relational reasoning for mergeable replicated data types
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Relational Reasoning for Mergeable Replicated Data Types KC Sivaramakrishnan joint work with Gowtham Kaki, Swarn Priya, Suresh Jagannathan 1 INTERNET INTERNET INTERNET INTERNET Serializability Linearizability Weak

  1. module Counter : sig type t val read : t -> int 7 val add : t -> int -> t +1 *3 val sub : t -> int -> t val mult : t -> int -> t val merge : lca:t -> v1:t -> v2:t -> t 8 21 end = struct type t = int let read x = x let add x d = x + d let sub x d = x - d let mult x n = x * n let merge ~lca ~v1 ~v2 = lca + (v1 - lca) + (v2 - lca) end � 12

  2. module Counter : sig type t val read : t -> int 7 val add : t -> int -> t +1 *3 val sub : t -> int -> t val mult : t -> int -> t val merge : lca:t -> v1:t -> v2:t -> t 8 21 end = struct type t = int let read x = x let add x d = x + d let sub x d = x - d 22 let mult x n = x * n let merge ~lca ~v1 ~v2 = lca + (v1 - lca) + (v2 - lca) end � 12

  3. module Counter : sig type t val read : t -> int 7 val add : t -> int -> t +1 *3 val sub : t -> int -> t val mult : t -> int -> t val merge : lca:t -> v1:t -> v2:t -> t 8 21 end = struct type t = int let read x = x let add x d = x + d let sub x d = x - d 22 22 let mult x n = x * n let merge ~lca ~v1 ~v2 = lca + (v1 - lca) + (v2 - lca) end � 12

  4. module Counter : sig type t val read : t -> int 7 val add : t -> int -> t +1 *3 val sub : t -> int -> t val mult : t -> int -> t val merge : lca:t -> v1:t -> v2:t -> t 8 21 end = struct type t = int let read x = x let add x d = x + d let sub x d = x - d 22 22 let mult x n = x * n let merge ~lca ~v1 ~v2 = 22 = 7 + (8-1) + (21 -7) lca + (v1 - lca) + (v2 - lca) end � 12

  5. module Counter : sig type t val read : t -> int 7 val add : t -> int -> t +1 *3 val sub : t -> int -> t val mult : t -> int -> t val merge : lca:t -> v1:t -> v2:t -> t 8 21 end = struct type t = int let read x = x let add x d = x + d let sub x d = x - d 22 22 let mult x n = x * n let merge ~lca ~v1 ~v2 = 22 = 7 + (8-1) + (21 -7) lca + (v1 - lca) + (v2 - lca) end • 3-way merge function makes the counter suitable for distribution � 12

  6. module Counter : sig type t val read : t -> int 7 val add : t -> int -> t +1 *3 val sub : t -> int -> t val mult : t -> int -> t val merge : lca:t -> v1:t -> v2:t -> t 8 21 end = struct type t = int let read x = x let add x d = x + d let sub x d = x - d 22 22 let mult x n = x * n let merge ~lca ~v1 ~v2 = 22 = 7 + (8-1) + (21 -7) lca + (v1 - lca) + (v2 - lca) end • 3-way merge function makes the counter suitable for distribution • Does not appeal to individual operations => independently extend data-type � 12

  7. Systems ➞ PL � 13

  8. Systems ➞ PL • CRDTs need to take care of systems level concerns such as exactly once delivery � 13

  9. Systems ➞ PL • CRDTs need to take care of systems level concerns such as exactly once delivery • 3-way merge handles it automatically � 13

  10. Systems ➞ PL 7 • CRDTs need to take care of +1 *3 systems level concerns such as exactly once delivery 8 21 • 3-way merge handles it automatically 22 22 � 13

  11. Systems ➞ PL 7 • CRDTs need to take care of +1 *3 systems level concerns such as exactly once delivery 8 21 • 3-way merge handles it automatically 22 22 ?? � 13

  12. Systems ➞ PL 7 • CRDTs need to take care of +1 *3 systems level concerns such as exactly once delivery 8 21 • 3-way merge handles it automatically 22 22 22 ?? 22 = 21 + (21-21) + (22 -21) � 13

