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6.824 2015 Lecture 5: Paxos

Note: These lecture notes were slightly modified from the ones posted on the 6.824 course website from Spring 2015.

Intro

Starting a new group of lectures on stronger fault tolerance

  • Today:
    • cleaner approach to replication: RSM via Paxos
    • Lab 3
  • Subsequent lectures:
    • How to use Paxos to build systems (Harp, EPaxos, Spanner)

Paxos

Links:

Recall: RSM

  • maintain replicas by executing operations in the same order
  • requires all replicas to agree on the (set and) order of operations

Lab 2 critique

  • primary/backup with viewserver
  • pro:
    • conceptually simple
    • just two msgs per op (request, reply)
    • primary can do computation, send result to backup
    • only two k/v servers needed to tolerate one failure
    • works with network partition
  • con:
    • ViewServer is a single point of failure
    • order can be messy: e.g., new view, data to backup, ack, &c
    • tension if backup is slow / temporarily unavail
      1. primary can wait for backup -- slow
      2. viewserver can declare backup dead -- expensive, hurts fault tolerance

We would like a general-purpose ordering scheme with:

  • no single point of failure
  • graceful handling of slow / intermittent replicas
  • handling of network partitions

Paxos will be a key building block for this.

  • some number of nodes participate in an instance of Paxos
    • Q: What is this instance?
    • A: "Each new command requires a separate Paxos agreement, which the paper calls an instance. So the database replicas might agree that the first command to execute is 'command one', and they use Paxos to agree on that. Then that instance of Paxos is done. A while later another client sends 'command two'; the replicas start up an entirely separate instance of Paxos to agree on this second client command.' --RTM
  • each node knows the address of every other node for that instance
  • each instance of Paxos can reach consensus on at most one value typically, a system uses many instances of Paxos each instance usually decides one operation assumptions: asynchronous, non-Byzantine

What does Paxos provide? How does it work?

  • "black-box" interface to a Paxos instance, on each node:
    • Propose a value (e.g., operation)
    • Check what value has been decided, if any
    • [ Lab 3A: src/paxos/paxos.go: Start, Status ]
  • Correctness:
    • if agreement reached, all agreeing nodes see same value
  • Fault-tolerance:
    • can tolerate non-reachability of a minority of nodes (correctness implies they won't agree at all)
  • Liveness:
    • a majority must be alive and able to communicate reliably (minorities are not live)

How to build a system using Paxos?

  1. Primary/Backup like Lab 2, but use Paxos to replicate the ViewServer
    • [ next Tuesday's lecture will be about such a system ]
  2. Lab 3: no ViewServer, all replicas use Paxos instead of primary/backup

Replicating either the ViewServer or K/V server with Paxos is similar.

Will look at a sketch of how to do a Paxos-based K/V server.

The basic idea:

  • [ Diagram: clients, replicas, log in each replica, K/V layer, Paxos layer]
  • no viewserver
  • three replicas
  • clients can send RPCs to any replica (not just primary)
  • server appends each client op to a replicated log of operations
    • Put, Get (and more later)
  • log entries (instances) are numbered sequentially
  • Paxos ensures agreement on content of each log entry
  • separate Paxos agreement for each of these log entries
    • separate instance of Paxos algorithm is run for log entry #i
    • Q: Can one log entry be agreed on at the same time with another? What if they depend on one another like Put(k1, a) and Append(k1, b)?
    • A: Yes! They can be agreed upon on the same time.
    • A: you can have agreed on log entry #i before agreeing on log entry #i+1
      • This means the reply associated with the Get or Put request in log entry i+1 will have to wait for the other log entries to be set (interesting)
  • servers can throw away log entries that all other servers have agreed on (and responded to?)
    • but if a server crashes, the other servers will know to keep their log entries around for when it comes back
  • protocol does not require designated proposers or leaders for correctness
    • these only help w/ performance
    • low probability of proposing "livelock" that can be overcome by having proposers wait a random amount of time
  • once a Paxos node agrees on a value it never changes its mind

Example:

  • client sends Put(a, b) to S1
  • S1 picks a log entry 3
  • S1 uses Paxos to get all servers to agree that entry 3 holds Put(a,b)

Example:

  • client sends Get(a) to S2
  • S2 picks log entry 4
  • S2 uses Paxos to get all servers to agree that entry 4 holds Get(a)
  • S2 scans log up to entry 4 to find latest Put(a, ...)
    • TODO: O(n) worst case for doing a Get because you can have Put followed by a gazillion PutAppend's (or you can have just one Put stored way back?).
      • Can the replicas index their log? I suppose. If they all store it fully.
  • S2 replies with that value
    • S2 can cache content of DB up through last log scan

Q: Why a log?

