CHUBBYand

PAXOS

Sergio Bernales

1Dennis Kafura – CS5204 – Operating Systems

chubby and paxos

Overview

Chubby Why? How? More precise how

Paxos Why? How? More precise why

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chubby and paxos

10 – 30,000 Clients

Different client services Who is in charge? Guarantee it? What if you go down?

i.e. DNS Server

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chubby and paxos

Service Requirements

Easy for client developers (of the variety kind) Reliable and Available Master is clear –hint, hint Paxos Thousands of clients, tiny things Growing clients = growing transactions

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chubby and paxos

Chubby

Coarse-grained lock service Caching Event Notification Access Control Lists /ls/foo/wombat/pouch

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chubby and paxos

System Structure

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Cache: data, metadata, absent file, handles(invalidated when data is changed)

chubby and paxos

Files, Directories, Handles

/ls/foo/wombat/pouch ACL

Controls: reading, writing, ACL changes ACL authentication in RPC

Meta-data of node (64 bits entries)

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Instance NumberContent generation number

Lock generationACL generation

chubby and paxos

Sequencer

Sequence numbers provided only for interactionc that use locks

Request sequencer Name of lock Acquisition mode (exclusive, shared) Lock generation number

Lock delay

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chubby and paxos

API (1 of 2)

Open(/ls/foo/wombat/pouch )* Close()

Poison() GetContentAndStats()

getStat() ReadDir()

SetContents() Compare and swap

Delete() Acquire(), TryAcquire(), Release()

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chubby and paxos

API (2 of 2)

GetSequencer() SetSequencer() CheckSequencer()

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chubby and paxos

Sessions

Keeps locks active Data consistent Survives failures KeepAlives

Most calls are KeepAlives

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chubby and paxos

Events

Subcribe to Contents modified Child node added Chubby master fail Handle has become invalid

Lock acquired Conflicting lock request

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chubby and paxos

Efficiency

Atomic operations No current directories No last access times Proxies KeepAlive replies used to transmit events and

cache invalidations

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chubby and paxos

Back to the DNS Server

Chubby caches Chubby batches

Don’t use as fileserver Do use for config files, elect masters

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chubby and paxos

Master Fail - Over

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chubby and paxos

Recover

1. Pick a new client epoch number(sent by clients)2. Master responds to master location requests3. Rebuilds session memory strucuture4. KeepAlives allowed5. Notifies clients of failure6. Master waits sessions syncs7. Fully operational8. While operating, handles are verified9. Ephemeral files removed

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chubby and paxos

Intro Paxos

A consensus algorithm1. Only a value that has been proposed may be

chosen2. Only a single value is chosen3. Unless value actually chosen, process don’t learn

about it If someone disagrees, no one has agreed

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chubby and paxos

Paxos

Proposers, Acceptors, Learners

Multiples of each System assumes failure Data never corrupted

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chubby and paxos

Choosing a Value

P1- An acceptor must accept the first proposal it receives

Problem Several values proposed, none chosen Possible to accept multiple

A solution: number proposals

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chubby and paxos

Choosing a Value

P2- If a proposal with value v is chosen, then every higher numbered proposal that is chosen has value v

P2a- If a proposal with value v is chosen, then every higher accepted by any proposer has value v

P2b- If a proposal with value v is chosen, then every higher accepted by any proposer has value v

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chubby and paxos

Choosing a Value

P2c- For any v and n, if a proposal with value v and number n is issued, then there is a set S consisting of a majority of acceptors such that either (a) no acceptor in S has accepted any proposal numbered less than n, or (b) v is the value of the highest-numbered proposal among all proposals numbered less than n accepted by the acceptors in S

P1a- An acceptor can accept a proposal, iff if has not reponded to a prepare request having a number greater than n

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chubby and paxos

Algorithm

Phase 1a) Proposer selects proposal number n and sends prepare

request to majority of acceptorsb) If acceptor receives request number n greater than

that of any other prepare request it has accepted, it sends back promise not to accept any more and also it’s last n

Phase 2a) Proposer receives prepare request form majority of

acceptors, then sends the proposal number n plus the actual value v

b) If acceptor receives accept request for proposal n, accepts it only if it has not responded to another prepare request.

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chubby and paxos

Learn a Chosen Value

Chosen value propagated All acceptors send messages to learners Distinguished learner gets the message, passes it on Distinguished learnerS Learner ask acceptors

Distinguished Acceptors

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