Events, Workflows, Sagas? Keep Your Event-driven Architecture Sane at Developer Week Nuremberg

When dealing with (micro-)services, we often face business processes that span multiple bounded contexts. Different ways of implementing this lead to massively divergent results, varying above all in complexity and manageability. Especially when applying Event-driven Architecture, its asynchronous nature clashes with a desire for global control and synchronous responses, sometimes leading to chimeras that combine the “worst of both worlds”.

This talk will establish some guiding principles, and use them to assess common approaches. It will explore the importance of responsibility boundaries and of bringing the data to the behavior.

A common mistake in the event-driven world is to replicate monolith workflows, resulting in an excessive need for orchestration or workflow tooling, or - in the worst case - in a “distributed monolith”. How can this be avoided? The talk will introduce the concept of micro-workflows and show, how challenging and re-designing your processes will lead to better systems. And lastly, we will ask ourselves: Should the saga pattern be considered harmful?

Transcript

2. Events, Workflows, Sagas? Keep Your Event-driven Architecture Sane. 2
3. @lutzhuehnken@mastodon.social In 2008, a large German publishing company announced that they’ll switch their IT to Apple Computers. 3
4. @lutzhuehnken@mastodon.social In 2008, a large German publishing company announced that they’ll switch their IT to Apple Computers. Somewhere down in the press statement, it read “the desktop computers will run Windows XP or Vista”. 4
5. @lutzhuehnken@mastodon.social Why me? - Build my first message-driven, asynchronous system for the Bundesbank starting 2001 - Worked on various event-driven systems, e.g. for Zalando, ING, ista, Maersk… - Currently building a bank with event-driven architecture! 5
6. @lutzhuehnken@mastodon.social 6
7. @lutzhuehnken@mastodon.social 7
8. @lutzhuehnken@mastodon.social 8
9. @lutzhuehnken@mastodon.social 9
10. @lutzhuehnken@mastodon.social So what have we looked at so far? Implementing a workflow by: - Making one service the coordinator, or - Using a workflow engine for coordination, and - Implementing a saga (can be done with either of the above) What do all approaches have in common? 10
11. @lutzhuehnken@mastodon.social - Introduce a complex piece of software - makes it harder to run, change, test - Introduce a split responsibility - Are not event-driven 11
12. @lutzhuehnken@mastodon.social Technical complexity: - Must be able to resume after crash - Must be able to handle timeouts - Must be distributed to avoid single point of failure - Needs distributed coordination (stateful) - (Some of) your business logic is now in a YAML or XML file 12
13. @lutzhuehnken@mastodon.social Organizational complexity: - Business logic is split - Error handling is split, leading to reduced clarity of responsibility if something goes wrong 13
14. @lutzhuehnken@mastodon.social Reminder: Events vs. Commands 14 Is.. Command An intention to perform an operation or change a state Event A fact, something that undisputedly happened in the past
15. @lutzhuehnken@mastodon.social Reminder: Events vs. Commands 15 Is.. Expected Response Communication Pattern Command An intention to perform an operation or change a state A confirmation that the command has been executed, or an error message Request-Response Event A fact, something that undisputedly happened in the past None Fire-And-Forget
16. @lutzhuehnken@mastodon.social 16
17. @lutzhuehnken@mastodon.social 17
18. @lutzhuehnken@mastodon.social 18
19. @lutzhuehnken@mastodon.social Q: So how to implement complex workflows over multiple microservices? A: Don’t. Instead, break up the workflow. To match actual event-driven architecture. Monolith -> Microservices Workflow -> Microworkflows 19
20. @lutzhuehnken@mastodon.social 20
21. @lutzhuehnken@mastodon.social 21
22. @lutzhuehnken@mastodon.social Q: But what about visibility of the end-to-end process? A: Don’t mix control and observability! 22
23. @lutzhuehnken@mastodon.social 23
24. @lutzhuehnken@mastodon.social Q: But what about timeouts? A: - flaky connections, retries - within service - processing times - SLA - service needs to scale, e.g. based on consumer lag - overall process - observer 24
25. @lutzhuehnken@mastodon.social Q: But what about error handling? A: Errors must be handled within a service. The service gives a promise. This is a fundamental decoupling in event-driven architecture. 25
26. @lutzhuehnken@mastodon.social Q: Surely, this only works for unrealistic, simplistic examples? A: No, it’s just much harder to fit it on slides in a non-confusing way 󰤇. 26
27. @lutzhuehnken@mastodon.social Image: https://blog.codecentric.de/wer-microservices-richtig-macht-braucht-keine-workflow-engine-und-kein-bpmn 27
28. @lutzhuehnken@mastodon.social Image: https://blog.codecentric.de/wer-microservices-richtig-macht-braucht-keine-workflow-engine-und-kein-bpmn 28
29. @lutzhuehnken@mastodon.social Challenge accepted! Send your workflow to: challenge@microworkflows.org 29
30. @lutzhuehnken@mastodon.social Technology is not the problem, it’s a mindset issue. Think Promises. 30
31. @lutzhuehnken@mastodon.social Some ideas seem outlandish, but they’re nothing new. Prior art: - Unix pipes - Actors - Stream Processing 31
32. @lutzhuehnken@mastodon.social Think Unix Philosophy 1. Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features". 2. Expect the output of every program to become the input to another, as yet unknown, program. ... 32 $ Cat file3.txt | grep “dwx” | tee file4.txt | wc –l
33. @lutzhuehnken@mastodon.social Think Actors 33 Supervisor Actor Client
34. @lutzhuehnken@mastodon.social Think Stream Processing 34
35. @lutzhuehnken@mastodon.social Enjoy the organizational clarity 35 ❌ ❌
36. @lutzhuehnken@mastodon.social Enjoy the (load) testing 36 While you’re freed from having e.g. the workflow engine in your end-to-end test, there are other challenges. Also: Testing a microworkflow is still an integration test. But it’s nice to be able to test e.g. throughput in a very isolated way.
38. @lutzhuehnken@mastodon.social Orchestration vs. Choreography While these terms might be fitting to describe the concepts, most comparisons are not useful, but based on some folkloristic concerns. Hence I tried to avoid them. 38
39. @lutzhuehnken@mastodon.social Example Heuristic “Complex workflows..” Problem: Taking a workflow as given, not as something that is designed and can be changed. 39
40. @lutzhuehnken@mastodon.social Think reducing complexity, not managing it. https://twitter.com/bitfield/status/1411731604335214592?ref_src=twsrc%5Etfw 40
41. @lutzhuehnken@mastodon.social Think Agile. And then putting SAFE on top of it. 41
42. @lutzhuehnken@mastodon.social Workflows are useful tools. Workflow engines too, probably. But not for implementing complex workflows on top of Microservices. That use case shouldn’t exist. Consider going event-driven instead. Break the workflow down into microworkflows. 42
43. @lutzhuehnken@mastodon.social People are quick to jump to sagas and workflow engines! - Is it someone who’s familiar with it, but not with EDA, and is scared to leave their comfort zone? - Is it someone who wants to sell you something? 43
44. @lutzhuehnken@mastodon.social Summary: Workflows and Sagas are at odds with event driven architecture. If you really want to get the benefits of event-driven architecture, go all-in, don’t impose an orchestrator on top of it. Keep your event-driven architecture sane. 44
45. Ask your question now! Follow me on Mastodon @lutzhuehnken@mastodon.social Follow me on LinkedIn https://linkedin.com/in/lutzh Read my blog https://www.reactivesystems.eu/ 45 Also: I’ll be around until tomorrow 15:00hrs. Ask me anything, don’t be shy.