Choose Your Own Consistency

A concurrent talk about CRDTs and Strong Consistency in Riak 2.0 --delivered with Chris Meiklejohn (@cmeik) at Erlang Factory Lite Toronto.

Transcript

1. Choose your own Consistency
2. A concurrent talk by: @cmeik & @tsantero
3. Leveraging Riak’s New Data Types Conflict-Free Replicated Datatypes Chris Meiklejohn @cmeik EFL Toronto 2013
4. cmeiklejohn
5. Riak Made Consistent Strong Consistency in Riak 2.0 Tom Santero @tsantero EFL Toronto 2013
6. @ basho.com tsantero
7. Riak Overview
8. Riak Overview Erlang implementation of Dynamo
9. Riak Overview Erlang implementation of Dynamo
10. Riak Object
11. Key Value
12. Riak Overview Consistent hashing
16. Riak Overview Dynamic membership
17. Riak Overview Replication factor
18. Replica Replica Replica
19. High Availability “...any non-failing node can respond to any request” --Gilbert & Lynch
20. Eventual Consistency “Eventual consistency is a consistency model used in distributed computing that informally guarantees that, if no new updates are made to a given data item, eventually all accesses to that item will return the last updated value.” --Wikipedia
21. Riak Overview Two Writes: {Writer, Value, Time}
22. [{a, v1, t1}] [{b, v1, t2}] [{a, v1, t1}]
23. Riak Overview Last Writer Wins Allow Mult
24. Riak Overview Last Writer Wins [{b, v1, t2}] [{b, v1, t2}] [{b, v1, t2}]
25. Riak Overview Allow Mult [{a, v1, t1}, {b, v1, t2}] [{a, v1, t1}, {b, v1, t2}] [{a, v1, t1}, {b, v1, t2}]
26. User specificed Merge
27. CRDTs
28. CRDTs Convergent Replicated Data Types
29. CRDTs Commutative Replicated Data Types
30. CRDTs Synchronization-free data structures
31. CRDTs Monotonic and confluent; convergent
32. CRDTs Create siblings; resolve via merge.
33. The Theory
37. Bounded Join Semilattices
38. Bounded Join Semilattices Partially ordered set; least upper bound; ACI.
39. Bounded Join Semilattices Associativity: (X · Y) · Z = X · (Y · Z)
40. Bounded Join Semilattices Commutativity: X · Y = Y · X
41. Bounded Join Semilattices Idempotence: X · X = X
42. Bounded Join Semilattices Objects grow over time; merge computes LUB
43. Bounded Join Semilattices Monotonic and confluent; convergent
44. Bounded Join Semilattices Map into another via monotone functions
45. Bounded Join Semilattices Examples
46. b a c a, b a, c a, b, c Set; merge function: union. b, c
47. 3 5 7 5 7 7 Increasing natural; merge function: max.
48. F F T F T T Booleans; merge function: or.
49. x <= y montone f(x) <= f(y)
50. CvRDT Examples OR-SET
51. [ [{1, a}], [] ] [ [{1, a}], [] ]
52. [ [{1, a}], [] ] [ [{1, a}], [] ] [ [{1, a}], [{1, a}] ] [ [{1, a}], [{1, a}] ]
53. [ [{1, a}], [] ] [ [{1, a}], [] ] [ [{1, a}], [{1, a}] ] [ [{1, a}], [{1, a}] ] [ [{1, a}, {2, a}], [{1, a}] ]
54. [ [{1, a}], [] ] [ [{1, a}], [] ] [ [{1, a}], [{1, a}] ] [ [{1, a}], [{1, a}] ] [ [{1, a}, {2, a}], [{1, a}] ] [ [{1, a}, {2, a}], [{1, a}] ]
55. [ [{1, a}], [] ] [ [{1, a}], [] ]
56. [ [{1, a}], [] ] [ [{1, a}], [] ] [ [{1, a}, {2, b}], [] ]
57. [ [{1, a}], [] ] [ [{1, a}], [] ] [ [{1, a}, {2, b}], [] ] [ [{1, a}], [{1, a}] ]
