go-ethereum/docs/_dapp/native-bindings.md

title
Go Contract Bindings

[Please note, events are not yet implemented as they need some RPC subscription features that are still under review.]

The original roadmap and/or dream of the Ethereum platform was to provide a solid, high performing client implementation of the consensus protocol in various languages, which would provide an RPC interface for JavaScript DApps to communicate with, pushing towards the direction of the Mist browser, through which users can interact with the blockchain.

Although this was a solid plan for mainstream adoption and does cover quite a lot of use cases that people come up with (mostly where people manually interact with the blockchain), it eludes the server side (backend, fully automated, devops) use cases where JavaScript is usually not the language of choice given its dynamic nature.

This page introduces the concept of server side native Dapps: Go language bindings to any Ethereum contract that is compile time type safe, highly performant and best of all, can be generated fully automatically from a contract ABI and optionally the EVM bytecode.

This page is written in a more beginner friendly tutorial style to make it easier for people to start out with writing Go native Dapps. The used concepts will be introduced gradually as a developer would need/encounter them. However, we do assume the reader is familiar with Ethereum in general, has a fair understanding of Solidity and can code Go.

Token contract

To avoid falling into the fallacy of useless academic examples, we're going to take the official Token contract as the base for introducing the Go native bindings. If you're unfamiliar with the contract, skimming the linked page should probably be enough, the details aren't relevant for now. In short the contract implements a custom token that can be deployed on top of Ethereum. To make sure this tutorial doesn't go stale if the linked website changes, the Solidity source code of the Token contract is also available at token.sol.

Go binding generator

Interacting with a contract on the Ethereum blockchain from Go (or any other language for a matter of fact) is already possible via the RPC interfaces exposed by Ethereum clients. However, writing the boilerplate code that translates decent Go language constructs into RPC calls and back is extremely time consuming and also extremely brittle: implementation bugs can only be detected during runtime and it's almost impossible to evolve a contract as even a tiny change in Solidity can be painful to port over to Go.

To avoid all this mess, the go-ethereum implementation introduces a source code generator that can convert Ethereum ABI definitions into easy to use, type-safe Go packages. Assuming you have a valid Go development environment set up, godep installed and the go-ethereum repository checked out correctly, you can build the generator with:

$ cd $GOPATH/src/github.com/ethereum/go-ethereum
$ godep go install ./cmd/abigen

Generating the bindings

The single essential thing needed to generate a Go binding to an Ethereum contract is the contract's ABI definition JSON file. For our Token contract tutorial you can obtain this either by compiling the Solidity code yourself (e.g. via @chriseth's online Solidity compiler), or you can download our pre-compiled token.abi.

To generate a binding, simply call:

$ abigen --abi token.abi --pkg main --type Token --out token.go

Where the flags are:

• --abi: Mandatory path to the contract ABI to bind to
• --pgk: Mandatory Go package name to place the Go code into
• --type: Optional Go type name to assign to the binding struct
• --out: Optional output path for the generated Go source file (not set = stdout)

This will generate a type-safe Go binding for the Token contract. The generated code will look something like token.go, but please generate your own as this will change as more work is put into the generator.

Accessing an Ethereum contract

To interact with a contract deployed on the blockchain, you'll need to know the address of the contract itself, and need to specify a backend through which to access Ethereum. The binding generator provides out of the box an RPC backend through which you can attach to an existing Ethereum node via IPC, HTTP or WebSockets.

We'll use the foundation's Unicorn token contract deployed on the testnet to demonstrate calling contract methods. It is deployed at the address 0x21e6fc92f93c8a1bb41e2be64b4e1f88a54d3576.

