Paxos Week: Return of the State Machine Doug Woos Logistics notes - - PowerPoint PPT Presentation

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Paxos Week: Return of the State Machine Doug Woos Logistics notes - - PowerPoint PPT Presentation

Mar 15, 2024 237 likes •1.04k views

Paxos Week: Return of the State Machine Doug Woos Logistics notes No in-class lecture Monday Problem Set 2 due tonight Lab 3 out Paxos Made Simple Discussion Paxos vs. Primary/Backup Paxos vs. 2PC What about reconfig? The story of Paxos

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Paxos Week: Return of the State Machine

Doug Woos

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Logistics notes

No in-class lecture Monday Problem Set 2 due tonight Lab 3 out

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Paxos Made Simple Discussion

Paxos vs. Primary/Backup Paxos vs. 2PC What about reconfig? The story of Paxos

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State machine replication

Reminder: want to agree on order of ops Can think of operations as a log

Op1 Op2 Op3 Op4 Op5 Op6

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Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2

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Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2 I want to do “Put k1 v1” I want to do “Put k2 v2”

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SLIDE 7

Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2 I want to do “Put k1 v1” I want to do “Put k2 v2” Paxos for Op1

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Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2

Put k1 v1

I want to do “Put k2 v2”

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SLIDE 9

Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2

Put k1 v1

I want to do “Put k2 v2” Paxos for Op2

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Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2

Put k1 v1 Put k2 v2

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Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2

Put k1 v1 Put k2 v2

Paxos?

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Lab 3

Paxos = Paxos Made Simple

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Lab 3

Paxos = Phase 1

  • Send prepare messages
  • Pick value to accept

Phase 2

  • Send accept messages
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Can we do better?

Phase 1: “leader election”

  • Deciding whose value we will use

Phase 2: “commit”

  • Leader makes sure it’s still leader, commits value

What if we split these phases?

  • Lets us do operations with one round-trip
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Op1 Op2 Op3 Op4 Op5 Op6

S1 S3 S2

Put k1 v1 Put k2 v2

PMMC

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Roles in PMMC

Replicas (like learners)

  • Keep log of operations, state machine, configs

Leaders (like proposers)

  • Get elected, drive the consensus protocol

Acceptors (simpler than in Paxos Made Simple!)

  • “Vote” on leaders
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A note about ballots in PMMC

(leader, seqnum) pairs Isomorphic to the system we discussed Mon, Wed

1 2 3

0, 4, 8, 12, 16, … 1, 5, 9, 13, 17, … 2, 6, 10, 14, 18, … 3, 7, 11, 15, 19, …

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SLIDE 18

A note about ballots in PMMC

(leader, seqnum) pairs Isomorphic to the system we discussed Mon, Wed

1 2 3

(0, 0), (0, 1), (0, 2), (0, 3), (0, 4), … (1, 0), (1, 1), (1, 2), (1, 3), (1, 4), … (2, 0), (2, 1), (2, 2), (2, 3), (2, 4), … (3, 0), (3, 1), (3, 2), (3, 3), (3, 4), …

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SLIDE 19

A note about ballots in PMMC

(leader, seqnum) pairs Isomorphic to the system we discussed Mon, Wed

1 2 3

0.0, 1.0, 2.0, 3.0, 4.0, … 0.1, 1.1, 2.1, 3.1, 4.1, … 0.2, 1.2, 2.2, 3.2, 4.2, … 0.3, 1.3, 2.3, 3.3, 4.3, …

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Paxos Made Moderately Complex Made Simple

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Paxos Made Moderately Complex Made Simple

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Acceptors

Acceptor ballot_num: 0 accepted:[]

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Acceptors

Acceptor ballot_num: _ accepted:[] p1a(0.1)

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Acceptors

Acceptor ballot_num: 0.1 accepted:[] p1a(0.1)

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Acceptors

Acceptor ballot_num: 0.1 accepted:[] p1a(0.1) p1b([])

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Acceptors

Acceptor ballot_num: 0.1 accepted:[]

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Acceptors

Acceptor ballot_num: 0.1 accepted:[] p1a(0.0)

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Acceptors

Acceptor ballot_num: 0.1 accepted:[] p1a(0.0) Nope!

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Acceptors

Acceptor ballot_num: 0.1 accepted:[]

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Acceptors

Acceptor ballot_num: 0.1 accepted:[] p2a(<0.1, 0, A>)

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Acceptors

Acceptor ballot_num: 0.1 accepted:[<0.1, 0, A>] p2a(<0.1, 0, A>)

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Acceptors

Acceptor ballot_num: 0.1 accepted:[<0.1, 0, A>] p2a(<0.1, 0, A>) OK!

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Acceptors

Acceptor ballot_num: 0.1 accepted:[<0.1, 0, A>]

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Acceptors

Acceptor ballot_num: 0.1 accepted:[<0.1, 0, A>] p2a(<0.0, 0, B>)

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Acceptors

Acceptor ballot_num: 0.1 accepted:[<0.1, 0, A>] p2a(<0.0, 0, B>) Nope!

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Acceptors

Acceptor ballot_num: 0.1 accepted:[<0.1, 0, A>]

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Acceptors

  • Ballot numbers increase
  • Only accept values from current ballot
  • Never remove ballots
  • If a value v is chosen by a majority on ballot b, then

any value accepted by any acceptor in the same slot

  • n ballot b’ > b has the same value
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Paxos Made Moderately Complex Made Simple

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Paxos Made Moderately Complex Made Simple

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Leader: Getting Elected

Leader active: false ballot_num: 0.0 proposals: []

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Leader: Getting Elected

Leader active: false ballot_num: 0.0 proposals: [] Acceptor Acceptor Acceptor p1a(0.0)

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Leader: Getting Elected

Leader active: false ballot_num: 0.0 proposals: [] Acceptor Acceptor Acceptor Nope! Nope!