58. [ [{1, a}], [] ] [ [{1, a}], [] ] [ [{1, a}, {2, b}], [] ] [ [{1, a}], [{1, a}] ] [ [{1, a}, {2, b}], [{1, a}] ]
59. riak_dt git clone git@github.com:basho/riak_dt.git
60. -­‐type  crdt()  ::  term(). -­‐type  operation()  ::  term(). -­‐type  actor()  ::  term(). -­‐type  value()  ::  term(). -­‐type  error()  ::  term(). -­‐callback  new()  -­‐>  crdt(). -­‐callback  value(crdt())  -­‐>  term(). -­‐callback  value(term(),  crdt())  -­‐>  value(). -­‐callback  update(operation(),  actor(),  crdt())  -­‐>                         {ok,  crdt()}  |  {error,  error()}. -­‐callback  merge(crdt(),  crdt())  -­‐>  crdt(). -­‐callback  equal(crdt(),  crdt())  -­‐>  boolean(). -­‐callback  to_binary(crdt())  -­‐>  binary(). -­‐callback  from_binary(binary())  -­‐>  crdt(). riak_dt/src/riak_dt.erl
61. Riak 1.4
62. Riak 1.4 Counters: G-Counter, PN-Counter
63. Riak 1.4 Counters: non-idempotent; O(Actors)
64. riak_dt/src/riak_dt_gcounter.erl -­‐module(riak_dt_gcounter). -­‐export([new/0,  new/2,  value/1,  value/2,  update/3,  merge/2,  equal/2,  to_binary/1,  from_binary/1]). -­‐export_type([gcounter/0,  gcounter_op/0]). -­‐opaque  gcounter()  ::  orddict:orddict(). -­‐type  gcounter_op()  ::  increment  |  {increment,  pos_integer()}.
65. riak_dt/src/riak_dt_pncounter.erl -­‐module(riak_dt_pncounter). -­‐export([new/0,  new/2,  value/1,  value/2,                  update/3,  merge/2,  equal/2,  to_binary/1,  from_binary/1]). -­‐export_type([pncounter/0,  pncounter_op/0]). -­‐opaque  pncounter()    ::  [{Actor::riak_dt:actor(),  Inc::pos_integer(),                                                      Dec::pos_integer()}]. -­‐type  pncounter_op()  ::  riak_dt_gcounter:gcounter_op()  |  decrement_op(). -­‐type  decrement_op()  ::  decrement  |  {decrement,  pos_integer()}. -­‐type  pncounter_q()    ::  positive  |  negative.
66. Riak 2.0
67. Riak 2.0 Requires bucket types; HTTP and protobuff API
68. Riak 2.0 Caveats: 2i, MapReduce, Yokozuna
69. Riak 2.0 Sets: Add, Remove, Membership; Idempotent
70. Riak 2.0 Sets: Add wins; O(Actors + Elements)
71. riak_dt/src/riak_dt_gset.erl -­‐module(riak_dt_gset). -­‐behaviour(riak_dt). %%  API -­‐export([new/0,  value/1,  update/3,  merge/2,  equal/2,                  to_binary/1,  from_binary/1,  value/2]). -­‐export_type([gset/0,  binary_gset/0,  gset_op/0]). -­‐opaque  gset()  ::  members(). -­‐type  binary_gset()  ::  binary(). -­‐type  gset_op()  ::  {add,  member()}.
72. riak_dt/src/riak_dt_orset.erl -­‐module(riak_dt_orset). -­‐behaviour(riak_dt). %%  API -­‐export([new/0,  value/1,  update/3,  merge/2,  equal/2,                  to_binary/1,  from_binary/1,  value/2,  precondition_context/1]). -­‐export_type([orset/0,  binary_orset/0,  orset_op/0]). -­‐opaque  orset()  ::  orddict:orddict(). -­‐type  binary_orset()  ::  binary().  %%  A  binary  that  from_binary/1  will   operate  on. -­‐type  orset_op()  ::  {add,  member()}  |  {remove,  member()}  |                                        {add_all,  [member()]}  |  {remove_all,  [member()]}  |                                        {update,  [orset_op()]}. -­‐type  actor()  ::  riak_dt:actor(). -­‐type  member()  ::  term().
