OpenPayment

I have created this as an example of using Event sourced application state.

Key principles: -

• Application state can be completely constructed from the stream
• Application state changes triggered only by a received event
• Any change received from an external system produces an event

Account

The account is simply to have an entity to demonstrate simple actions a service might need to do. The related items (Customer, Card, etc) are referenced by a URL which shows how you might relate data from other services into this service.

Event stream

For the example and tests there is an in memory stream. For an actual service, a service designed for event streams like kafka would be best. You could use a fast, fault-tolerant store like cassandra if that was already in the system. The key aspects of the event stream system are: -

• consistent ordering
• entire stream persisted

Message queue systems initially look attractive for this but typically have a retention period which only allows a limited time coverage.

To reload the state from the stream simply requires clearing the state and processing the events sequentially. The In memory stream just tracks the index of the next event to send. So this is set back to 0.

Snapshots can be taken to store the entire application state at a point in time. This means that only events from that point forward will be required to rebuild the current state.

Typical log showing event handling

go run ./cmd/simpleAccountWebservice/main.go
{"severity":"debug","eventId":"c1273386-7fa5-458c-ae10-8aa75d090add","eventType":"account add","streamLen":1,"time":"2020-11-01T20:48:27Z","message":"event published"}
{"severity":"debug","time":"2020-11-01T20:48:27Z","message":"processed account add request"}
{"severity":"debug","eventId":"c1273386-7fa5-458c-ae10-8aa75d090add","eventType":"account add","time":"2020-11-01T20:48:27Z","message":"event received"}
{"severity":"debug","eventId":"c1273386-7fa5-458c-ae10-8aa75d090add","time":"2020-11-01T20:48:27Z","message":"received event"}
{"severity":"debug","eventId":"c1273386-7fa5-458c-ae10-8aa75d090add","eventType":"account add","time":"2020-11-01T20:48:27Z","message":"received account event"}
{"severity":"debug","eventId":"c1273386-7fa5-458c-ae10-8aa75d090add","eventType":"account add","time":"2020-11-01T20:48:27Z","message":"processed event"}
{"severity":"debug","eventId":"c1273386-7fa5-458c-ae10-8aa75d090add","time":"2020-11-01T20:48:27Z","message":"event processed by simple account"}
{"severity":"debug","eventId":"c1273386-7fa5-458c-ae10-8aa75d090add","time":"2020-11-01T20:48:27Z","message":"event processed by all subscribers"}
{"severity":"debug","eventId":"331f3f46-dfb2-4dcf-8e1a-7be217df9ccb","eventType":"account update","streamLen":2,"time":"2020-11-01T20:48:37Z","message":"event published"}
{"severity":"debug","time":"2020-11-01T20:48:37Z","message":"processed account update request"}
{"severity":"debug","eventId":"331f3f46-dfb2-4dcf-8e1a-7be217df9ccb","eventType":"account update","time":"2020-11-01T20:48:37Z","message":"event received"}
{"severity":"debug","eventId":"331f3f46-dfb2-4dcf-8e1a-7be217df9ccb","time":"2020-11-01T20:48:37Z","message":"received event"}
{"severity":"debug","eventId":"331f3f46-dfb2-4dcf-8e1a-7be217df9ccb","eventType":"account update","time":"2020-11-01T20:48:37Z","message":"received account event"}
{"severity":"debug","eventId":"331f3f46-dfb2-4dcf-8e1a-7be217df9ccb","eventType":"account update","time":"2020-11-01T20:48:37Z","message":"processed event"}
{"severity":"debug","eventId":"331f3f46-dfb2-4dcf-8e1a-7be217df9ccb","time":"2020-11-01T20:48:37Z","message":"event processed by simple account"}
{"severity":"debug","eventId":"331f3f46-dfb2-4dcf-8e1a-7be217df9ccb","time":"2020-11-01T20:48:37Z","message":"event processed by all subscribers"}
{"severity":"debug","accountNumber":"123456789","time":"2020-11-01T20:48:41Z","message":"processed account get request"}
{"severity":"debug","eventId":"47741764-c292-4aee-b807-d89de0ff98bf","eventType":"account delete","streamLen":3,"time":"2020-11-01T20:48:56Z","message":"event published"}
{"severity":"debug","time":"2020-11-01T20:48:56Z","message":"processed account delete request"}
{"severity":"debug","eventId":"47741764-c292-4aee-b807-d89de0ff98bf","eventType":"account delete","time":"2020-11-01T20:48:57Z","message":"event received"}
{"severity":"debug","eventId":"47741764-c292-4aee-b807-d89de0ff98bf","time":"2020-11-01T20:48:57Z","message":"received event"}
{"severity":"debug","eventId":"47741764-c292-4aee-b807-d89de0ff98bf","eventType":"account delete","time":"2020-11-01T20:48:57Z","message":"received account event"}
{"severity":"debug","eventId":"47741764-c292-4aee-b807-d89de0ff98bf","eventType":"account delete","time":"2020-11-01T20:48:57Z","message":"processed event"}
{"severity":"debug","eventId":"47741764-c292-4aee-b807-d89de0ff98bf","time":"2020-11-01T20:48:57Z","message":"event processed by simple account"}
{"severity":"debug","eventId":"47741764-c292-4aee-b807-d89de0ff98bf","time":"2020-11-01T20:48:57Z","message":"event processed by all subscribers"}
{"severity":"error","error":"Account does not exist","accountNumber":"123456789","time":"2020-11-01T20:49:00Z","message":"failed to get account"}