To run the snippet below, please ensure a Geth instance is running and attached to the Morden test network where the above mentioned contract was deployed. Also please update the path to the IPC socket below to the one reported by your own local Geth node.

package main

import (
	"fmt"
	"log"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/ethclient"
)

func main() {
	// Create an IPC based RPC connection to a remote node
	conn, err := ethclient.Dial("/home/karalabe/.ethereum/testnet/geth.ipc")
	if err != nil {
		log.Fatalf("Failed to connect to the Ethereum client: %v", err)
	}
	// Instantiate the contract and display its name
	token, err := NewToken(common.HexToAddress("0x21e6fc92f93c8a1bb41e2be64b4e1f88a54d3576"), conn)
	if err != nil {
		log.Fatalf("Failed to instantiate a Token contract: %v", err)
	}
	name, err := token.Name(nil)
	if err != nil {
		log.Fatalf("Failed to retrieve token name: %v", err)
	}
	fmt.Println("Token name:", name)
}

And the output (yay):

Token name: Testnet Unicorn

If you look at the method invoked to read the token name token.Name(nil), it required a parameter to be passed, even though the original Solidity contract requires none. This is a *bind.CallOpts type, which can be used to fine tune the call.

• Pending: Whether to access pending contract state or the current stable one
• GasLimit: Place a limit on the computing resources the call might consume

Transacting with an Ethereum contract

Invoking a method that changes contract state (i.e. transacting) is a bit more involved, as a live transaction needs to be authorized and broadcast into the network. Opposed to the conventional way of storing accounts and keys in the node we attach to, Go bindings require signing transactions locally and do not delegate this to a remote node. This is done so to facilitate the general direction of the Ethereum community where accounts are kept private to DApps, and not shared (by default) between them.

Thus to allow transacting with a contract, your code needs to implement a method that given an input transaction, signs it and returns an authorized output transaction. Since most users have their keys in the Web3 Secret Storage format, the bind package contains a small utility method (bind.NewTransactor(keyjson, passphrase)) that can create an authorized transactor from a key file and associated password, without the user needing to implement key signing himself.

Changing the previous code snippet to send one unicorn to the zero address:

package main

import (
	"fmt"
	"log"
	"math/big"
	"strings"

	"github.com/ethereum/go-ethereum/accounts/abi/bind"
	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/ethclient"
)

const key = `paste the contents of your *testnet* key json here`

func main() {
	// Create an IPC based RPC connection to a remote node and instantiate a contract binding
	conn, err := ethclient.Dial("/home/karalabe/.ethereum/testnet/geth.ipc")
	if err != nil {
		log.Fatalf("Failed to connect to the Ethereum client: %v", err)
	}
	token, err := NewToken(common.HexToAddress("0x21e6fc92f93c8a1bb41e2be64b4e1f88a54d3576"), conn)
	if err != nil {
		log.Fatalf("Failed to instantiate a Token contract: %v", err)
	}
	// Create an authorized transactor and spend 1 unicorn
	auth, err := bind.NewTransactor(strings.NewReader(key), "my awesome super secret password")
	if err != nil {
		log.Fatalf("Failed to create authorized transactor: %v", err)
	}
	tx, err := token.Transfer(auth, common.HexToAddress("0x0000000000000000000000000000000000000000"), big.NewInt(1))
	if err != nil {
		log.Fatalf("Failed to request token transfer: %v", err)
	}
	fmt.Printf("Transfer pending: 0x%x\n", tx.Hash())
}

And the output (yay):

Transfer pending: 0x4f4aaeb29ed48e88dd653a81f0b05d4df64a86c99d4e83b5bfeb0f0006b0e55b

Note, with high probability you won't have any testnet unicorns available to spend, so the above program will fail with an error. Send at least 2.014 testnet(!) Ethers to the foundation testnet tipjar 0xDf7D0030bfed998Db43288C190b63470c2d18F50 to receive a unicorn token and you'll be able to see the above code run without an error!

Similar to the method invocations in the previous section which only read contract state, transacting methods also require a mandatory first parameter, a *bind.TransactOpts type, which authorizes the transaction and potentially fine tunes it:

• From: Address of the account to invoke the method with (mandatory)
• Signer: Method to sign a transaction locally before broadcasting it (mandatory)
• Nonce: Account nonce to use for the transaction ordering (optional)
• GasLimit: Place a limit on the computing resources the call might consume (optional)
• GasPrice: Explicitly set the gas price to run the transaction with (optional)
• Value: Any funds to transfer along with the method call (optional)

The two mandatory fields are automatically set by the bind package if the auth options are constructed using bind.NewTransactor. The nonce and gas related fields are automatically derived by the binding if they are not set. An unset value is assumed to be zero.

