Convergent Replicated Data Types

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@vanstee [email protected] Patrick Van Stee

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“Structuring data so conflict resolution is automatic”

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Dynamo Trade consistency for availability and partition tolerance

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Node 1 Node 2 { a: “200” } { a: “-3000” } :(

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Node 1 Node 2 { a: “200” } { a: “-3000” } { a: “200”, a: “-3000” } Riak

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CRDT •Counter •Set •Graph •List •Tree

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CRDT •Counter •Set •Graph •List •Tree

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Counter •Can only be increased •Merge: use largest value

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Chat Room Karma Fault-Tolerant Distributed

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Serious Business { SB[PVS]: 0} Serious Coding { SB[PVS]: 1} # PVS++ { SC[PVS]: 0} { SC[PVS]: 5} # PVS += 5 { SB[PVS]: 2} # PVS++ Merge { SB[PVS]: 2, SC[PVS]: 5} { SB[PVS]: 1, SC[PVS]: 5} { SB[PVS]: 2, SC[PVS]: 5} { PVS: 7}

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What about decrements? Use separate counters for positive and negative. Take the difference for the result.

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Serious Business { SB[PVS+]: 0, SB[PVS-]: 0 } Serious Coding { SB[PVS+]: 1, SB[PVS-]: 0 } # PVS++ { SB[PVS+]: 0, SB[PVS-]: 0 } { SB[PVS+]: 5, SB[PVS-]: 0 } # PVS += 5 { SB[PVS+]: 1, SB[PVS-]: 1 } # PVS-- Merge { PVS+: 6, PVS-: 1 } { PVS+: 6, PVS-: 0 } { PVS+: 6, PVS-: 1 } { PVS: 5 }

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Set •Can only be added to •Merge: take the union

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Serious Business { PVS: [ ] } Serious Coding { PVS: [] } { PVS: [] } { PVS: [ ] } Merge { PVS: [ , ] } { PVS: [ , ] } { PVS: [ , , ] } { PVS: [ , , ] }

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Hold on, Can we remove stuff? Use separate sets for added and removed elements.

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Serious Business { PVS+: [ ], PVS-: [] } Serious Coding { PVS+: [], PVS-: [] } { PVS+: [], PVS-: [] } { PVS+: [ ], PVS-: [] } Merge { PVS+: [ , ], PVS-: [ ] } { PVS+: [ ], PVS-: [ ] } { PVS+: [ , ], PVS-: [] } { PVS+: [ , ], PVS-: [ ] } { PVS: [ ] }

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But wait, we can only remove once?

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U-Set •Append unique values to all set members •Garbage collection becomes an issue