The document describes distributed sagas, a protocol for coordinating microservices. Distributed sagas define a directed acyclic graph of requests and compensating requests that represent a single business transaction. A saga execution coordinator executes the requests, tracking their state in a fault-tolerant log. This allows transactions to complete fully or be compensated if a request fails, without the need for two-phase commit across services. Distributed sagas provide isolation and modularity to microservices architectures compared to other approaches to distributed transactions.

1. Distributed Sagas
A Protocol for Coordinating Microservices

2. Caitie McCaffrey
Distributed Systems Engineer
caitiem.com
@caitie

3. Service ServiceService
Monoliths

4. Microservices & NoSQL

5. 2015

6. 2015
“Application-level
Mechanisms for maintaining
database integrity”

7. Reserve a Hotel Front End

8. Reserve a Hotel Front End
Feral Concurrency
Control Mechanisms

9. Reserve a Hotel Front End
Feral Concurrency
Control Mechanisms

10. Reserve a Car Front End

11. Reserve a Flight Front End

12. Trips Front End

13. Trips Front End
Trips

14. Trips Front End
Trips
Feral Concurrency
Control Mechanisms

15. Trips Front End
Trips
Feral Concurrency
Control Mechanisms

16. Trips Front End
Trips
Feral Concurrency
Control Mechanisms

17. Front End
Trips
Architecture

18. Death Star Architectures

19. Can We Do Better
than Feral Concurrency Control?

20. 2012

21. 2012
“Spanner is Google’s scalable, multi-
version, globally distributed, and
synchronously-replicated database”

22. 2015

23. 2015
“The biggest barrier…is that
consistency mechanisms must
integrate across many
stateful services”

24. Two Phase Commit

25. 2PC: Prepare Front End
Trips
Propose Book Hotel
Propose Book Car
Propose Book
Flight

26. 2PC: Prepare Front End
Trips
Vote Book Hotel
Vote Book Car
Vote Book
Flight

27. 2PC: Commit Front End
Trips
Commit/Abort Book Hotel
Commit/Abort Book Car
Commit/Abort
Book Flight

28. 2PC: Commit Front End
Trips
Done
Done
Done

29. 2PC: Commit Front End
Trips
Done
Done
Done
Doesn’t Scale
•O (N^2) Messages in the worst case
•Coordinator is a Single Point of Failure
•Reduced Throughput

30. Distributed Sagas
A Protocol for Coordinating Microservices

31. 1987

32. 1987
“Sagas are Long Lived Transactions
[in a single relational Database]”

33. 1987
“A Saga is a Long Lived Transaction that
can be written as a sequence of transactions
that can be interleaved.
All transactions in the sequence complete
successfully or compensating transactions
are ran to amend a partial execution.”

34. Distributed Sagas
A Protocol for Coordinating Microservices

35. A Distributed Saga is a Collection of Requests
Book Hotel Book Car Book Flight Charge Money

36. A Distributed Saga is a Collection of Requests
Book Hotel Book Car Book Flight Charge Money
and Compensating Requests
Cancel Hotel Cancel Car Cancel Flight Refund Money

37. A Distributed Saga is a Collection of Requests
Book Hotel Book Car Book Flight Charge Money
and Compensating Requests
Cancel Hotel Cancel Car Cancel Flight Refund Money
that represent a single business level action