Pre-configured contract sessions

As mentioned in the previous two sections, both reading as well as state modifying contract calls require a mandatory first parameter which can both authorize as well as fine tune some of the internal parameters. However, most of the time we want to use the same parameters and issue transactions with the same account, so always constructing the call/transact options or passing them along with the binding can become unwieldy.

To avoid these scenarios, the generator also creates specialized wrappers that can be pre- configured with tuning and authorization parameters, allowing all the Solidity defined methods to be invoked without needing an extra parameter.

These are named analogous to the original contract type name, just suffixed with Sessions:

// Wrap the Token contract instance into a session
session := &TokenSession{
	Contract: token,
	CallOpts: bind.CallOpts{
		Pending: true,
	},
	TransactOpts: bind.TransactOpts{
		From:     auth.From,
		Signer:   auth.Signer,
		GasLimit: big.NewInt(3141592),
	},
}
// Call the previous methods without the option parameters
session.Name()
session.Transfer("0x0000000000000000000000000000000000000000"), big.NewInt(1))

Deploying contracts to Ethereum

Interacting with existing contracts is nice, but let's take it up a notch and deploy a brand new contract onto the Ethereum blockchain! To do so however, the contract ABI we used to generate the binding is not enough. We need the compiled bytecode too to allow deploying it.

To get the bytecode, either go back to the online compiler with which you may generate it, or alternatively download our token.bin. You'll need to rerun the Go generator with the bytecode included for it to create deploy code too:

$ abigen --abi token.abi --pkg main --type Token --out token.go --bin token.bin

This will generate something similar to token.go. If you quickly skim this file, you'll find an extra DeployToken function that was just injected compared to the previous code. Beside all the parameters specified by Solidity, it also needs the usual authorization options to deploy the contract with and the Ethereum backend to deploy the contract through.

Putting it all together would result in:

package main

import (
	"fmt"
	"log"
	"math/big"
	"strings"
	"time"

	"github.com/ethereum/go-ethereum/accounts/abi/bind"
	"github.com/ethereum/go-ethereum/ethclient"
)

const key = `paste the contents of your *testnet* key json here`

func main() {
	// Create an IPC based RPC connection to a remote node and an authorized transactor
	conn, err := rpc.NewIPCClient("/home/karalabe/.ethereum/testnet/geth.ipc")
	if err != nil {
		log.Fatalf("Failed to connect to the Ethereum client: %v", err)
	}
	auth, err := bind.NewTransactor(strings.NewReader(key), "my awesome super secret password")
	if err != nil {
		log.Fatalf("Failed to create authorized transactor: %v", err)
	}
	// Deploy a new awesome contract for the binding demo
	address, tx, token, err := DeployToken(auth, conn), new(big.Int), "Contracts in Go!!!", 0, "Go!")
	if err != nil {
		log.Fatalf("Failed to deploy new token contract: %v", err)
	}
	fmt.Printf("Contract pending deploy: 0x%x\n", address)
	fmt.Printf("Transaction waiting to be mined: 0x%x\n\n", tx.Hash())

	// Don't even wait, check its presence in the local pending state
	time.Sleep(250 * time.Millisecond) // Allow it to be processed by the local node :P

	name, err := token.Name(&bind.CallOpts{Pending: true})
	if err != nil {
		log.Fatalf("Failed to retrieve pending name: %v", err)
	}
	fmt.Println("Pending name:", name)
}

And the code performs as expected: it requests the creation of a brand new Token contract on the Ethereum blockchain, which we can either wait for to be mined or as in the above code start calling methods on it in the pending state :)

Contract pending deploy: 0x46506d900559ad005feb4645dcbb2dbbf65e19cc
Transaction waiting to be mined: 0x6a81231874edd2461879b7280ddde1a857162a744e3658ca7ec276984802183b

Pending name: Contracts in Go!!!

