Developer mode

It is often convenient for developers to work in an environment where changes to client or application software can be deployed and tested rapidly and without putting real-world users or assets at risk. For this purpose, Geth has a --dev flag that spins up Geth in "developer mode". This creates a single-node Ethereum test network with no connections to any external peers. It exists solely on the local machine. Starting Geth in developer mode does the following:

This configuration enables developers to experiment with Geth's source code or develop new applications without having to sync to a pre-existing public network. Blocks are only mined when there are pending transactions. Developers can break things on this network without affecting other users. This page will demonstrate how to spin up a local Geth testnet and a simple smart contract will be deployed to it using the Remix online integrated development environment (IDE).

Prerequisites

It is assumed that the user has a working Geth installation (see installation guide). It would also be helpful to have basic knowledge of Geth and the Geth console. See Getting Started. Some basic knowledge of Solidity and smart contract deployment would be useful.

Start Geth in Dev Mode

Starting Geth in developer mode is as simple as providing the --dev flag. It is also possible to create a realistic block creation frequency by setting --dev.period 12 instead of creating blocks only when transactions are pending. Additional configuration options required to follow this tutorial.

Remix will be used to deploy a smart contract to the node which requires information to be exchanged externally with Geth's own domain. To permit this, enable http and the net namespace must be enabled and the Remix URL must be provided to --http.corsdomain. Some other namespaces will also be enabled for this tutorial. The full command is as follows:

geth --dev --http --http.api eth,web3,net --http.corsdomain "https://remix.ethereum.org"

The terminal will display the following logs, confirming Geth has started successfully in developer mode:

This terminal must be left running throughout the entire tutorial. In a second terminal, attach a Javascript console. By default, the ipc file is saved in the datadir:

geth attach <datadir>/geth.ipc

The Javascript terminal will open with the following welcome message:

For simplicity this tutorial uses Geth's built-in account management. First, the existing accounts can be displayed using eth.accounts:

eth.accounts

An array containing a single address will be displayed in the terminal, despite no accounts having yet been explicitly created. This is the "developer" account. The developer address is the recipient of the total amount of ether created at the local network genesis. Querying the ether balance of the developer account will return a very large number. The developer account can be invoked as eth.accounts[0].

The following command can be used to query the balance. The return value is in units of Wei, which is divided by 1¹⁸ to give units of ether. This can be done explicitly or by calling the web3.FromWei() function:

eth.getBalance(eth.accounts[0])/1e18

// or

web3.fromWei(eth.getBalance(eth.accounts[0]))

Using web3.fromWei() is less error-prone because the correct multiplier is built in. These commands both return the following:

A new account can be created using Clef. Some of the ether from the developer account can then be transferred across to it. A new account is generated using the newaccount function on the command line:

clef newaccount --keystore <path-to-keystore>

The terminal will display a request for a password, twice. Once provided, a new account will be created, and its address will be printed to the terminal. The account creation is also logged in the Geth terminal, including the location of the keyfile in the keystore. It is a good idea to back up the password somewhere at this point. If this were an account on a live network, intended to own assets of real-world value, it would be critical to back up the account password and the keystore in a secure manner.

To reconfirm the account creation, running eth.accounts in the Javascript console should display an array containing two account addresses, one being the developer account and the other being the newly generated address. The following command transfers 50 ETH from the developer account to the new account:

eth.sendTransaction({from: eth.accounts[0], to: eth.accounts[1], value: web3.toWei(50, "ether")})

A transaction hash will be returned to the console. This transaction hash will also be displayed in the logs in the Geth console, followed by logs confirming that a new block was mined (remember, in the local development network, blocks are mined when transactions are pending). The transaction details can be displayed in the Javascript console by passing the transaction hash to eth.getTransaction():

eth.getTransaction("0x62044d2cab405388891ee6d53747817f34c0f00341cde548c0ce9834e9718f27")

The transaction details are displayed as follows:

Now that the user account is funded with ether, a contract can be created and deployed to the Geth node.

A simple smart contract

This tutorial will make use of a classic example smart contract, Storage.sol. This contract exposes two public functions, one to add a value to the contract storage and one to view the stored value. The contract, written in Solidity, is provided below:

pragma solidity >=0.8.2 <0.9.0;

contract Storage {
    uint256 number;

    function store(uint256 num) public {
        number = num;
    }

    function retrieve() public view returns (uint256) {
        return number;
    }
}

Solidity is a high-level language that makes code executable by the Ethereum virtual machine (EVM) readable to humans. This means that there is an intermediate step between writing code in Solidity and deploying it to Ethereum. This step is called "compilation", and it converts human-readable code into EVM-executable bytecode. This byte-code is then included in a transaction sent from the Geth node during contract deployment. This can all be done directly from the Geth Javascript console; however, this tutorial uses an online IDE called Remix to handle the compilation and deployment of the contract to the local Geth node.