  • Why not require all replicas to agree on each op in lock-step?
  • Allows one replica to fall behind, then catch up
    • e.g., if it is slow
    • other replicas do not have to wait
  • Allows one replica to crash and catch up
    • if it keeps state on disk
    • can replay missed operations
  • Allows pipelining/overlap of agreement
    • agreement turns out to require multiple message rounds

Q: What about agreement -- we need all replicas to have same op in each log slot

  • Provided by Paxos, as we will see next

Agreement is hard (1):

  • May be multiple proposals for the op in a particular log slot
  • Sx (server x) may initially hear of one, Sy may hear of another
  • Clearly one must later change its mind
  • Thus: multiple rounds, tentative initially
  • How do we know when agreement is permanent -- no longer tentative?

Agreement is hard (2):

  • TODO: If S1 and S2 agree, and S3 and S4 don't respond, are we done?
  • Agreement has to be able to complete even w/ failed servers
  • We can't distinguish failed server from network partition
  • So maybe S3/S4 are partitioned have "agreed" on a different operation!

Two main ideas in Paxos:

  1. Many rounds may be required but they will converge on one value
  2. A majority is required for agreement -- prevent "split brain"
    • Key point: any two majorities overlap
    • so any later majority will share at least one server w/ any earlier majority
    • so any later majority can find out what earlier majority decided (discussed below)

Lab 3B K/V server creates a separate Paxos instance for each client Put, Get (much harder)

  • rest of lecture focuses on agreement for a specific instance

Paxos sketch

  • each node consists of three logical entities:
    • proposer
    • acceptor
    • learner
  • each proposer wants to get agreement on its value
    • could try to use a "designated leader" to avoid dueling proposers
    • OK to have multiple proposers, so leader election can be approximate
  • proposer contacts acceptors, tries to assemble a majority
    • if a majority respond, we're done
  • in our K/V server example, roughly:
    • proposer gets RPC from client, proposes operation
    • acceptors are internal to Paxos, help decide consensus
    • learner figures out what operation was decided to run, responds to client

Broken strawman: can we do Paxos in a single round?

  • acceptor "accepts" the first value that it hears from proposer
  • when is consensus reached?
    • can we take the value with the most votes?
    • no, need a majority of accepts for the same value: floor(n/2)+1
    • otherwise, consensus on 2 different values (lossy/partitioned network)
  • Problem:
    • suppose we have 3 servers: S1, S2, S3
    • what if each server proposes + accepts its own value?
      • no majority, stuck
      • but maybe we can detect this situation and recover?
    • Worse: S3 crashes -> we may have reached majority, but we'll never know
  • need a way for acceptors to change their mind, if no consensus reached yet

Basic Paxos exchange

     proposer          acceptors

           prepare(n) ->
        <- prepare_ok(n, n_a, v_a)

           accept(n, v') ->
        <- accept_ok(n)

           decided(v') ->

Why n?

  • to distinguish among multiple rounds, e.g., proposer crashes, simul props
  • want later rounds to supersede earlier ones
  • numbers allow us to compare early/late
  • n values must be unique and roughly follow time
  • n = <time, server ID>
    • e.g., ID can be server's IP address
  • "round" is the same as "proposal" but completely different from "instance"
    • round/proposal numbers are WITHIN a particular instance

Definition: server S accepts n/v

  • it responded accept_ok to accept(n, v)

Definition: n/v is chosen

  • a majority of servers accepted n/v

The crucial property:

  • if a value was chosen, any subsequent choice must be the same value
    • i.e., protocol must not change its mind
    • maybe a different proposer &c, but same value!
    • this allows us to freely start new rounds after crashes &c
  • tricky b/c "chosen" is system-wide property
    • e.g., majority accepts, then proposer crashes
    • => no node can tell locally that agreement was reached

So:

