Improve Deserialization of InstanceEvents

[ChunMengLu]

Bug report
I customized eventstore and found that it is not good enough for deserialization.
I use jackson and need to customize a Deserializer like RegistrationDeserializer.

[joshiste]

Could you please elaborate a bit on your customizing and why you need to deserialize the InstanceEvents using jackson?
What changes to the InstanceEvents would you apply to improve it?

[ChunMengLu]

Because the memory currently used for InstanceEvents storage, spring boot admin will be lost after restart, so I use rocksdb to store. Use jackson for serialization.

[ChunMengLu]

Currently I use custom deserialization:

/**
 * InstanceEvent Deserializer
 *
 * @author L.cm
 */
public class InstanceEventDeserializer extends StdDeserializer<InstanceEvent> {

	public InstanceEventDeserializer() {
		super(InstanceEvent.class);
	}

	@Override
	public InstanceEvent deserialize(JsonParser jsonParser, DeserializationContext context) throws IOException {
		JsonNode node = jsonParser.readValueAsTree();
		String type = node.at("/type").asText();
		InstanceId instance = InstanceId.of(node.at("/instance").asText());
		long version = node.at("/version").asLong();
		Instant timestamp = Instant.parse(node.at("/timestamp").asText());

		if ("DEREGISTERED".equals(type)) {
			return new InstanceDeregisteredEvent(instance, version, timestamp);
		} else if ("ENDPOINTS_DETECTED".equals(type)) {
			List<Endpoint> endpoints = JsonUtil.convertValue(node.at("/endpoints"), JsonUtil.getListType(Endpoint.class));
			return new InstanceEndpointsDetectedEvent(instance, version, timestamp, Endpoints.of(endpoints));
		} else if ("INFO_CHANGED".equals(type)) {
			Map<String, Object> value = JsonUtil.convertValue(node.at("/info"), Map.class);
			return new InstanceInfoChangedEvent(instance, version, timestamp, Info.from(value));
		} else if ("REGISTERED".equals(type)) {
			Registration registration = JsonUtil.convertValue(node.at("/registration"), Registration.class);
			return new InstanceRegisteredEvent(instance, version, timestamp, registration);
		} else if ("REGISTRATION_UPDATED".equals(type)) {
			Registration registration = JsonUtil.convertValue(node.at("/registration"), Registration.class);
			return new InstanceRegistrationUpdatedEvent(instance, version, timestamp, registration);
		} else if ("STATUS_CHANGED".equals(type)) {
			Map<String, Object> value = JsonUtil.convertValue(node.at("/statusInfo"), Map.class);
			return new InstanceStatusChangedEvent(instance, version, timestamp, StatusInfo.from(value));
		}
		return null;
	}
}

[joshiste]

@ChunMengLu Currently I think the best is to provide a jackson module with your deserializer by SBA as I'm currently refusing to taint the code with the jackson polymorphic deserialization stuff.
Do you want to work on a PR for this?

[srempfer]

I started the development of #684 and as pre-requirement I need a Jackson module for the InstanceEvent stuff.
I'll contribute the module as separate PR for this issue because I not know how long the Redis based event store will take to implement.

[srempfer]

I just referenced a working state to the issue.

@joshiste: Is that what you had in mind? Do you prefer...

• ...the Mixin variant like I did it for InstanceEventMixin/InstanceEndpointsDetectedEventMixin
• ...the @jsonxyz annotations direct in the domain classes like I did for InstanceRegistrationUpdatedEvent
• ... a deserializer like the above InstanceEventDeserializer

My preference is the own "InstanceEventDeserializer" because of the minimal changes required.
It also don't pollutes the domain classes with the jackson stuff and you don't need a mixin for every InstanceEvent.
In case of get it not proper working with the 'InstanceEventDeserializer' approach I would create a mixin for the polymorphic stuff and add 'JsonCreator' annotations to the events like its already done for InstanceId

[joshiste]

My preference would be to pollute the core domain as little as possible with Jackson annotations. So either a custom deserializer or a mixin would be fine.