RE: Generate events from consecutive entity (aggregate?) statuses

My question is:

why there is, as far as I understood, something (the job) that polls the aggregate for its status instead of simply having the aggregates that fires events on status changes?

The event carries the status change:

class AggregateNameChangedEvent
{
    DatetTime When;
    String OldName;
    String NewName;
}

.m

From: [email protected] [mailto:[email protected]] On Behalf Of Matteo Moci
Sent: venerdì 24 maggio 2013 23.26
To: [email protected]
Subject: [domaindrivendesign] Generate events from consecutive entity (aggregate?) statuses




Hi all,
I am quite new to ddd and all its related aspects like EventSourcing, CQRS, etc...
and lately I discovered this list as really helpful to understand both the basic or subtle and more advanced concepts.

Currently, I'd like to have some feedback about the design of a part of a system, since I have lots to learn in this context.

The system continuously monitors external entities (aggregates) status:
this means that, for example, a job J is launched now and asks for the status of an Aggregate A to another system,
and stores its snapshot in an internal repository.

Based on the previous snapshot of A in the repository, after storing the current A snapshot, the job J should generate a list of events related to the A status change: for every type of change that A can have, there is a specific type of event that should be produced and sent in the system for further processing.

If the A aggregate at time 0 had a field name = "Anna" and at time 1 is name = "Anna F.", the event fired should be something like "entity A changed name to 'Anna F.' at time 1".

Storing all the sequence of snapshots of every aggregate provides the system also a way of replaying them, producing again the events that were produced in first instance. This will help also testing, I think.

How does that sound to you?
Do you have some advice or relevant links to learn from?

Thanks!
Matteo

PS: I pasted a snippet here [1] to illustrate the snapshot repository APIs that I came up with until now.
[1] https://gist.github.com/mox601/5646540


--
Matteo Moci
http://it.linkedin.com/in/matteomoci
http://about.me/matteomoci/bio