Events, Workflows, Sagas? Keep Your Event-driven Architecture Sane. 1

In 2008, a large German publishing company announced that they’ll switch their IT to Apple Computers. 2

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”. 3

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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? 8

- Introduce a complex piece of software - makes it harder to run, change, test - Introduce a split responsibility - Are not event-driven 9

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 10

Organizational complexity: - Business logic is split - Error handling is split, leading to reduced clarity of responsibility if something goes wrong 11

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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 15

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Q: But what about visibility of the end-to-end process? A: Don’t mix control and observability! 18

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But what about timeouts? - flaky connections, retries - within service - processing times - SLA - service needs to scale, e.g. based on consumer lag - overall process - observer 20

But what about error handling? Errors must be handled within a service. The service gives a promise. This is a fundamental decoupling in event-driven architecture. 21

Surely, this only works for unrealistic, simplistic examples. No, it’s just much harder to fit it on slides in a non-confusing way. 22

Image: https://blog.codecentric.de/wer-microservices-richtig-macht-braucht-keine-workflow-engine-und-kein-bpmn 23

Image: https://blog.codecentric.de/wer-microservices-richtig-macht-braucht-keine-workflow-engine-und-kein-bpmn 24

Technicalities are not the problem, it’s a mindset issue. Think Promises. Enjoy the organizational clarity that it produces. 25

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. ... 26

Think reducing complexity, not managing it. https://twitter.com/bitfield/status/1411731604335214592?ref_src=twsrc%5Etfw 27

Think Agile. And then putting SAFE on top of it. 28

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Orchestration vs. Choreography While these terms might be fitting to describe the concepts, most comparisons are not useful, but based on some folkloristic concerns (e.g. taking a workflow as given, not as something that is designed and can be changed). Hence I tried to avoid them. 30

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. 31

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? - Is it someone who really understands EDA, and what you want to achieve, and, after considering all trade-offs, gives genuine, well-founded advice? 32

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. 33

Thank you! Mastodon https://social.tchncs.de/@lutzhuehnken Twitter https://twitter.com/lutzhuehnken Blog https://www.reactivesystems.eu/ 34