Why should you care?

Auditable domains — finance, healthcare workflows, inventory — need “what happened” not only “what is”.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.orderEvents.insertMany([
  { streamId: "ord-1", version: 1, type: "OrderCreated", data: { userId: "u1" }, at: new Date() },
  { streamId: "ord-1", version: 2, type: "ItemAdded", data: { sku: "TV-55", qty: 1 }, at: new Date() },
  { streamId: "ord-1", version: 3, type: "OrderPaid", data: { amount: 42999 }, at: new Date() }
])
db.orderEvents.createIndex({ streamId: 1, version: 1 }, { unique: true })
// Projection:
db.orderState.updateOne(
  { _id: "ord-1" },
  { $set: { userId: "u1", items: [{ sku: "TV-55", qty: 1 }], status: "paid", amount: 42999 } },
  { upsert: true }
)
db.orderEvents.find({ streamId: "ord-1" }).sort({ version: 1 })

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• orderEvents is append-only with unique streamId+version.
• orderState is a projection for fast reads.
• Replaying events rebuilds state if projections corrupt.
• Never update old events — append compensating ones.

Practice next

1. Insert the three events.
2. Build/update the projection document.
3. Query the event stream in version order.
4. Rebuild orderState from scratch by folding events in a script.
5. Watch orderEvents with change streams to update projections.

Remember

Append immutable events per stream. Project current state separately. Unique (streamId, version) is critical.

Ledger-grade order audit

Support replays ord-1 events to see exactly when an item was added and paid.

Outcome: Disputes resolve with a full timeline, not a guess.