The Flow Emulator is a lightweight tool that emulates the behaviour of the real Flow network.
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The Emulator

The emulator exposes a gRPC server that implements the Flow Access API, which is designed to have near feature parity with the real network API.

The Flowser Emulator Explorer

There is also an block explorer GUI for the emulator, that will help you speed up development when using the emulator.

Running

Configuration

The Flow Emulator can be run in different modes and settings, all of them are described in the table bellow.

Please note that if you will run the emulator using the Flow CLI you must use flags to pass configuration values and if you plan to run the emulator with Docker you must use the environment variables (Env) to pass configuration values.

Flag	Env	Default	Description
`--port`, `-p`	`FLOW_PORT`	`3569`	gRPC port to listen on
`--rest-port`	`FLOW_RESTPORT`	`8888`	REST API port to listen on
`--admin-port`	`FLOW_ADMINPORT`	`8080`	Admin API port to listen on
`--dev-wallet`	`DEV_WALLET`	`false`	Enable local Dev Wallet server
`--dev-wallet-port`	`DEV_WALLET_PORT`	`8701`	Port to run Dev Wallet server on
`--verbose`, `-v`	`FLOW_VERBOSE`	`false`	Enable verbose logging (useful for debugging)
`--log-format`	`FLOW_LOGFORMAT`	`text`	Output log format (valid values `text`, `JSON`)
`--block-time`, `-b`	`FLOW_BLOCKTIME`	`0`	Time between sealed blocks. Valid units are `ns`, `us` (or `µs`), `ms`, `s`, `m`, `h`
`--contracts`	`FLOW_CONTRACTS`	`false`	Start with contracts like FUSD, NFT and an NFT Marketplace, when the emulator starts
`--service-priv-key`	`FLOW_SERVICEPRIVATEKEY`	random	Private key used for the service account
`--service-pub-key`	`FLOW_SERVICEPUBLICKEY`	random	Public key used for the service account
`--service-sig-algo`	`FLOW_SERVICEKEYSIGALGO`	`ECDSA_P256`	Service account key signature algorithm
`--service-hash-algo`	`FLOW_SERVICEKEYHASHALGO`	`SHA3_256`	Service account key hash algorithm
`--init`	`FLOW_INIT`	`false`	Generate and set a new service account
`--rest-debug`	`FLOW_RESTDEBUG`	`false`	Enable REST API debugging output
`--grpc-debug`	`FLOW_GRPCDEBUG`	`false`	Enable gRPC server reflection for debugging with grpc_cli
`--persist`	`FLOW_PERSIST`	false	Enable persistence of the state between restarts
`--dbpath`	`FLOW_DBPATH`	`./flowdb`	Specify path for the database file persisting the state
`--simple-addresses`	`FLOW_SIMPLEADDRESSES`	`false`	Use sequential addresses starting with `0x1`
`--token-supply`	`FLOW_TOKENSUPPLY`	`1000000000.0`	Initial FLOW token supply
`--transaction-expiry`	`FLOW_TRANSACTIONEXPIRY`	`10`	Transaction expiry, measured in blocks
`--storage-limit`	`FLOW_STORAGELIMITENABLED`	`true`	Enable account storage limit
`--storage-per-flow`	`FLOW_STORAGEMBPERFLOW`		Specify size of the storage in MB for each FLOW in account balance. Default value from the flow-go
`--min-account-balance`	`FLOW_MINIMUMACCOUNTBALANCE`		Specify minimum balance the account must have. Default value from the flow-go
`--transaction-fees`	`FLOW_TRANSACTIONFEESENABLED`	`false`	Enable transaction fees
`--transaction-max-gas-limit`	`FLOW_TRANSACTIONMAXGASLIMIT`	`9999`	Maximum gas limit for transactions
`--script-gas-limit`	`FLOW_SCRIPTGASLIMIT`	`100000`	Specify gas limit for script execution

Running the emulator with the Flow CLI

The emulator is bundled with the Flow CLI, a command-line interface for working with Flow.

Installation

Follow these steps to install the Flow CLI.

Starting the server

Starting the emulator by using Flow CLI also leverages CLI configuration file flow.json. You can use the flow.json to specify the service account which will be reused between restarts. Read more about CLI configuration here.

You can start the emulator with the Flow CLI:

flow emulator --init

Using the emulator in a project

You can start the emulator in your project context by running the above command in the same directory as flow.json. This will configure the emulator with your project's service account, meaning you can use it to sign and submit transactions. Read more about the project and configuration here.

Managing emulator state

It's possible to manage emulator state by using the admin API. You can at any point create a new named snapshot of the state and then at any later point revert emulator state to that reference.

In order to use the state managment functionality you need to run the emulator with persistent state:

flow emulator --persist

Create a new snapshot by doing an HTTP request:

GET http://localhost:8080/emulator/snapshot/{name}

Please note the example above uses the default admin API port

At any later point you can reload to that snapshot by executing the same HTTP request as before. You need to use the same value for name parameter.

The snapshot functionality is a great tool for testing where you can first initialize a base snapshot with seed values, execute the test and then revert to that initialized state.

Launching dev-wallet with the emulator

You can start the dev-wallet with the --dev-wallet flag. Default dev-wallet port is 8701.

After starting dev-wallet, you can set your fcl config to use it like below:

import * as fcl from "@onflow/fcl"

fcl.config()
  // Point App at Emulator
  .put("accessNode.api", "http://localhost:8080") 
  // Point FCL at dev-wallet (default port)
  .put("discovery.wallet", "http://localhost:8701/fcl/authn")

You can read more about setting up dev-wallet at FCL Dev Wallet Project

Running the emulator with Docker

Docker builds for the emulator are automatically built and pushed to gcr.io/flow-container-registry/emulator, tagged by commit and semantic version. You can also build the image locally.

docker run gcr.io/flow-container-registry/emulator

The full list of environment variables can be found here. You can pass any environment variable by using -e docker flag and pass the valid value.

Custom Configuration Example:

docker run -e FLOW_PORT=9001 -e FLOW_VERBOSE=true -e FLOW_SERVICEPUBLICKEY=<hex-encoded key> gcr.io/flow-container-registry/emulator

To generate a service key, use the keys generate command in the Flow CLI.

flow keys generate

Development

Read contributing document.

arturenault/flow-emulator