73. riak_dt/src/riak_dt_orswot.erl -­‐module(riak_dt_orswot). -­‐behaviour(riak_dt). -­‐export_type([orswot/0,  orswot_op/0,  binary_orswot/0]). -­‐opaque  orswot()  ::  {riak_dt_vclock:vclock(),  entries()}. -­‐type  binary_orswot()  ::  binary().  %%  A  binary  that  from_binary/1  will  operate   on. -­‐type  orswot_op()  ::    {add,  member()}  |  {remove,  member()}  |                                            {add_all,  [member()]}  |  {remove_all,  [member()]}  |                                            {update,  [orswot_op()]}. -­‐type  orswot_q()    ::  size  |  {contains,  term()}. -­‐type  actor()  ::  riak_dt:actor(). -­‐type  entries()  ::  [{member(),  minimal_clock()}]. -­‐type  minimal_clock()  ::  [dot()]. -­‐type  dot()  ::  {actor(),  Count::pos_integer()}. -­‐type  member()  ::  term().
74. Riak 2.0 Maps: Recursive; Associative Array; Nestable
75. Riak 2.0 Maps: Update wins; O(Actors + Elements)
76. riak_dt/src/riak_dt_map.erl -­‐module(riak_dt_map). -­‐behaviour(riak_dt). %%  API -­‐export([new/0,  value/1,  value/2,  update/3,  merge/2,                  equal/2,  to_binary/1,  from_binary/1,  precondition_context/1]). -­‐export_type([map/0,  binary_map/0,  map_op/0]). -­‐type  binary_map()  ::  binary().  %%  A  binary  that  from_binary/1  will  accept -­‐type  map()  ::  {riak_dt_vclock:vclock(),  valuelist()}. -­‐type  field()  ::  {Name::term(),  Type::crdt_mod()}. -­‐type  crdt_mod()  ::  riak_dt_pncounter  |  riak_dt_lwwreg  |                                        riak_dt_od_flag  |                                        riak_dt_map  |  riak_dt_orswot. -­‐type  valuelist()  ::  [{field(),  entry()}]. -­‐type  entry()  ::  {minimal_clock(),  crdt()}. -­‐type  crdt()    ::    riak_dt_pncounter:pncounter()  |  riak_dt_od_flag:od_flag()  |                                    riak_dt_lwwreg:lwwreg()  |                                    riak_dt_orswot:orswot()  |                                    riak_dt_map:map(). -­‐type  map_op()  ::  {update,  [map_field_update()  |  map_field_op()]}.
77. Riak 2.0 Maps: LWW-Register, Booleans, Sets, and Maps
78. Riak 2.0 LWW-Register: last writer wins
79. -­‐module(riak_dt_lwwreg). -­‐export([new/0,  value/1,  value/2,  update/3,  merge/2,                  equal/2,  to_binary/1,  from_binary/1]). -­‐export_type([lwwreg/0,  lwwreg_op/0]). -­‐opaque  lwwreg()  ::  {term(),  non_neg_integer()}. -­‐type  lwwreg_op()  ::  {assign,  term(),  non_neg_integer()}    |  {assign,   term()}. -­‐type  lww_q()  ::  timestamp. riak_dt/src/riak_dt_lwwreg.erl
80. Riak 2.0 Boolean: Enabled, Disabled; O(Actors)
81. -­‐module(riak_dt_enable_flag). -­‐behaviour(riak_dt). -­‐export([new/0,  value/1,  value/2,  update/3,  merge/2,  equal/2,  from_binary/1,  to_binary/1]). riak_dt/src/riak_dt_enable_flag.erl
82. This project is funded by the European Union, 7th Research Framework Programme, ICT call 10, grant agreement n°609551.
