Read Models
As you can see we have annotated this class with @ReadModel. Dewdrop on startup looks to identify ALL the classes that it can find that implement @ReadModel and then keeps track of them. When it finds a ReadModel it will assume that we automatically want to create that ReadModel unless we tell it not to.
If we have a situation where we don’t want a ReadModel to be created and live forever, we can mark the ReadModel as ephemeral which tells Dewdrop to only create this class when we see a query executed for this. Consider it a lazy ReadModel, or a just in time ReadModel. If we want to have an ephemeral ReadModel we can then choose what that means by adding the destroyInMinutesUnused field to our @ReadModel annotation on our class. For example:
@ReadModel(ephemeral = true, destroyInMinutesUnused = ReadModel.DESTROY_IMMEDIATELY)
@Stream(name = "DewdropAccountAggregate", subscribed = true)
@Stream(name = "DewdropUserAggregate", subscribed = false)
public class DewdropAccountDetailsReadModel {
@DewdropCache
Map<UUID, DewdropAccountDetails> cache;
@QueryHandler
public Result<DewdropAccountDetails> handle(DewdropGetAccountByIdQuery query) {
DewdropAccountDetails dewdropAccountDetails = cache.get(query.getAccountId());
if (dewdropAccountDetails != null) { return Result.of(dewdropAccountDetails); }
return Result.empty();
}
}
Here we’ve told Dewdrop that we want to create this ReadModel when we see a query executed for this ReadModel. We also told Dewdrop that we want to destroy this ReadModel immediately after it was used. This is a good way to keep your ReadModels from growing too large and eating up memory. However, this would be a bad strategy to use in a stream that has a lot of events associated with. This would become an expensive operation, and would take a long time to load if there were thousands of events. The three states of destroyInMinutesUnused to note are:
DESTROY_IMMEDIATELY- This will destroy the ReadModel immediately after it was used.NEVER_DESTROY- This will add the ReadModel to the cache, but it will never be destroyed until restart (just like a non-ephemeral ReadModel).n- Just add a value here for the count of minutes you want it to live on for before destruction.
Destroying a ReadModel after an hour of use is a good idea for situations where you have a UI that is being heavily used by a user and you want to keep the ReadModel alive for snappy performance. However, after that user is done we can then destroy this ReadModel and clear up memory.