Slide 1
Slide 1 text
Moving elements
in list CRDTs
Martin Kleppmann
University of Cambridge, UK
[email protected]
Slide 2
Slide 2 text
No content
Slide 3
Slide 3 text
No content
Slide 4
Slide 4 text
No content
Slide 5
Slide 5 text
No content
Slide 6
Slide 6 text
No content
Slide 7
Slide 7 text
No content
Slide 8
Slide 8 text
No content
Slide 9
Slide 9 text
No content
Slide 10
Slide 10 text
No content
Slide 11
Slide 11 text
List (sequence, array) CRDTs: WOOT, Treedoc, Logoot,
RGA, Causal Trees, LSEQ, …
Slide 12
Slide 12 text
List (sequence, array) CRDTs: WOOT, Treedoc, Logoot,
RGA, Causal Trees, LSEQ, …
Emulating “move” as “delete-and-reinsert”:
concurrent moves of the same item
à duplication
Slide 13
Slide 13 text
Concurrent move of the same item to different
positions — what should happen?
Slide 14
Slide 14 text
Converge to one of the destination positions
(pick one arbitrarily but deterministically)
Slide 15
Slide 15 text
“pick one arbitrarily”
= last-writer wins register!
posphone joe
:= “head of the list”
merge
posphone joe
== “head of the list”
posphone joe
:= “after buy milk”
Slide 16
Slide 16 text
List CRDT with move operation
posphone joe
:= “after buy milk”
need one register per list item
state = AWSet({ (v1
, LWWRegister(p1
)),
(v2
, LWWRegister(p2
)),
… })
need a stable way of referencing list positions
Treedoc: path through binary tree
Logoot: list of (integer, replicaID) pairs
RGA: s4vector
Causal Trees: logical timestamp
etc…
Composition of any list CRDT + AWSet +
LWWRegister = another CRDT
Slide 17
Slide 17 text
Moving ranges of elements
Slide 18
Slide 18 text
Moving ranges of elements
Slide 19
Slide 19 text
Desired outcome
Slide 20
Slide 20 text
Actual outcome
Fixing this: an open problem!