Eventarc

Eventarc delivers events from Google services as CloudEvents: a Firestore document was created, an object was written, an API was called. consumeEvents creates the trigger and runs your handler for each event.

React to new Firestore documents

src/Auditor.ts

import * as GCP from "@alchemy.run/gcp";
import * as Effect from "effect/Effect";
import * as HttpServerResponse from "effect/http/HttpServerResponse";
import { Shop } from "./resources.ts";

export default class Auditor extends GCP.Function<Auditor>()(
  "Auditor",
  { main: import.meta.url },
  Effect.gen(function* () {
    const shop = yield* Shop;
    const db = yield* GCP.Firestore.WriteDatabase(shop);

    yield* GCP.Eventarc.consumeEvents(
      "OrderCreated",
      {
        location: "us-central1",
        eventFilters: [
          { attribute: "type", value: "google.cloud.firestore.document.v1.created" },
          { attribute: "database", value: shop.databaseId },
          { attribute: "document", value: "orders/{id}", operator: "match-path-pattern" },
        ],
      },
      (event) =>
        db.set(`audit/${event.id}`, { document: event.attributes.document ?? "" }).pipe(
          Effect.orDie,
        ),
    );

    return { fetch: Effect.succeed(HttpServerResponse.text("ok")) };
  }).pipe(
    Effect.provide(GCP.Firestore.WriteDatabaseHttp),
    Effect.provide(GCP.Run.EventarcEventSource),
  ),
) {}

eventFilters choose which events you get. event.attributes names the document; event.data holds the change itself.

Match the database’s location

Firestore only sends events to triggers in the database’s own location. Pin both:

src/resources.ts

export const Shop = GCP.Firestore.Database("Shop", {
  location: "us-central1",
  type: "FIRESTORE_NATIVE",
});

The service stays private

The trigger signs each delivery with the service’s own identity, and Alchemy grants that identity what it needs to receive events. Nobody else can call the event path.

Handle repeats

Eventarc delivers at least once. Use set keyed by the event, or create and ignore DocumentAlreadyExists, so a second delivery changes nothing.

Example

examples/gcp-eventarc-firestore writes an audit entry for every new order.

Where next

• Firestore — the database that emits the events.
• Eventarc reference — triggers, channels, and pipelines.