Connect your app to the Realtime Database Emulator

Before connecting your app to the Realtime Database emulator, make sure that you understand the overall Firebase Local Emulator Suite workflow, and that you install and configure the Local Emulator Suite and review its CLI commands.

Choose a Firebase project

The Firebase Local Emulator Suite emulates products for a single Firebase project.

To select the project to use, before you start the emulators, in the CLI run firebase use in your working directory. Or, you can pass the --project flag to each emulator command.

Local Emulator Suite supports emulation of real Firebase projects and demo projects.

We recommend you use demo projects wherever possible. Benefits include:

• Easier setup, since you can run the emulators without ever creating a Firebase project
• Stronger safety, since if your code accidentally invokes non-emulated (production) resources, there is no chance of data change, usage and billing
• Better offline support, since there is no need to access the internet to download your SDK configuration.

Instrument your app to talk to the emulators

Android, iOS, and Web SDKs

Set up your in-app configuration or test classes to interact with the Realtime Database as follows.

        // 10.0.2.2 is the special IP address to connect to the 'localhost' of
        // the host computer from an Android emulator.
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        database.useEmulator("10.0.2.2", 9000);
EmulatorSuite.java

    // In almost all cases the ns (namespace) is your project ID.
let db = Database.database(url:"http://localhost:9000?ns=YOUR_DATABASE_NAMESPACE")
ViewController.swift

var db = firebase.database();
if (location.hostname === "localhost") {
  // Point to the RTDB emulator running on localhost.
  db.useEmulator("localhost", 9000);
} 
emulator-suite.js

import { getDatabase, useDatabaseEmulator } from "firebase/database";

const db = getDatabase();
if (location.hostname === "localhost") {
  // Point to the RTDB emulator running on localhost.
  useDatabaseEmulator(db, "localhost", 9000);
} 
rtdb_emulator_connect.js

No additional setup is needed to test Cloud Functions triggered by Realtime Database events using the emulator. When the Realtime Database and Cloud Functions emulators are both running, they automatically work together.

Admin SDKs

The Firebase Admin SDKs automatically connect to the Realtime Database emulator when the FIREBASE_DATABASE_EMULATOR_HOST environment variable is set:

export FIREBASE_DATABASE_EMULATOR_HOST="localhost:9000"

If your code is running inside the Cloud Functions emulator your project ID and other configuration will be automatically set when calling initalizeApp.

When connecting to the Realtime Database emulator from any other environment, you will need to specify a project ID. You can pass a project ID to initializeApp directly or set the GCLOUD_PROJECT environment variable. Note that you do not need to use your real Firebase project ID, the Realtime Database emulator will accept any project ID:

admin.initializeApp({ projectId: "your-project-id" });

export GCLOUD_PROJECT="your-project-id"

Clear your database between tests

To flush the Realtime Database between activities, you can clear the database reference. You can use this approach as an alternative to simply shutting down the emulator process.

// With a DatabaseReference, write null to clear the database.
database.getReference().setValue(null);
EmulatorSuite.java

// With a DatabaseReference, write nil to clear the database.
    Database.database().reference().setValue(nil);

// With a database Reference, write null to clear the database.
firebase.database().ref().set(null);
emulator-suite.js

import { getDatabase, ref, set } from "firebase/database";

// With a database Reference, write null to clear the database.
const db = getDatabase();
set(ref(db), null);
rtdb_emulator_flush.js

Naturally, your code should await confirmation that the flush finished or failed using the asynchronous event handling features of your platform.

Having implemented a step like this, you can sequence your tests and trigger your functions with confidence that old data will be purged between runs and you're using a fresh baseline test configuration.

Import and export data

The database and Cloud Storage emulators allow you to export data from a running emulator instance. Define a baseline set of data to use in your unit tests or continuous integration workflows, then export it to be shared among the team.

firebase emulators:export ./dir

In tests, on emulator startup, import the baseline data.

firebase emulators:start --import=./dir

You can instruct the emulator to export data on shutdown, either specifying an export path or simply using the path passed to the --import flag.

firebase emulators:start --import=./dir --export-on-exit

These data import and export options work with the firebase emulators:exec command as well. For more, refer to the emulator command reference.

Visualize Security Rules activity

As you work through prototype and test loops, you can use visualization tools and reports provided by the Local Emulator Suite.

Visualize Rules evaluations

As you add Security Rules to your prototype you can debug them with Local Emulator Suite tools.

After running a suite of tests, you can access test coverage reports that show how each of your rules was evaluated. To get the reports, query an exposed endpoint on the emulator while it's running. For a browser-friendly version, use the following URL:

http://localhost:9000/.inspect/coverage?ns=<database_name>

This breaks your rules into expressions and subexpressions that you can mouseover for more information, including number of executions and values returned. For the raw JSON version of this data, include the following URL in your query:

http://localhost:9000/.inspect/coverage.json?ns=<database_name>

What next?

• For a curated set of videos and detailed how-to examples, follow the Firebase Emulators Training Playlist.
• Investigate advanced use cases involving Security Rules testing and the Firebase Test SDK: Test Security Rules (Realtime Database).
• Since triggered functions are a typical integration with Realtime Database, learn more about the Cloud Functions for Firebase emulator at Run functions locally.