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Leader: Getting Elected

Leader active: false ballot_num: 1.0 proposals: [] Acceptor Acceptor Acceptor

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Leader: Getting Elected

Leader active: false ballot_num: 1.0 proposals: [] Acceptor Acceptor Acceptor Or…

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Leader: Getting Elected

Leader active: false ballot_num: 0.0 proposals: [] Acceptor Acceptor Acceptor OK([])! OK([])!

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Leader: Getting Elected

Leader active: true ballot_num: 0.0 proposals: [] Acceptor Acceptor Acceptor

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Paxos Made Moderately Complex Made Simple

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Paxos Made Moderately Complex Made Simple

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Leader: Handling proposals

Leader active: true ballot_num: 0.0 proposals: [] Acceptor Acceptor Acceptor Replica Op1 should be A (A = “Put k1 v1”)

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Leader: Handling proposals

Leader active: true ballot_num: 0.0 proposals: [<1, A>] Acceptor Acceptor Acceptor Replica

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Leader: Handling proposals

Leader active: true ballot_num: 0.0 proposals: [<1, A>] Acceptor Acceptor Acceptor p2a(<0.0, 1, A>) Replica

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Leader: Handling proposals

Leader active: true ballot_num: 0.0 proposals: [<1, A>] Acceptor Acceptor Acceptor Replica Nope! Nope!

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Leader: Handling proposals

Leader active: false ballot_num: 0.0 proposals: [<1, A>] Acceptor Acceptor Acceptor Replica

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Leader: Handling proposals

Leader active: false ballot_num: 0.0 proposals: [<1, A>] Acceptor Acceptor Acceptor Replica Or…

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Leader: Handling proposals

Leader active: true ballot_num: 0.0 proposals: [<1, A>] Acceptor Acceptor Acceptor Replica OK! OK!

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Leader: Handling proposals

Leader active: true ballot_num: 0.0 proposals: [<1, A>] Acceptor Acceptor Acceptor Replica Replica Replica Op1 is A

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Paxos Made Moderately Complex Made Simple

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Election revisited

Acceptor ballot_num: 2.1 accepted:[<2.1, 1, A>] Leader active: false ballot_num: 3.0 proposals: [<1, B>]

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Election revisited

Acceptor ballot_num: 2.1 accepted:[<2.1, 1, A>] Leader active: false ballot_num: 3.0 proposals: [<1, B>] p1a(3.0)

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Election revisited

Acceptor ballot_num: 3.0 accepted:[<2.1, 1, A>] Leader active: false ballot_num: 3.0 proposals: [<1, B>]

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Election revisited

Acceptor ballot_num: 3.0 accepted:[<2.1, 1, A>] Leader active: false ballot_num: 3.0 proposals: [<1, B>] OK([<2.1, 1, A>])

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Election revisited

Acceptor ballot_num: 3.0 accepted:[<2.1, 1, A>] Leader active: true ballot_num: 3.0 proposals: [<1, A>]

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Leaders

  • Only propose one value per ballot and slot
  • If a value v is chosen by a majority on ballot b, then

any value proposed by any leader in the same slot on ballot b’ > b has the same value

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Paxos Made Moderately Complex Made Simple

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Paxos Made Moderately Complex Made Simple

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Replicas

Op1 Op2 Op3 Op4 Op5 Op6 Put k1 v1 Put k2 v2

Replica

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Replicas

Op1 Op2 Op3 Op4 Op5 Op6 Put k1 v1 Put k2 v2 App k1 v1 App k2 v2

slot_out slot_in Replica

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Replicas

Op1 Op2 Op3 Op4 Op5 Op6 Put k1 v1 Put k2 v2 App k1 v1 App k2 v2

slot_out slot_in Replica Leader decision(3, “App k1 v1”)

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Replicas

Op1 Op2 Op3 Op4 Op5 Op6 Put k1 v1 Put k2 v2 App k1 v1 App k2 v2

slot_out slot_in Replica Leader

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Replicas

Op1 Op2 Op3 Op4 Op5 Op6 Put k1 v1 Put k2 v2 App k1 v1 App k2 v2

slot_out slot_in Replica Leader decision(4, “Put k3 v3”)

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Replicas

Op1 Op2 Op3 Op4 Op5 Op6 Put k1 v1 Put k2 v2 App k1 v1 Put k3 v3

slot_out slot_in Replica Leader

App k2 v2

propose(5, “App k2 v2”)

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Paxos Made Moderately Complex Made Simple

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When to run for office

When should a leader try to get elected?

  • At the beginning of time
  • When the current leader seems to have failed

Paper describes an algorithm, based on pinging the leader and timing out If you get preempted, don’t immediately try for election again!

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Reconfiguration

All replicas must agree on who the leaders and acceptors are How do we do this?

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Reconfiguration

All replicas must agree on who the leaders and acceptors are How do we do this?

  • Use the log!
  • Commit a special reconfiguration command
  • New config applies after WINDOW slots
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Reconfiguration

What if we need to reconfigure now and client requests aren’t coming in?

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Reconfiguration

What if we need to reconfigure now and client requests aren’t coming in?

  • Commit no-ops until WINDOW is cleared
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Other complications

State simplifications

  • Can track much less information, esp. on replicas

Garbage collection

  • Unbounded memory growth is bad
  • Lab 3: track finished slots across all instances,

garbage collect when everyone is ready Read-only commands

  • Can’t just read from replica (why?)
  • But, don’t need their own slot
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