DDD Exchange London 2018: Know the flow! Events, commands and long-running services.

@martinschimak => April 26th, 2018 => London => DDD Exchange

Transcript

1. Events, commands and long-running services @martinschimak -> April 26th, 2018 -> London -> DDD eXchange Know the flow!
2. Read Model Domain Event Aggregate Policy Command External System UI Command Invoked on raises raises Invoked on projected onto listened to by triggers triggers Inspired by the picture that explains (almost) everything :-) * *) © The ingenious Alberto Brandolini
3. Domain events are more prominently discussed than commands
4. The cool thing about domain events? They are facts. It already happened. Events let us look into the past. Payment received
5. Events create trust in a world of eventual consistency. Strong consistency Payment received Payment
6. We are dedicated to events. For good reasons. CQRS! Order summary Payment received Payment UI raises projected onto
7. Need more good reasons? Event Sourcing! Payment received Payment Payment received Payment received Payment received Payment received
8. Events are easy to grasp. Event Storming! Goods shipped Order placed Goods fetched Payment received „We need to achieve some facts to create value for customers!“ Events are a concept easy to grasp for everybody in the room.
9. Events are relaxed! Just feel fine. Don‘t want to achieve anything.
10. Events help to decouple services. Order summary Payment received Payment UI raises projected onto Payment Service Order Service
11. Events help with cross-cutting concerns Payment received Customer Notification Policy Inform customer listened to by triggers Payment Service Notification Service
12. Let‘s just rely on events in between services!
13. Relying on events only leads to suboptimal coupling Order placed Payment Service Payment received Inventory Service Goods fetched Shipment Service Goods shipped
14. Shipment Service Autonomous services can be deployed independently. To change the order of two services, we touch 4 sticky notes… but wait, all three services! Goods shipped Order placed Inventory Service Payment Service Payment received Goods fetched
15. Who made the decision to use events only?
16. Our policies decide to act on facts – with commands Domain Event Policy Command listened to by triggers
17. Who owns the policy in a pure event choreography? Payment received Start Fetching Policy Fetch goods listened to by triggers Payment Service Inventory Service
18. Who owns the policy in a commanding approach? Payment received Start Fetching Policy Fetch goods listened to by triggers Payment Service Inventory Service
19. Neither of both truly „owns“ that policy? Payment received Fetch goods listened to by triggers Payment Service Inventory Service Start Fetching Policy
20. Extracting the policy into a mediator service Payment received Start Fetching Policy Fetch goods listened to by triggers Payment Service Inventory Service Order Service
21. The order as the companies core business Place order Order placed Request payment Order Service Payment received Fetch goods Goods fetched Ship goods Goods shipped “When the order is placed…” “When the payment is received…” “When the goods are fetched…” “Then we are done!” Mark order as fulfilled Order fulfilled Payment Service Inventory Service Shipment Service
22. Double check: what happens when we charge later? Place order Order placed Request payment Order Service Payment received Fetch goods Goods fetched Ship goods Goods shipped “When the order is placed…” “When the payment is received…” “When the goods are fetched…” “Then we are done!” Mark order as fulfilled Order fulfilled Payment Service Inventory Service Shipment Service
23. Events decrease coupling? Often! Pro Tip: some commands may help, too!-)
24. Trade offs! Events and commands in between services. Payment received Policy Fetch goods listened to by triggers Payment Service Inventory Service Order Service
25. Commands are intent Nothing really happened so far. It‘s in the future. Place order
26. „Place order“ seen as an atomic transaction Order Invoked on Order Placed raises Place order
27. When hugging a tree, it is difficult to see the wood!
28. „Place order“ seen as a long-running service Order fulfilled Order canceled Payment received Goods shipped Place order Order placed
29. Atomic vs. composite command execution Place order Order placed Typically we see a „command“ as the intent to change a write model... … but the customer‘s or service clients intent is often targeted at a more valuable business result, which needs many steps to be achieved. Place order Order fulfilled Atomic, trans- actional execution Composite, long- running execution
30. Is that command executable as an atomic step? Request payment
31. Let‘s do some event storming… Request payment Payment received Payment requested Charge credit card Depending on account balance Withdraw amount from account Amount Withdrawn Credit card charged Credit card failed Update credit card details Account details Credit card details updated Whenever card details are updated After two weeks Depending on the amount Amount credited Mark payment received Mark payment canceled Payment canceled Credit amount to account Whenever payment is canceled Mark payment received Payment received
32. Credit card gateway Accounting system Payment Request payment Charge credit card Credit card charged Our result seen as a collection of commands Withdraw amount from account Amount Withdrawn Amount credited Credit amount to account Mark payment Payment received Payment canceled Payment requested Credit card failed Update credit card details Credit card details updated Update credit card details Credit card details updated Credit card failed
33. What happens if we expose those atomic commands? Charge credit card Credit card charged Credit card failed Payment Service Retry with new credit card details Who owns the policy?
34. Payment seen as a long-running service Request payment Payment received Payment Service
35. What happens if we expose troubles with some steps? Request payment Payment received Credit card failed Payment Service Retry with new credit card details Who owns the policy?