Compile and deploy using Remix

In a web browser, open https://remix.ethereum.org. This opens an online smart contract development environment. On the left-hand side of the screen there is a side-bar menu that toggles between several toolboxes that are displayed in a vertical panel. On the right-hand side of the screen, there is an editor and a terminal. This layout is similar to the default layout of many other IDEs such as VSCode. The contract defined in the previous section, Storage.sol is already available in the Contracts directory in Remix. It can be opened and reviewed in the editor.

The Solidity logo is present as an icon in the Remix sidebar. Clicking this icon opens the Solidity compiler wizard, which can be used to compile Storage.sol ready. With Solidity.sol open in the editor window, simply click the Compile 1_Storage.sol button. A green tick will appear next to the Solidity icon to confirm that the contract has been compiled successfully. This means the contract bytecode is available.

Below the Solidity icon is a fourth icon that includes the Ethereum logo. Clicking this opens the Deploy menu. In this menu, Remix can be configured to connect to the local Geth node. In the drop-down menu labelled ENVIRONMENT, select Custom - External Http Provider. This will open an information pop-up with instructions for configuring Geth - these can be ignored as they were completed earlier in this tutorial. However, at the bottom of this pop-up is a box labelled External HTTP Provider Endpoint. This should be set to Geth's 8545 port on localhost (http://127.0.0.1:8545). Click OK. The ACCOUNT field should automatically populate with the address of the account created earlier using the Geth Javascript console.

To deploy Storage.sol, click DEPLOY.

The following logs in the Geth terminal confirm that the contract was successfully deployed.

Interact with contract using Remix

The contract is now deployed on a local testnet version of the Ethereum blockchain. This means there is a contract address that contains an executable bytecode that can be invoked by sending transactions with instructions, also in bytecode, to that address. Again, this can all be achieved by constructing transactions directly in the Geth console or even by making external http requests using tools such as Curl. Here, Remix is used to retrieve the value; then the same action is taken using the Javascript console.

After deploying the contract in Remix, the Deployed Contracts tab in the sidebar automatically populates with the public functions exposed by Storage.sol. To send a value to the contract storage, type a number in the field adjacent to the store button, then click the button.

In the Geth terminal, the following logs confirm that the transaction was successful (the actual values will vary from the example below):

The transaction hash can be used to retrieve the transaction details using the Geth Javascript console, which will return the following information:

The from address is the account that sent the transaction, and the to address is the deployment address of the contract. The value entered into Remix is now in storage at that contract address. This can be retrieved using Remix by calling the retrieve function - to do this simply click the retrieve button. Alternatively, it can be retrieved using web3.getStorageAt using the Geth Javascript console. The following command returns the value in the contract storage (replace the given address with the correct one displayed in the Geth logs).

web3.eth.getStorageAt("0x407d73d8a49eeb85d32cf465507dd71d507100c1", 0)

This returns a value that looks like the following:

The returned value is a left-padded hexadecimal value. For example, the return value 0x000000000000000000000000000000000000000000000000000000000000000038 corresponds to a value of 56 entered as a uint256 to Remix. After converting from hexadecimal string to decimal number the returned value should be equal to that provided to Remix in the previous step.

Reusing --datadir

This tutorial used an ephemeral blockchain that is completely destroyed and started afresh during each dev-mode session. However, it is also possible to create persistent blockchain and account data that can be reused across multiple sessions. This is done by providing the --datadir flag and a directory name when starting Geth in dev-mode.

geth --datadir dev-chain --dev --http --http.api web3,eth,net --http.corsdomain "https://remix.ethereum.org"

Re-using accounts

Geth will fail to start in dev-mode if keys have been manually created or imported into the keystore in the --datadir directory. This is because the account cannot be automatically unlocked. To resolve this issue, the password defined when the account was created can be saved to a text file and its path passed to the --password flag on starting Geth, for example, if password.txt is saved in the top-level go-ethereum directory:

geth --datadir dev-chain --dev --http --http.api web3,eth,net --http.corsdomain "https://remix.ethereum.org" --password password.txt

Using a Custom Genesis Configuration

It is possible to use a custom genesis block configuration in development mode. To obtain a compatible configuration, run geth --dev dumpgenesis. The resulting genesis has proof-of-stake and all pre-merge hard forks activated at block 0. Precompile addresses are funded to prevent them being removed from the state per EIP158.

Users are free to modify the generated template provided they keep the pre-merge hard-forks and proof-of-stake transition activated at block 0.

Summary

This tutorial has demonstrated how to spin up a local developer network using Geth. Having started this development network, a simple contract was deployed to the developer network. Then, Remix was connected to the local Geth node and used to deploy and interact with a contract. Remix was used to add a value to the contract storage, and then the value was retrieved using Remix and also using the lower-level commands in the Javascript console.