Show data that updates live

Goal: a screen should reflect new data the moment it changes — a dashboard count, a list of open items — without the user refreshing or you writing polling code.

Make the query observable

A read model can be served as an observable stream. Instead of returning a snapshot, the query returns an ISubject<T> that pushes a new value whenever the underlying read model changes:

[ReadModel]
[FromEvent<BookBorrowed>]
[FromEvent<BookReturned>]
public record OnLoan([property: Key] BookId Id, string Title)
{
    public static ISubject<IEnumerable<OnLoan>> AllOnLoan(IMongoCollection<OnLoan> collection) =>
        collection.Observe();
}

Observe() turns the collection into a live subject. As events update the read model, subscribers receive the new set.

Consume it on the frontend

The generated proxy for an observable query exposes a .use() hook (and the Components data table that wraps it). It re-renders on every push — no polling:

const [onLoan] = AllOnLoan.use();
// onLoan.data is always the latest set

Or drop it straight into a live table — see Displaying data.

How it works

When a command appends BookBorrowed, the projection updates the OnLoan read model, and the observable pushes the new value to every subscriber. The brief gap between the append and the push is the normal eventual consistency window — usually imperceptible.

See also

• Read Models — consistency and watching read models.
• Displaying data — rendering an observable query in Components.