Dynamo concepts in depth. Pavlo Baron, codecentric AG Friday, - - PowerPoint PPT Presentation

Dynamo concepts in depth. Pavlo Baron, codecentric AG

Friday, August 31, 12

Pavlo Baron

pavlo.baron@codecentric.de @pavlobaron

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The shopping cart case

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The 2 AM alarm call case

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The Tower of Babel case

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The Neo vs. Smiths case

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The Pavlo case

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So Dynamo isn’t about speed. It’s about immediate, reliable writes. It’s about

• peration relaxation.

It’s about distribution and fault tolerance. It’s about almost linear scalability.

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Time and timestamps

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Clocks V(i), V(j): competing Conflict resolution: 1: siblings, client 2: merge, system 3: voting, system

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Node 1 Node 2 Node 3 1,0,0 1,2,0 3,2,0 1,3,3 1,1,0 1,0,1 1,2,2 1,2,3 2,2,0 4,3,3 4,4,3 4,3,4

Vector clocks

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Node 2 Node 3 Node 4 1,1,0,0 1,0,1,0 1,0,0,1 1,3,0,3 1,2,0,2 1,2,0,3

Vector clocks

Node 1 1,0,0,0 1,2,0,0 1,0,2,0

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O(1) for data lookups / delta tracking

#

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Merkle Trees N, M: nodes HT(N), HT(M): hash trees M needs update:

• btain HT(N)

calc delta(HT(M), HT(N)) pull keys(delta)

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Node a.1 Node a.2

a ab ac abc abd acb acc

Merkle Trees

a ab ad abe abd ada adb

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Node a.1 Node a.2

a ab abc abd

Merkle Trees

a ab ad abd ada adb

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“Equal” nodes based decentralized distribution

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Consensus, agreement, voting, quorum

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Consistent hashing - the ring X bit integer space 0 <= N <= 2 ^ X

• r: 2 x Pi

0 <= A <= 2 x Pi x(N) = cos(A) y(N) = sin(A)

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Quorum V: vnodes holding a key W: write quorum R: read quorum DW: durable write quorum W > 0.5 * V R + W > V

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Key = “foo” # = N, W = 2

N

Insert key (sloppy quorum)

replicate

• k

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leave

Add node

c

• p

y copy leave leave copy

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Key = “foo” # = N, R = 2

N

Lookup key (sloppy quorum)

Value = “bar”

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leave

Remove node

copy

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Node 1 Node 2 Node 3 update

Gossip – node down/up

Node 4 update update, 4 down read read, 4 up update

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Eventual consistency

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BASE Basically Available, Soft-state, Eventually consistent Opposite to ACID

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Read your write consistency

write v 2 read v2

FE1 v 2 Data store v 3 v 1

write v 1 read v1

FE2

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Session 2 Session 1

Session consistency

write v 2 read v2

FE v 2 Data store v 3 v 1

write v 1 read v1

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Monotonic read consistency

read v 2 read v2

FE1 v 2 Data store v 3 v 1

read v 3 read v4

FE2 v 4

read v3

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Monotonic write consistency

write v 1 write v2

FE1 Data store v 1

read v 3 read v3

FE2 v 4 v 2 v 3

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Eventual consistency

read v 1 read v2

FE1 Data store v 3

write v 3

FE2

read v3

v 1

read v2

v 2

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Hinted handofg N: node, G: group including N node(N) is unavailable replicate to G or store data(N) locally hint handofg for later node(N) is alive handofg data to node(N)

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Key = “foo”

N replicate

Key = “foo”, # = N -> handofg hint = true

Direct replica fails

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Replica recovers

handofg

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N

Key = “foo”, # = N -> handofg hint = true

All replicas fail

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All replicas recover

replicate handofg

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Latency is an adjustment screw

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Availability is an adjustment screw

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CAP – the variations CA – irrelevant CP – eventually unavailable ofgering maximum consistency AP – eventually inconsistent ofgering maximum availability

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CAP – the tradeofg A C

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CP

Replica 1 Replica 2

v 1

read write v 2 read

v 1 v 2 v 2

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CP (partition)

Replica 1 Replica 2

v 1

read write v 2 read

v 1 v 2

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AP

Replica 1 Replica 2

v 1

read write v 2 read

v 1 v 2 v 2

replicate

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AP (partition)

Replica 1 Replica 2

v 1

read write v 2 read

v 1 v 2 v 2

hint handofg

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Frequent structure changes

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Thank you

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Many graphics I’ve created myself Some images originate from istockphoto.com except few ones taken from Wikipedia and product pages

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