38. Distributed Saga
Requests

39. Requests
Can Abort
Book Car

40. Requests
Can Abort
Book Car

41. Requests
Must Be Idempotent
Book Car

42. Requests
Must Be Idempotent
Book Car

43. Requests
Must Be Idempotent
Book Car

44. Requests
Must Be Idempotent
Book Car

45. Requests
Must Be Idempotent
Book Car

46. Requests
Must Be Idempotent
Book Car
Timeout

47. Requests
Must Be Idempotent
Book Car

48. Requests
Must Be Idempotent
Book Car

49. Distributed Saga
Compensating Requests

50. Compensating Requests
Semantically undoes the effect of a request

51. Cancel Car
Compensating
Requests
Cannot Abort

52. Cancel Car
Compensating
Requests
Can Not Abort

53. Cancel Car
Compensating
Requests
Must Be Idempotent

54. Compensating
Requests
Must Be Commutative
with Requests
Book Car
Cancel Car
Cancel Car
Book Car
is the same as

55. Compensating
Requests
Book Car
Must Be Commutative
with Requests

56. Compensating
Requests
Book Car
Timeout
Must Be Commutative
with Requests

57. Compensating
Requests
Book Car
Book Car
Must Be Commutative
with Requests

58. Compensating
Requests
Book Car
Book Car
Cancel Car
Must Be Commutative
with Requests

59. Compensating
Requests
Book Car
Book Car
Cancel Car
Must Be Commutative
with Requests

60. Compensating
RequestsRequests
Idempotent Idempotent
Commutative
Can Not Abort
Can Abort

61. Distributed Saga Guarantee
Book Hotel Book Car Book Flight Charge Money
All requests were completed successfully

62. Distributed Saga Guarantee
Book Hotel Book Car Book Flight Charge Money
All requests were completed successfully
Or a subset of requests and the corresponding
compensating requests were executed
Book Hotel Book Car Cancel Hotel Cancel Car

63. Distributed Saga Guarantee
Book Hotel Book Car Book Flight Charge Money
All requests were completed successfully
Or a subset of requests and the corresponding
compensating requests were executed
Book Hotel Book Car Cancel Hotel Cancel Car
No Atomicity
No Isolation

64. Distributed Saga Guarantee
Book Hotel Book Car Book Flight Charge Money
All requests were completed successfully
Or a subset of requests and the corresponding
compensating requests were executed
Book Hotel Book Car Cancel Hotel Cancel Car
No Atomicity
No Isolation
Book Hotel
Book Car
Visible before
Saga Completes

65. Distributed Saga Guarantee
Book Hotel Book Car Book Flight Charge Money
All requests were completed successfully
Or a subset of requests and the corresponding
compensating requests were executed
Book Hotel Book Car Cancel Hotel Cancel Car

66. Distributed Saga
defining a

67. Directed Acyclic Graph
Distributed Saga Car FlightHotel
Payment
Start Saga
End Saga

68. Name: Hotel
Request: Book Hotel
Compensating Request: Cancel Hotel
Status: Not Completed
VertexDistributed Saga

69. Start & End Vertices
Distributed Saga
Start Saga
End Saga
Car FlightHotel
Payment
Start Saga
End Saga

70. Distributed Saga Log
fault-tolerant & highly available

71. Saga Execution Coordinator
SEC
Car FlightHotel
Payment
Start Saga
End Saga
SagaLog

72. Distributed Saga
Executing a

73. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Book Trip Request
SEC

74. Car FlightHotel
Payment
Start Saga
End Saga
SEC
SagaLog
Start Saga
Book Trip Request

75. Car FlightHotel
Payment
Start Saga
End Saga
SEC
SagaLog
Done
Start Saga
Book Trip Request

76. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Start Hotel
Start Saga

77. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Done
Start Saga
Start Hotel

78. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Book Hotel
Request
Start Saga
Start Hotel

79. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Book Hotel
Response
Start Saga
Start Hotel

80. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
End Hotel
Start Saga
Start Hotel

81. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Done
Start Saga
Start Hotel
End Hotel

82. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Start Car
Start Saga
Start Hotel
End Hotel

83. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Done
Book Car
Request
Start Saga
Start Hotel
End Hotel
Start Car

84. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Book Car
Response
End Car

85. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Done
End Car

86. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
End Car

87. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
DoneEnd Car
Start Flight
Book Flight
Request

88. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
Book Flight
Response
End Flight

89. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
Done
End Flight

90. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
Start Payment
End Flight

91. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
Done
End Flight
Start Payment
Payment
Request

92. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
End Payment
End Flight
Start Payment
Payment
Response

93. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
End Flight
Start Payment
End Payment
Done

94. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
End Flight
Start Payment
End Payment
End Saga

95. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
End Flight
Start Payment
End Payment
Done
End Saga

96. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
End Car
Start Flight
End Flight
Start Payment
End Payment
End Saga

97. Distributed Saga
Failure of a

98. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
SEC
Book Car
Request
Start Saga
Start Hotel
End Hotel
Start Car
Start Flight

99. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Book Car
Response
Abort CarStart Car
Start Flight

100. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
DoneStart Car
Start Flight
Abort Car

101. Car FlightHotel
Payment
Start Saga
End Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
DoneStart Car
Start Flight
Abort Car
Rollback Recovery

102. Car FlightHotel
Payment
End Comp Saga
Start Comp Saga
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car

103. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Start Comp Saga
End Comp Saga

104. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Payment Log
Entries?Start Car
Start Flight
Abort Car
Start Comp Saga
End Comp Saga

105. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
{ }Start Car
Start Flight
Abort Car
Start Comp Saga
End Comp Saga

106. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Car Log
Entries?
Start Comp Saga
End Comp Saga

107. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
{ Start, Abort }
Start Comp Saga
End Comp Saga

108. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Hotel Log
Entries?
Start Comp Saga
End Comp Saga

109. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
{ Start, End }
Start Comp Saga
End Comp Saga

110. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
{ Start, End }
Cancel Hotel
Request
Start Comp Saga
End Comp Saga

111. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Cancel Hotel
Response
Comp Hotel
Start Comp Saga
End Comp Saga

112. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Done
Comp Hotel
Start Comp Saga
End Comp Saga

113. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
Flight Log
Entries?
Start Comp Saga
End Comp Saga

114. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
{ Start }
Start Comp Saga
End Comp Saga

115. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
{ Start }
Book Flight
Request
Start Comp Saga
End Comp Saga

116. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
End Flight
Book Flight
Response
Start Comp Saga
End Comp Saga

117. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
Done
Cancel Flight
Request
End Flight
Start Comp Saga
End Comp Saga

118. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
Comp Flight
Cancel Flight
Response
Start Comp Saga
End Comp Saga
End Flight

119. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
Done
Comp Flight
Start Comp Saga
End Comp Saga
End Flight
Comp Flight

120. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
End Saga
Start Comp Saga
End Comp Saga
Comp Flight
End Flight
Comp Flight

121. Car FlightHotel
Payment
SagaLog
Start Saga
SEC
Start Hotel
End Hotel
Start Car
Start Flight
Abort Car
Comp Hotel
Done
End Saga
Start Comp Saga
End Comp Saga
Comp Flight
End Flight
Comp Flight

122. Distributed Saga Guarantee
Book Hotel Book Car Book Flight Charge Money
All requests were completed successfully
Or a subset of requests and the corresponding
compensating requests were executed
Book Hotel Book Car Cancel Hotel Cancel Car

123. Saga Execution Coordinator
Recovering from
Failure

124. Front End
SEC
Start Saga
End Saga
SagaLog

125. Front End
SEC
Start Saga
End Saga
SagaLog

126. Front End
SEC
Start Saga
End Saga
SagaLog

127. Front End
SEC
SagaLog
Start Saga
End Saga

128. with Distributed Sagas
Isolation of Complex Code
Fron
Tr
Complex Code
Lives Everywhere
Front
SEC
SagaLog
Complex Code
Lives Here
Complex Code
Lives Here

129. Fron
Tr
with Distributed Sagas
Modular Services
Front
SEC
SagaLog
Only Flight
Booking
Flight Booking &
Feral Concurrency
Control Mechanisms

130. Fron
Tr
with Distributed Sagas
Service Composition
Front
SEC
SagaLog
Just Add A
New Saga! New Service &
Feral Concurrency
Control Mechanisms

131. Distributed Sagas Makes
Building & Modifying
Microservices Easier

132. Thank you
@caitie