36. „Delegating our problems to our clients forces them to deal with the mitigation. They become god services.“ Credit card failed Retry with new credit card details Charge credit card listened to by triggers Payment Service Order Service
37. Smart endpoints (with dumb pipes) … … don‘t delegate problems they better deal with themselves!
38. A long-running smart endpoint … Request payment Payment received Payment canceled 1) creates value – serving a result its client is really interested in 2) assumes responsibility – instead of delegating troubles with internal steps to its client 3) may go asynchronous – if needed for composite execution Payment Service
39. Long-running services help to protect bounded contexts Order service Payment service Request payment Payment canceled Payment received
40. How to get productive with long-running services?
41. Heavily inspired by Mathias Verraes! Thank you! Considering the CQRS pattern Read Model Projections Write Model Invariant Protection Client/Human Interaction Query Command Domain Event
42. Write Model Read Model Query Domain Event Command Observe Act Decide
43. How do we implement the payment service? Request payment Payment received Payment requested Charge credit card Depending on account balance Withdraw amount from account Amount Withdrawn Credit card charged Credit card failed Update credit card details Account details Credit card details updated Whenever card details are updated After two weeks Depending on the amount Amount credited Mark payment received Mark payment canceled Payment canceled Credit amount to account Whenever payment is canceled Mark payment received Payment received
44. Write Model Read Model Query Domain Event Command Decide Observe Act
45. Write Model Read Model Process Model Domain Event Command Query Policy plexiti de|coding domain language Observe Act Decide
46. Process Model Write Model Domain Event Command Process Manager Read Model Policy Observe Decide
47. How to get productive with process managers? Handling of time & timeouts Retries Compensation Message-Driven
48. e.g. with Axon Messaging & Saga Management @Saga class Payment { private var paymentAmount = 0F @StartSaga @SagaEventHandler(associationProperty = "paymentId") fun on(event: PaymentRequested) { // save state, evaluate policy, create command } } Private, persistent process state Managed event correlation Process policy and construct command is cool, but Java works as well! :-)
49. What about business process execution engines? Monitoring & Operations Handling of time & timeouts Versatile Retries Explicitness & Visibility for Flows Versioning for Flows and Policies Transparent Compensation Parallelisation and synchronisation
50. Payment service as a BPMN model
51. Payment service as a BPMN model
52. Make the implicit explicit!
53. Explicit and simple timer management
54. Transparent and simple compensation handling
55. Transparent business process monitoring and operations 897 2 Temporarily failing commands Executions currently waiting for events
56. But are business process engines usable in a modern architecture?
57. Credit amount back to account Withdraw amount from cust. account Charge amount by credit card I want that kind of „reactive“ business process execution Charge amount by credit card Credit amount back to account Update credit card details Check customer account balance Payment requested Depending on account balance Charge credit card Withdraw amount from account Credit card charged Whenever credit card is charged Mark payment received Payment received Amount Withdrawn Depending on the amount Mark payment received Payment received Mark paym. partly covered Payment partly covered Whenever payment p. covered Charge credit card Credit card failed Whenever credit card failed Update credit card details Credit card details updated Whenever card details are updated Charge credit card After two weeks Mark payment canceled Payment canceled Whenever payment is canceled Credit amount to account Amount credited Every two days Remind customer Customer reminded Withdraw amount from cust. account
58. I want autonomous services - respecting boundaries Order service Payment service Request payment Payment received Payment canceled Request payment
59. I want transparent commanding - atomic or composite Order service Request payment
60. With this I get flexible and local technology decisions Order service Payment service Request payment Payment received Payment canceled Request payment Your local implementation reacting to events and commands
61. Is it feasible? Sure it is.
62. e.g. with Camunda an open source process engine Domain Entities, value objects, domain events etc… + atomic command behaviour + composite, long-running command behaviour Application Infrastructure Process Engine
63. Reality Check. Order fulfillment process of a very large online retailer.
64. BDD style business process specification
65. Charge amount by credit card BDD style business process test - and visual feedback Charge amount by credit card Update credit card details Check customer account balance
66. An emerging ecosystem of saga, state & process managers CADENCE Baker
67. I like graphical models to know and discuss „what is“…
68. … and I like sticky notes to discuss „what should be“! Request payment Payment received Payment requested Charge credit card Depending on account balance Withdraw amount from account Amount Withdrawn Credit card charged Credit card failed Update credit card details Account details Credit card details updated Whenever card details are updated After two weeks Depending on the amount Amount credited Mark payment received Mark payment canceled Payment canceled Credit amount to account Whenever payment is canceled Mark payment received Payment received
69. Need code? Reactive processes with Axon & Camunda. Order service Payment service Request payment Payment received Payment canceled Request payment plexiti de|coding domain language https://github.com/plexiti/axon-camunda-poc @martinschimak