencyAvailability despite failures

Increased complexitySemantic informationNot transparent

Bottleneck moved to commitHard to make peer-to-peerUnless (unacceptable?) restrictions

Unavoidable


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Replication: optimistic approaches

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