Stateless all the thing, they say. In the last few years we’ve been brainwashed: design stateless systems, otherwise they cannot scale, they cannot be highly available, and they are hard to maintain and evolve. In a nutshell stateful is bad. However complex software systems need to do collaborative processing, that is stateful by definition. Stateless myth busted! Collaborative domains deal with long business transactions and need to interact with distributed resources. The traditional distributed transactions approach, even if tempting, is a time bomb. This is when Sagas come into play. Sagas allow to model complex collaborative domains without the need for distributed transactions and/or orchestration across multiple resources. Join Mauro on a journey that aims to disclose what sagas are, how they can be used to model a complex collaborative domain, and what role they play when it comes to designing systems with failure and eventual consistency in mind. (It’s all right, I know we’re you’ve been)

1. @mauroservienti
welcome to the
(state) machine
It’s all right, I know where you’ve been

2. @mauroservienti
All I want to do when I wake up in
the morning is…
(cit. Rosanna, Toto, Toto IV)

3. @mauroservienti

4. @mauroservienti
tickets web site
buy a ticket
stage dive… :-)

5. @mauroservienti
All what they want to do is…

6. @mauroservienti

7. @mauroservienti
display available tickets
reserve with the theater
charge my credit card
deal with insurance
ship tickets
deal with their stuff

8. @mauroservienti
sometimes order doesn’t matter
sometimes it does!

9. @mauroservienti
spot the

10. @mauroservienti
my ticket
Warehouse
Insurance

11. @mauroservienti
issuesOwnership is lost

12. @mauroservienti

13. @mauroservienti
chop
up

14. @mauroservienti

15. @mauroservienti

16. @mauroservienti
The (state) machine
ticket ordering process

17. @mauroservienti
reserve with the theater
charge my credit card
deal with insurance
ship tickets
deal with their stuff
The (state) machine

18. @mauroservienti
reservation
payment
insurance
shipping
stuff
The (state) machine

19. @mauroservienti
Process Manager
Connecting the dots…

20. @mauroservienti
Credit Card
Gateway/ACL
Orders Manager
Checkout request
Block off money
request/response
Reservation
Gateway/ACL
Reservation
request/response
Credit Card
Gateway/ACL
Charge credit card
request/response
Insurance
Gateway/ACL
Tickets Insurance
request/response
Shipping
Gateway/ACL
Ship tickets
request/response
2-phase commit / timeout
…

21. @mauroservienti
like a Punch Card, a
Process Manager
owns the process

22. @mauroservienti
Process Manager (a different point of view)
New requirement: collect tickets at the venue
OrderId ShippingId ShippingStatus Etc
12 1337 Delivered …
58 1338 Pending …
Orders table

23. @mauroservienti
Process Managers: Source of coupling
•Single Point of Failure
•Violates Single Responsibility Principle
•Contention/Performance Bottleneck
•Conflicting Changes/Merge Conflicts
•Single Unit of Deployment
•Data exchange across boundaries

24. @mauroservienti
user mental model
tends to be UI driven

25. @mauroservienti
there is no Spoon Orders Manager

26. @mauroservienti
Autonomy
Collaborative Domains

27. @mauroservienti
Sagas

28. @mauroservienti
multiple workflows,
each providing
compensating
actions for every
step of the workflow
where it can fail

29. @mauroservienti
multiple workflows,
each providing
compensating
actions for every
step of the workflow
where it can fail

30. @mauroservienti
multiple workflows,
each providing
compensating
actions for every
step of the workflow
where it can fail

31. @mauroservienti
Shipping
Insurance
Finance
Reservation
1-day timeout to release money
Payment
Gateway
Finance
Order Checked-out
Initiate Payment Policy
Finance
Hold money
Venue
Gateway
Reserve seats
Seats Reserved
Payment
Gateway
Charge money
Payment Succeeded
Order Shipped
…whatever
…
Courier
Gateway
Pick-up Request
Tickets Ordering Saga

32. @mauroservienti
Shipping
Insurance
Finance
Reservation
1-day timeout to release money
Payment
Gateway
Finance
Order Checked-out
Initiate Payment Policy
Finance
Hold money
Venue
Gateway
Reserve seats
Seats Reserved
Payment
Gateway
Charge money
Payment Succeeded
Order Shipped
…whatever
…
Courier
Gateway
Pick-up Request
Payment Policy
Tickets Ordering Saga

33. @mauroservienti
New requirement
• 12-hours delivery SLA with our customers
• if the default shipping courier doesn't respond
• contact another one to deliver the package

34. @mauroservienti
Shipping
Insurance
Finance
Reservation
1-day timeout to release money
Payment
Gateway
Finance
Order Checked-out
Initiate Payment Policy
Finance
Hold money
Venue
Gateway
Reserve seats
Seats Reserved
Payment
Gateway
Charge money
Payment Succeeded
Order Shipped
…whatever
…
Courier
Gateway
Pick-up Request
Tickets Ordering Saga

35. @mauroservienti
Shipping
Order Shipped
…
First Courier
Gateway
Pick-up Request
8-hours timeout
Shipping
Have we received a response?
Shipping
Yes
No
Second Courier
Gateway
Pick-up Request
Shipping
Shipping Policy
2-hours escalation timeout
Shipping
Have we received a response?
Yes No Shipment Failed
…

36. @mauroservienti
Payment
Policy
Shipping
Policy
Reservation
Policy
Seats Reserved
Payment Succeeded
Release money
Order Checked-out
fail
1-day timeout to release money
Happy path and not so happy path :)

37. @mauroservienti
Payment
Policy
Customer Care
…
Reservation
Policy
Payment Failed
Order Checked-out
fail
Happy path and not so happy path :)

38. @mauroservienti
Sagas (a different point of view)
OrderId ShippingId ShippingStatus Etc
12 1337 Delivered …
58 1338 Pending …
Orders table
Sales Shipping Finance
Id ProductId
12 ABC
58 ACD
Orders table
OrderId Address
12 Haifa, Israel
58 Rotterdam, Holland
Shipping table
OrderId Status
12 Paid
58 Overdue
Invoices table
VS

39. @mauroservienti
Sagas
•Business Process is distributed
•No overarching Process Manager
•Respect Single Responsibility Principle
•Independent scale out
•Simpler/not conflicting evolution
•Independent Units of Deployment

40. @mauroservienti
Takeaways
• Boundaries are key to success
• Finding boundaries is the hardest part

41. @mauroservienti
Takeaways
• Boundaries are key to success
• Mental model can badly influence design
• Users/Business analysts tend to think in term
of data presentation

42. @mauroservienti
Takeaways
• Boundaries are key to success
• Mental model can badly influence design
• Behaviors define how to designs processes
• Group data that change together
• Follow the coupling
• Identify processes and chop them up by “service”

43. @mauroservienti
Takeaways
• Boundaries are key to success
• Mental model can badly influence design
• Behaviors define how to designs processes
• Use Delayed Messaging to model time
• Simplifies making decisions in an async world

44. @mauroservienti | #EDDD
Mauro Servienti
Solution Architect @ Particular Software
the makers of NServiceBus
mauro.servienti@particular.net
@mauroservienti
//milestone.topics.it

45. @mauroservienti | #EDDD
Thank you!