  • proposer doesn't send out value with prepare (but sends it a bit later)
  • acceptors send back any value they have already accepted
  • if there is one, proposer proposes that value
    • to avoid changing an existing choice
  • if no value already accepted,
    • proposer can propose any value (e.g., a client request)
  • proposer must get prepare_ok from majority
    • to guarantee intersection with any previous majority,
    • to guarantee proposer hears of any previously chosen value

Now the protocol -- see the handout

    proposer(v):
      choose n, unique and higher than any n seen so far
      send prepare(n) to all servers including self
      if prepare_ok(n, n_a, v_a) from majority:
        v' = v_a with highest n_a; choose own v otherwise
        send accept(n, v') to all
        if accept_ok(n) from majority:
          send decided(n, v') to all

    acceptor state:
      must persist across reboots
      n_p (highest prepare seen)
      n_a, v_a (highest accept seen)

    acceptor's prepare(n) handler:
      if n > n_p
        n_p = n
        reply prepare_ok(n, n_a, v_a)
      else
        reply prepare_reject

    acceptor's accept(n, v) handler:
      if n >= n_p
        n_p = n
        n_a = n
        v_a = v
        reply accept_ok(n)
      else
        reply accept_reject

Example 1 (normal operation):

    `S1`, `S2`, `S3` 
    [ but `S3` is dead or slow ]

    `S1`: -> starts proposal w/ n=1 v=A
    `S1`: <- p1   <- a1vA    <- dA
    `S2`: <- p1   <- a1vA    <- dA
    `S3`: dead...

    "p1" means Sx receives prepare(n=1)
    "a1vA" means Sx receives accept(n=1, v=A)
    "dA" means Sx receives decided(v=A)
  • S1 and S2 will reply with prepare_ok(1, 0, null) to the p1 message.
  • If dA is lost, one of the nodes waiting can run Paxos again and try a new n higher than the previous one.
    • the prepare_ok(2, 1, 'A') reply will come back,
    • then the node is forced to send a2vA and hopefully this time, after the node gets the accept_ok message, it will send out dA messages that won't get lost again
  • a value is said to be chosen when a majority of acceptors in the accept handler take the accept branch and accept the value
    • however, not everyone will know this, so that's why the decide message is sent out

These diagrams are not specific about who the proposer is

  • it doesn't really matter
  • the proposers are logically separate from the acceptors
  • we only care about what acceptors saw and replied

Note Paxos only requires a majority of the servers

  • so we can continue even though S3 was down
  • proposer must not wait forever for any acceptor's response

What would happen if network partition?

  • i.e., S3 was alive and had a proposed value B
  • S3's prepare would not assemble a majority

The homework question

How does Paxos ensure that the following sequence of events can't happen? What actually happens, and which value is ultimately chosen?

  proposer 1 crashes after sending two accept() requests
  proposer 2 has a different value in mind

  A: p1 a1foo
  B: p1       p2 a2bar
  C: p1 a1foo p2 a2bar

  C's prepare_ok to B really included "foo"
    thus a2foo, and so no problem

The point:

  • if the system has already reached agreement, majority will know value
    • in this example, A and C agreed on 'foo'
  • any new majority of prepares will intersect that majority
    • in this example, AC intersects BC in C
  • so subsequent proposer will learn of already-agreed-on value
    • in this example, C will reply with a prepare_ok(n=2, n_a=1, v_a=foo) to proposer
  • and subsequent proposer will send already-agreed-on value in accept msg
    • in this example, proposer will send accept(n=2, v=foo)

Example 2 (concurrent proposers):

    A1 starts proposing n=10 by sending prepare(n=10) 
    A1 sends out just one accept v=10
    [ => A1 must have received prepare_ok from majority ]
    A3 starts proposing n=11
      but A1 does not receive its proposal
      [ => A1 never replies to A3 with prepare_ok(n=11, n_a=10, v=10) because it never got the prepare ]
      A3 only has to wait for a majority of proposal responses

    A1: p10 a10v10 
    A2: p10        p11
    A3: p10        p11  a11v11

    A1 and A3 have accepted different values!

What will happen?

  • Q: What will A2 do if it gets a10v10 accept msg from A1?
    • Recall: a10v10 means accept(n=10,v=10) which is sent by proposer after he receives a majority of <-prepare_ok's on n=10
    • A: A2 will reject because it has a higher np from p11
  • Q: What will A1 do if it gets a11v11 accept msg from A3?
    • A: A1 will reply accept_ok and change its value to 11 because n = 11 > np = 10
  • In other words, a value has not been chosen yet because no majority accepted the same value yet with the same proposal number.

