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249 lines
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249 lines
10 KiB
---
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title: Go API
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sort_key: D
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---
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Ethereum was originally conceptualized to be the base layer for [Web3][web3-link], providing
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the backbone for a new generation of decentralized, permissionless and censorship resistant
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applications called [dapps][dapp-link]. The first step towards this vision was the development
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of clients providing an RPC interface into the peer-to-peer protocols. This allowed users to
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transact between accounts and interact with smart contracts using command line tools.
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Geth was one of the original clients to provide this type of gateway to the Ethereum network.
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Before long, web-browser-like graphical interfaces (e.g. Mist) were created to extend clients, and
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client functions were built into websites built using the time-tested HTML/CSS/JS stack.
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However, to support the most diverse, complex dapps, developers require programmatic access to client
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functions through an API. This opens up client technologies as re-usable, composable units that
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can be applied in creative ways by a global community of developers.
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To support this, Geth ships official Go packages that can be embedded into third party
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desktop and server applications. There is also a [mobile API](/docs/dapp/mobile) that can be
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used to embed Geth into mobile applications.
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This page provides a high-level overview of the Go API.
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*Note, this guide will assume some familiarity with Go development. It does not cover general topics
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about Go project layouts, import paths or any other standard methodologies. If you are new to Go,
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consider reading [Getting Started with Go][go-guide] first.*
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## Overview
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Geth's reusable Go libraries focus on three main usage areas:
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- Simplified client side account management
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- Remote node interfacing via different transports
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- Contract interactions through auto-generated bindings
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The libraries are updated synchronously with the Geth Github repository.
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The Go libraries can be viewed in full at [Go Packages][go-pkg-link].
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Péter Szilágyi (@karalabe) gave a high level overview of the Go libraries in
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a talk at DevCon2 in Shanghai in 2016. The slides are still a useful resource
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([available here][peter-slides]) and the talk itself can be viewed by clicking
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the image below (it is also archived on [IPFS][ipfs-link]).
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[](https://www.youtube.com/watch?v=R0Ia1U9Gxjg)
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## Go packages
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The `go-ethereum` library is distributed as a collection of standard Go packages straight from go-ethereum's
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GitHub repository. The packages can be used directly via the official Go toolkit, without needing any
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third party tools.
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The canonical import path for Geth is `github.com/ethereum/go-ethereum`, with all packages residing
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underneath. Although there are [lots of them][go-ethereum-dir] most developers will only care about
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a limited subset.
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All the Geth packages can be downloaded using:
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```
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$ go get -d github.com/ethereum/go-ethereum/...
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```
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More Go API support for dapp developers can be found on the [Go Contract Bindings](/docs/dapp/native-bindings)
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and [Go Account Management](/docs/dapp/native-accounts) pages.
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## Tutorial
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This section includes some basic usage examples for the `ethclient` and `gethclient` packages available as
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part of the Go API. The `ethclient` package provides a client that implements the full Ethereum JSON-RPC API,
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whereas `gethclient` offers the Geth-specific API.
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### Instantiating a client
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The client is an instance of the `Client` struct which has associated functions that wrap requests to the Ethereum
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or Geth RPC API endpoints.
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A client is instantiated by passing a raw url or path to an ipc file to the client's `Dial` function. In the following
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code snippet the path to the ipc file for a local Geth node is provided to `ethclient.Dial()`.
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```go
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// create instance of ethclient and assign to cl
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cl, err := ethclient.Dial("/tmp/geth.ipc")
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if err != nil {
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panic(err)
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}
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_ = cl
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```
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### Interacting with the client
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The client can now be used to handle requests to the Geth node using the full JSON-RPC API. For example, the function
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`BlockNumer()` wraps a call to the `eth_blockNumber` endpoint. The function `SendTransaction` wraps a call to
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`eth_sendTransaction`. The full list of client methods can be found [here][ethclient-pkg].
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Frequently, the functions take an instance of the `Context` type as their leading argument. This defines context about requests sent from the application such as deadlines, cancellation signals etc. More information on this can
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be found in the [Go documentation](https://pkg.go.dev/golang.org/x/net/context). An empty context instance can be
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created using `Context.Background()`.
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### Querying client for data
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A simple starting point is to fetch the chain ID from the client. This e.g. is needed when signing a transaction as is to be seen in the next section.