  13. Does the 3-way merge idea generalise?

  14. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end � 15

  15. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations � 15

  16. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] � 15

  17. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] pop() � 1 [2] � 15

  18. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] pop() � 1 pop() � 1 [2] [2] � 15

  19. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] pop() � 1 pop() � 1 [2] [2] pop() [ ] � 15

  20. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] pop() � 1 pop() � 1 [2] [2] pop() pop() [ ] [ ] � 15

  21. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] pop() � 1 pop() � 1 [2] [2] pop() pop() [ ] [ ] • Convergence is not sufficient; Intent is not preserved � 15

  22. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] [1] pop() � 1 pop() � 1 [2] [2] pop() pop() [ ] [ ] • Convergence is not sufficient; Intent is not preserved � 15

  23. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] [1] pop() � 1 pop() � 1 push(2) [2] [2] [1,2] pop() pop() [ ] [ ] • Convergence is not sufficient; Intent is not preserved � 15

  24. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] [1] pop() � 1 pop() � 1 push(2) push(3) [2] [2] [1,2] [1,3] pop() pop() [ ] [ ] • Convergence is not sufficient; Intent is not preserved � 15

  25. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] [1] pop() � 1 pop() � 1 push(2) push(3) [2] [2] [1,2] [1,3] pop() push(3) pop() [ ] [ ] [1,2,3] • Convergence is not sufficient; Intent is not preserved � 15

  26. module type Queue = sig type 'a t val push : 'a t -> 'a -> 'a t val pop : 'a t -> ('a * 'a t) option (* at-least once semantics *) end • Try replicating queues by asynchronously transmitting operations [1,2] [1] pop() � 1 pop() � 1 push(2) push(3) [2] [2] [1,2] [1,3] pop() push(2) push(3) pop() [ ] [ ] [1,2,3] [1,3,2] • Convergence is not sufficient; Intent is not preserved � 15

  27. Concretising Intent � 16

  28. Concretising Intent • Intent is a woolly term ★ How can we formalise the intent of operations on a data structure? � 16

  29. Concretising Intent • Intent is a woolly term ★ How can we formalise the intent of operations on a data l structure? v1 v2 v � 16

  30. Concretising Intent • Intent is a woolly term ★ How can we formalise the intent of operations on a data l structure? • For a replicated queue, v1 v2 v � 16

  31. Concretising Intent • Intent is a woolly term ★ How can we formalise the intent of operations on a data l structure? • For a replicated queue, v1 v2 1. Any element popped in either v1 or v2 does not remain in v v � 16

  32. Concretising Intent • Intent is a woolly term ★ How can we formalise the intent of operations on a data l structure? • For a replicated queue, v1 v2 1. Any element popped in either v1 or v2 does not remain in v v 2. Any element pushed into either v1 or v2 appears in v � 16

  33. Concretising Intent • Intent is a woolly term ★ How can we formalise the intent of operations on a data l structure? • For a replicated queue, v1 v2 1. Any element popped in either v1 or v2 does not remain in v v 2. Any element pushed into either v1 or v2 appears in v 3. An element that remains untouched in l, v1, v2 remains in v � 16

  34. Concretising Intent • Intent is a woolly term ★ How can we formalise the intent of operations on a data l structure? • For a replicated queue, v1 v2 1. Any element popped in either v1 or v2 does not remain in v v 2. Any element pushed into either v1 or v2 appears in v 3. An element that remains untouched in l, v1, v2 remains in v 4. Order of pairs of elements in l, v1, v2 must be preserved in m, if those elements are present in v. � 16

  35. Relational Specification � 17

  36. Relational Specification • Let’s define relations R mem and R ob to capture membership and ordering ★ R mem [1,2,3] = {1,2,3} ★ R ob [1,2,3] = { (1,2), (1,3), (2,3) } � 17

  37. Relational Specification • Let’s define relations R mem and R ob to capture membership and ordering l ★ R mem [1,2,3] = {1,2,3} v1 v2 ★ R ob [1,2,3] = { (1,2), (1,3), (2,3) } v � 17