What if A3 (2nd proposer) were to crash at this point (and not restart)?

  • TODO: Not sure which "point" they are referring to.
  • Case 1: If A3 crashes after proposing n=11 and after receiving a majority of prepare_ok's but before sending out the other two accept's to A1 and A2, then:
    • A1 could come back online and send an accept(n=10,v=10) to A2
    • A2 will reject because it has a higher np = 11 (see above and see algorithm)
    • A1 will repropose with higher n and eventually convince both itself and A2 to accept v10
  • Case 2: A3 crashes after proposing n=11 and receiving a majority of prepare_ok's and after sending out an accept to A2
    • A2 will accept_ok on v11
    • Now v11 is chosen: any future majority of prepare_ok's will return v11

How about this:

A1: p10  a10v10               p12
A2: p10          p11  a11v11  
A3: p10          p11          p12   a12v10

Has the system agreed to a value at this point?

What's the commit point?

  • i.e., exactly when has agreement been reached?
  • i.e., at what point would changing the value be a disaster?
  • after a majority has the same v_a? no -- why not? above counterexample
  • after a majority has the same v_a/n_a? yes -- why sufficient? sketch:
    • suppose majority has same v_a/n_a
    • acceptors will reject accept() with lower n
    • for any higher n: prepare's must have seen our majority v_a/n_a (overlap)
    • what if overlap servers saw prepare(n) before accept(v_a, n_a)?
      • would reject v_a/n_a
      • thus wouldn't have a majority yet
      • proposer might be free to choose v != v_a

Why does the proposer need to pick v_a with highest n_a?

    A1: p10  a10vA               p12
    A2: p10          p11  a11vB  
    A3: p10          p11  a11vB  p12   a12v??

    n=11 already agreed on vB
    n=12 sees both vA and vB, but must choose vB

Two cases:

  1. There was a majority before n=11
    • n=11's prepares would have seen value and re-used it
    • so it's safe for n=12 to re-use n=11's value
  2. There was not a majority before n=11
    • n=11 might have obtained a majority
    • so it's required for n=12 to re-use n=11's value

Why does prepare handler check that n > n_p?

  • it's taking max(concurrent n's), for accept handler
  • responding to all prepare() with prepare_ok() would be also fine,
    • but proposers with n < n_p would be ignored by accept() anyway

Why does accept handler check n >= n_p?

  • required to ensure agreement
  • there's a unique highest n active
  • everyone favors the highest n
  • without n >= n_p check, you could get this bad scenario:

Scenario:

    A1: p1 p2 a1vA
    A2: p1 p2 a1vA a2vB
    A3: p1 p2      a2vB

Why does accept handler update n_p = n?

  • required to prevent earlier n's from being accepted
  • node can get accept(n,v) even though it never saw prepare(n)
  • without n_p = n, can get this bad scenario:

Scenario:

    A1: p1    a2vB a1vA p3 a3vA
    A2: p1 p2           p3 a3vA
    A3:    p2 a2vB

What if new proposer chooses n < old proposer?

  • i.e., if clocks are not synced
  • cannot make progress, though no correctness problem

What if an acceptor crashes after receiving accept?

A1: p1  a1v1
A2: p1  a1v1 reboot  p2  a2v?
A3: p1               p2  a2v?

A2 must remember v_a/n_a across reboot! on disk
  might be only intersection with new proposer's majority
  and thus only evidence that already agreed on v1

What if an acceptor reboots after sending prepare_ok?

  • does it have to remember n_p on disk?
  • if n_p not remembered, this could happen:

Example:

  `S1`: p10            a10v10
  `S2`: p10 p11 reboot a10v10 a11v11
  `S3`:     p11               a11v11
  • 11's proposer did not see value 10, so 11 proposed its own value
  • but just before that, 10 had been chosen!
  • b/c S2 did not remember to ignore a10v10

Can Paxos get stuck?

  • Yes, if there is not a majority that can communicate
  • How about if a majority is available?
    • Possible to livelock: dueling proposers, keep prepare'ing higher n's
      • One reason to try electing a leader: reduce chance of dueling proposers
    • With single proposer and reachable majority, should reach consensus