Relation between command handlers, aggregates, the repository and the event store in CQRS

The following is based on my own experience and my experiments with various frameworks like Lokad.CQRS, NCQRS, etc. I'm sure there are multiple ways to handle this. I'll post what makes most sense to me.

1. Aggregate Creation:

Every time a command handler needs an aggregate, it uses a repository. The repository retrieves the respective list of events from the event store and calls an overloaded constructor, injecting the events

var stream = eventStore.LoadStream(id)
var User = new User(stream)

If the aggregate didn't exist before, the stream will be empty and the newly created object will be in it's original state. You might want to make sure that in this state only a few commands are allowed to bring the aggregate to life, e.g. User.Create().

2. Storage of new Events

Command handling happens inside a Unit of Work. During command execution every resulting event will be added to a list inside the aggregate (User.Changes). Once execution is finished, the changes will be appended to the event store. In the example below this happens in the following line:

store.AppendToStream(cmd.UserId, stream.Version, user.Changes)

3. Order of Events

Just imagine what would happen, if two subsequent CustomerMoved events are replayed in the wrong order.

An Example

I'll try to illustrate the with a piece of pseudo-code (I deliberately left repository concerns inside the command handler to show what would happen behind the scenes):

Application Service:

UserCommandHandler
    Handle(CreateUser cmd)
        stream = store.LoadStream(cmd.UserId)
        user = new User(stream.Events)
        user.Create(cmd.UserName, ...)
        store.AppendToStream(cmd.UserId, stream.Version, user.Changes)

    Handle(BlockUser cmd)
        stream = store.LoadStream(cmd.UserId)
        user = new User(stream.Events)
        user.Block(string reason)
        store.AppendToStream(cmd.UserId, stream.Version, user.Changes)

Aggregate:

User
    created = false
    blocked = false

    Changes = new List<Event>

    ctor(eventStream)
        isNewEvent = false
        foreach (event in eventStream)
            this.Apply(event, isNewEvent)

    Create(userName, ...)
        if (this.created) throw "User already exists"
        isNewEvent = true
        this.Apply(new UserCreated(...), isNewEvent)

    Block(reason)
        if (!this.created) throw "No such user"
        if (this.blocked) throw "User is already blocked"
        isNewEvent = true
        this.Apply(new UserBlocked(...), isNewEvent)

    Apply(userCreatedEvent, isNewEvent)
        this.created = true
        if (isNewEvent) this.Changes.Add(userCreatedEvent)

    Apply(userBlockedEvent, isNewEvent)
        this.blocked = true
        if (isNewEvent) this.Changes.Add(userBlockedEvent)

Update:

As a side note: Yves' answer reminded me of an interesting article by Udi Dahan from a couple of years ago:

• Don’t Create Aggregate Roots

A small variation on Dennis excellent answer:

• When dealing with "creational" use cases (i.e. that should spin off new aggregates), try to find another aggregate or factory you can move that responsibility to. This does not conflict with having a ctor that takes events to hydrate (or any other mechanism to rehydrate for that matter). Sometimes the factory is just a static method (good for "context"/"intent" capturing), sometimes it's an instance method of another aggregate (good place for "data" inheritance), sometimes it's an explicit factory object (good place for "complex" creation logic).
• I like to provide an explicit GetChanges() method on my aggregate that returns the internal list as an array. If my aggregate is to stay in memory beyond one execution, I also add an AcceptChanges() method to indicate the internal list should be cleared (typically called after things were flushed to the event store). You can use either a pull (GetChanges/Changes) or push (think .net event or IObservable) based model here. Much depends on the transactional semantics, tech, needs, etc ...
• Your eventstream is a linked list. Each revision (event/changeset) pointing to the previous one (a.k.a. the parent). Your eventstream is a sequence of events/changes that happened to a specific aggregate. The order is only to be guaranteed within the aggregate boundary.