Bind Solidity directly

If you've followed the tutorial along until this point you've probably realized that every contract modification needs to be recompiled, the produced ABIs and bytecodes (especially if you need multiple contracts) individually saved to files and then the binding executed for them. This can become a quite bothersome after the Nth iteration, so the abigen command supports binding from Solidity source files directly (--sol), which first compiles the source code (via --solc, defaulting to solc) into it's constituent components and binds using that.

Binding the official Token contract token.sol would then entail to running:

$ abigen --sol token.sol --pkg main --out token.go

Note: Building from Solidity (--sol) is mutually exclusive with individually setting the bind components (--abi, --bin and --type), as all of them are extracted from the Solidity code and produced build results directly.

Building a contract directly from Solidity has the nice side effect that all contracts contained within a Solidity source file are built and bound, so if your file contains many contract sources, each and every one of them will be available from Go code. The sample Token solidity file results in token.go.

Project integration (i.e. go generate)

The abigen command was made in such a way as to play beautifully together with existing Go toolchains: instead of having to remember the exact command needed to bind an Ethereum contract into a Go project, we can leverage go generate to remember all the nitty-gritty details.

Place the binding generation command into a Go source file before the package definition:

//go:generate abigen --sol token.sol --pkg main --out token.go

After which whenever the Solidity contract is modified, instead of needing to remember and run the above command, we can simply call go generate on the package (or even the entire source tree via go generate ./...), and it will correctly generate the new bindings for us.

Blockchain simulator

Being able to deploy and access already deployed Ethereum contracts from within native Go code is an extremely powerful feature, but there is one facet with developing native code that not even the testnet lends itself well to: automatic unit testing. Using go-ethereum internal constructs it's possible to create test chains and verify them, but it is unfeasible to do high level contract testing with such low level mechanisms.

To sort out this last issue that would make it hard to run (and test) native DApps, we've also implemented a simulated blockchain, that can be set as a backend to native contracts the same way as a live RPC backend could be: backends.NewSimulatedBackend(genesisAccounts).

package main

import (
	"fmt"
	"log"
	"math/big"

	"github.com/ethereum/go-ethereum/accounts/abi/bind"
	"github.com/ethereum/go-ethereum/accounts/abi/bind/backends"
	"github.com/ethereum/go-ethereum/core"
	"github.com/ethereum/go-ethereum/crypto"
)

func main() {
	// Generate a new random account and a funded simulator
	key, _ := crypto.GenerateKey()
	auth := bind.NewKeyedTransactor(key)

	sim := backends.NewSimulatedBackend(core.GenesisAccount{Address: auth.From, Balance: big.NewInt(10000000000)})

	// Deploy a token contract on the simulated blockchain
	_, _, token, err := DeployMyToken(auth, sim, new(big.Int), "Simulated blockchain tokens", 0, "SBT")
	if err != nil {
		log.Fatalf("Failed to deploy new token contract: %v", err)
	}
	// Print the current (non existent) and pending name of the contract
	name, _ := token.Name(nil)
	fmt.Println("Pre-mining name:", name)

	name, _ = token.Name(&bind.CallOpts{Pending: true})
	fmt.Println("Pre-mining pending name:", name)

	// Commit all pending transactions in the simulator and print the names again
	sim.Commit()

	name, _ = token.Name(nil)
	fmt.Println("Post-mining name:", name)

	name, _ = token.Name(&bind.CallOpts{Pending: true})
	fmt.Println("Post-mining pending name:", name)
}

And the output (yay):

Pre-mining name: 
Pre-mining pending name: Simulated blockchain tokens
Post-mining name: Simulated blockchain tokens
Post-mining pending name: Simulated blockchain tokens

Note, that we don't have to wait for a local private chain miner, or testnet miner to integrate the currently pending transactions. When we decide to mine the next block, we simply Commit() the simulator.