84. Strong Consistency
85. Strong Consistency Why?
86. Strong Consistency Why? atomicity
87. Strong Consistency Why? recency
88. Strong Consistency Why? partial writes
89. A A A
90. A A A Val = <<“B”>>.
91. A A A Val = <<“B”>>.
92. B A A
93. B A A riakc_pb_socket:get(PBC, <<“Bucket”>>, <<“Key”>>).
94. B A A riakc_pb_socket:get(PBC, <<“Bucket”>>, <<“Key”>>).
95. B A A riakc_pb_socket:get(PBC, <<“Bucket”>>, <<“Key”>>). B B
96. Strong Consistency Single key atomic operations
97. Strong Consistency any get sees most recent put
98. Strong Consistency get/modify/put cycle fails if object is changed
99. Strong Consistency put w/o vclock fails if object exists
100. Consensus
101. Distributed Consensus “The problem of reaching agreement among remote processes is one of the most fundamental problems in distributed computing and is at the core of many algorithms for distributed data processing, distributed file management, and fault-tolerant distributed applications.” --Fischer, Lynch, Paterson
102. Consensus Guarantees: termination, agreement, validity
103. Termination processes eventually decide on a value
104. Agreement processes that decide do so on the same value
105. Validity values must have been proposed
106. Consensus Algorithms
107. Consensus Algorithms Paxos, ZAB, Raft
108. Paxos Lamport 1990: “The Part-Time Parliament”
110. ZAB ZooKeeper Atomic Broadcast
111. Raft Ousterhout, Ongaro 2013: In search of an understandable consensus algorithm
112. Paxos coordinated requests; leaders
113. Paxos 2 round trips / request
114. Node 1 Node 2 Node 3 N++ prepare(N) promise(N, V ) b promise(N, V ) c V = f(V , V , V ) b c a N commit(N, V ) N accept(N)
115. Multi-Paxos First Request
116. Node 1 Node 2 Node 3 N++; I = 0 prepare(N, I) promise(N, I, V ) b promise(N, I, V ) c V = f(V , V , V ) b c a N commit(N, I, V ) N accept(N, I)
117. Multi-Paxos Each Additional Request
118. Node 1 Node 2 Node 3 I++ commit(N, I, V) accept(N, I)
119. Multi-Paxos Each Request: ship entire state
120. Riak
121. Riak Key/Value
122. Riak Keys are Independent
123. Riak read-repair, active-anti entropy
124. Riak individual key = isolated state
125. Riak multi-paxos per key
126. Consensus Groups
127. Consensus Groups participants in decisioning; ensembles
128. Consensus Groups preference lists (preflists) in Riak
129. preflist
134. Consensus Groups one group/ensemble per preflist
135. Consensus Groups ring_size = 256, 256 ensembles
136. Ensembles
137. Ensembles leader election; Epochs; get/put operations
138. Get Operations leader reads local object; if Epoch old: refresh
139. Node 1 Node 2 Node 3 obj.epoch < epoch get(key) reply(Epoch , Seq , Val ) b Val = latest(Val , Val , Val ) Val.epoch = epoch write(Epoch, ++Seq, Val) ack(Epoch, Seq) b b reply(Epoch , Seq , Val ) c c c a b c
140. Node 1 Node 2 Node 3 obj.epoch == epoch Reply = local_get(Key)
141. Get Operations Worst Case: 2 roundtrips / request Best Case: 0 roundtrips / request
142. Put Operations leader reads local object; if Epoch old: refresh modify object commit modified object
143. Node 1 Node 2 Node 3 obj.epoch < epoch get(key) reply(Epoch , Seq , Val ) b Latest = latest(Val , Val , Val ) Val = modify(Latest) write(Epoch, ++Seq, Val) ack(Epoch, Seq) b b reply(Epoch , Seq , Val ) c c c a b c
144. Node 1 Node 2 Node 3 obj.epoch == epoch Latest = local_get(Key) Val = modify(Latest) write(Epoch, ++Seq, Val) ack(Epoch, Seq)
145. Put Operations Worst Case: 2 roundtrips / write Best Case: 1 roundtrips / write
146. Failed Quorums Leader Re-election ; new Epoch
147. Cluster Membership
148. Cluster Membership add/remove nodes
149. Cluster Membership consensus state
150. Cluster Membership multi-paxos joint consensus
151. Existing Ensemble Joining Ensemble riak_01 riak_02 riak_03 riak_07 riak_08 riak_09 [{riak_01}, {riak_02}, {riak_03}] [{riak_07}, {riak_08}, {riak_09}]
152. Joint-Consensus Ensemble [{riak_01}, {riak_02}, {riak_03}, {riak_07}, {riak_08}, {riak_09}]
153. Joint-Consensus Ensemble [{riak_01}, {riak_02}, {riak_03}, {riak_07}, {riak_08}, {riak_09}]
154. New Ensemble riak_07 riak_08 riak_09 [{riak_07}, {riak_08}, {riak_09}]
155. Q & A