# Read Models, Projections and Eventual Consistency — Event-Driven Architecture & CQRS

Source: https://www.skillbyai.com/en/event-driven-architecture/q-projections

> Build read models from events and handle the delay in the UI.

## Keeping read models up to date

A **projection** is code that listens to events (or change-data-capture records) and updates a **read model**: insert a row on `OrderPlaced`, update the status on `OrderShipped`, increment a counter on `ProductViewed`. One stream of events can feed many projections: a customer order list, a warehouse picking view, a search index, a revenue dashboard. Projections must be **idempotent** and able to be **rebuilt from scratch**, by replaying events into a fresh table, which also lets you add new read models later. The cost is **eventual consistency**: just after a command succeeds, the read model may not show the change yet. Handle it in the UI: return the new ID and show the result optimistically, poll or push an update, display "processing", or read from the write model for the user's own just-created item.

## A projection for the customer orders view

Upserts make it safe to replay or receive duplicates.

```python
def project(event, db):
    t, d = event["type"], event["data"]
    if t == "OrderPlaced":
        db.execute(
            "INSERT INTO customer_orders_view (order_id, customer_id, placed_at, item_summary, status) "
            "VALUES (%s, %s, %s, %s, 'PLACED') ON CONFLICT (order_id) DO NOTHING",
            (d["orderId"], d["customerId"], event["time"], summarise(d["items"])),
        )
    elif t == "OrderShipped":
        db.execute(
            "UPDATE customer_orders_view SET status = 'SHIPPED', eta = %s WHERE order_id = %s",
            (d.get("eta"), d["orderId"]),
        )
    elif t == "OrderCancelled":
        db.execute(
            "UPDATE customer_orders_view SET status = 'CANCELLED' WHERE order_id = %s",
            (d["orderId"],),
        )
```

## The railway departure board

The departure board is a read model: it is updated from the control room's decisions, usually within seconds. Occasionally a train has already left before the board catches up, so the station staff plan for that delay.

**Quiz:** A user creates an order and is immediately redirected to "My orders", but the new order is missing. What is the most likely cause?

- [ ] The command failed silently
- [ ] The database lost the write
- [ ] The schema registry rejected the event
- [x] The read model has not processed the event yet (eventual consistency)

*Answer:* The read model has not processed the event yet (eventual consistency). Projections lag slightly behind the write side; the UI must account for it.