  38. Relational Specification • Let’s define relations R mem and R ob to capture membership and ordering l ★ R mem [1,2,3] = {1,2,3} v1 v2 ★ R ob [1,2,3] = { (1,2), (1,3), (2,3) } v � 17

  39. Relational Specification • Let’s define relations R mem and R ob to capture membership and ordering l ★ R mem [1,2,3] = {1,2,3} v1 v2 ★ R ob [1,2,3] = { (1,2), (1,3), (2,3) } v 1.Any element popped in either v1 or v2 does not remain in v � 17

  40. Relational Specification • Let’s define relations R mem and R ob to capture membership and ordering l ★ R mem [1,2,3] = {1,2,3} v1 v2 ★ R ob [1,2,3] = { (1,2), (1,3), (2,3) } v 1.Any element popped in either v1 or v2 does not remain in v 2. Any element pushed into either v1 or v2 appears in v � 17

  41. Relational Specification • Let’s define relations R mem and R ob to capture membership and ordering l ★ R mem [1,2,3] = {1,2,3} v1 v2 ★ R ob [1,2,3] = { (1,2), (1,3), (2,3) } v 1.Any element popped in either v1 or v2 does not remain in v 2. Any element pushed into either v1 or v2 appears in v 3. An element that remains untouched in l, v1, v2 remains in v � 17

  42. Relational Specification l v1 v2 v � 18

  43. Relational Specification l v1 v2 v • RHS has to be confined to R mem (v) x R mem (v) since certain orders might be missing ★ Consider l = [0], v1= [0,1], v2 = [ ], v = [1] � 18

  44. Relational Specification l v1 v2 v • RHS has to be confined to R mem (v) x R mem (v) since certain orders might be missing ★ Consider l = [0], v1= [0,1], v2 = [ ], v = [1] • RHS is an underspecification since orders between concurrent insertions will only be present in R ob (v) ★ Consider l = [ ], v1= [0], v2 = [1], v = [0,1] � 18

  45. � 19

  46. R mem = {1,2} R ob = { (1,2) } [1,2] � 19

  47. R mem = {1,2} R ob = { (1,2) } [1,2] pop() � 1 R mem = {2} [2] R ob = { } � 19

  48. R mem = {1,2} R ob = { (1,2) } [1,2] pop() � 1 pop() � 1 R mem = {2} R mem = {2} [2] [2] R ob = { } R ob = { } � 19

  49. R mem = {1,2} R ob = { (1,2) } [1,2] pop() � 1 pop() � 1 R mem = {2} R mem = {2} [2] [2] R ob = { } R ob = { } R mem = {2} R mem = {2} [2] [2] R ob = { } R ob = { } � 19

  50. � 20

  51. R mem = {1} R ob = { } [1] � 20

  52. R mem = {1} R ob = { } [1] push(2) R mem = {1,2} [1,2] R ob = { (1,2) } � 20

  53. R mem = {1} R ob = { } [1] push(2) push(3) R mem = {1,2} R mem = {1,3} [1,2] [1,3] R ob = { (1,2) } R ob = { (1,3) } � 20

  54. R mem = {1} R ob = { } [1] push(2) push(3) R mem = {1,2} R mem = {1,3} [1,2] [1,3] R ob = { (1,2) } R ob = { (1,3) } Use < as an arbitration function between concurrent insertions � 20

  55. R mem = {1} R ob = { } [1] push(2) push(3) R mem = {1,2} R mem = {1,3} [1,2] [1,3] R ob = { (1,2) } R ob = { (1,3) } [1,2,3] [1,3,2] R mem = {1,2,3} R mem = {1,2,3} R ob = { (1,2), (1,3), (2,3) } R ob = { (1,2), (1,3). (2,3) } Use < as an arbitration function between concurrent insertions � 20

  56. Characteristic Relations A sequence of relations R T is called a characteristic relation of a data type T, if for every x : T and y : T , R T ( x ) = R T ( y ) i ff x and y are extensionally equal as interpreted under T . � 21

  57. Characteristic Relations A sequence of relations R T is called a characteristic relation of a data type T, if for every x : T and y : T , R T ( x ) = R T ( y ) i ff x and y are extensionally equal as interpreted under T . • R mem and R ob are the characteristic relations of queue � 21

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