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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 a 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
--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_WITHCONTRACTS 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 variable transaction fees and execution effort metering
as decribed in Variable Transaction Fees: Execution Effort FLIP
--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.

Using Emulator in Go

You can use the emulator as a module in your Go project. To install emulator, use go get:

go get github.com/onflow/flow-emulator

After installing the emulator module you can initialize it in the code:

var opts []emulator.Option
privKey, err := crypto.DecodePrivateKeyHex(crypto.ECDSA_P256, "")

opts = append(opts, emulator.WithServicePublicKey(
  privKey.PublicKey(),
  crypto.ECDSA_P256,
  crypto.SHA3_256,
))

blockchain, err := emulator.NewBlockchain(opts...)

You can then access all methods of the blockchain like so:

account, err := blockchain.GetAccount(address) 

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 management 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.

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

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