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```go
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chainid, err := cl.ChainID(context.Background())
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if err != nil {
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return err
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}
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```
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Unlike `ChainID`, many functions require arguments other than context. The Go API takes in and returns high-level types which are used in Geth internals as well to simplify programming and remove the need for knowing how data needs to be formatted exactly as per the JSON-RPC API spec. For example to find out the nonce for an account at a given block the address needs to be provided as a `common.Address` type and the block number as a `*big.Int`:
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```go
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addr := common.HexToAddress("0xb02A2EdA1b317FBd16760128836B0Ac59B560e9D")
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nonce, err := cl.NonceAt(context.Background(), addr, big.NewInt(14000000))
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```
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### Querying past events
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Contracts emit events during execution which can be queried from the client. The parameters for the event one is interested in have to be filled out in the `ethereum.FilterQuery` object. This includes which event topics are of interested, from which contracts and during which range of blocks. The example below queries `Transfer` events of all ERC-20 tokens for the last 10 blocks:
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```go
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blockNum, err := cl.BlockNumber(context.Background())
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if err != nil {
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return err
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}
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q := ethereum.FilterQuery{
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FromBlock: new(big.Int).Sub(blockNum, big.NewInt(10)),
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ToBlock: blockNum,
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Topics: [][]common.Hash{common.HexToHash("0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef")},
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}
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logs, err := cl.FilterLogs(context.Background(), q)
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if err != nil {
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return err
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}
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```
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### Sending a transaction
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Sending a transaction is achieved using the `SendTransaction()` function. `SendTransaction` takes an instance of
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`context.Context` as its leading argument and a signed transaction as its second argument. The signed transaction
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must be generated in advance. Building the signed transaction is a multi-stage
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process that requires first generating a key pair if none exists already, retrieving some chain data and defining sender and recipient
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addresses. Then these data can be collected into a transaction object and signed. The resulting signed transaction
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can then be passed to `SendTransaction`.
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The example below assumes the following key pair has already been generated:
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```go
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// SK and ADDR are the secret key and sender address
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SK = "0xaf5ead4413ff4b78bc94191a2926ae9ccbec86ce099d65aaf469e9eb1a0fa87f"
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ADDR = "0x6177843db3138ae69679A54b95cf345ED759450d"
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```
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The secret key and address can be used to send a transaction. In the example below 1 ETH is sent from the
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address `ADDR` to an arbitrary recipient.
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```go
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import (
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"context"
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"math/big"
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"github.com/ethereum/go-ethereum/common"
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"github.com/ethereum/go-ethereum/core/types"
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"github.com/ethereum/go-ethereum/crypto"
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"github.com/ethereum/go-ethereum/ethclient"
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"github.com/ethereum/go-ethereum/params"
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)
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// sendTransaction sends a transaction with 1 ETH to a specified address.
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func sendTransaction(cl *ethclient.Client) error {
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var (
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sk = crypto.ToECDSAUnsafe(common.FromHex(SK))
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to = common.HexToAddress("0xb02A2EdA1b317FBd16760128836B0Ac59B560e9D")
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value = new(big.Int).Mul(big.NewInt(1), big.NewInt(params.Ether))
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sender = common.HexToAddress(ADDR)
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gasLimit = uint64(21000)
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)
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// Retrieve the chainid (needed for signer)
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chainid, err := cl.ChainID(context.Background())
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if err != nil {
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return err
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}
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// Retrieve the pending nonce
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nonce, err := cl.PendingNonceAt(context.Background(), sender)
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if err != nil {
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return err
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}
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// Get suggested gas price
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tipCap, _ := cl.SuggestGasTipCap(context.Background())
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feeCap, _ := cl.SuggestGasPrice(context.Background())
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// Create a new transaction
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tx := types.NewTx(
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&types.DynamicFeeTx{
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ChainID: chainid,
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Nonce: nonce,
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GasTipCap: tipCap,
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GasFeeCap: feeCap,
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Gas: gasLimit,
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To: &to,
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Value: value,
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Data: nil,
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})
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// Sign the transaction using our keys
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signedTx, _ := types.SignTx(tx, types.NewLondonSigner(chainid), sk)
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// Send the transaction to our node
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return cl.SendTransaction(context.Background(), signedTx)
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}
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```
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### gethclient
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An instance of `gethclient` can be used in exactly the same way as `ethclient`. However, `gethclient`
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includes Geth-specific API methods. These additional methods are:
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```shell
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CallContract()
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CreatAccessList()
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GCStats()
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GetNodeInfo()
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GetProof()
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MemStats()
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SetHead()
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SubscribePendingTransactions()
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```
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*Note that both `ethclient` and `gethclient` have a `CallContract()` function - the difference is that
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the `gethclient` version includes an `overrides` argument.*
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Details relating to these endpoints can be found at [pkg.go.dev][go-api-docs] or the Geth [Github][ethclient-link].
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The code snippets in this tutorial were adapted from a more more in-depth set of examples available on
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[Github][web3go-link].
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## Summary
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There are a wide variety of Go APIs available for dapp developers that abstract away the complexity of interacting with Ethereum
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using a set of composable, reusable functions provided by Geth.
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[go-guide]: https://github.com/golang/go/wiki#getting-started-with-go
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[peter-slides]: https://ethereum.karalabe.com/talks/2016-devcon.html
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[go-ethereum-dir]: https://pkg.go.dev/github.com/ethereum/go-ethereum/#section-directories
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[ethclient-pkg]:https://pkg.go.dev/github.com/ethereum/go-ethereum/ethclient#Client
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[go-pkg-link]: https://pkg.go.dev/github.com/ethereum/go-ethereum#section-directories
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[ipfs-link]: https://ipfs.io/ipfs/QmQRuKPKWWJAamrMqAp9rytX6Q4NvcXUKkhvu3kuREKqXR
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[dapp-link]: https://ethereum.org/en/glossary/#dapp
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[web3-link]: https://ethereum.org/en/web3/
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[ethclient-link]: https://github.com/ethereum/go-ethereum/tree/master/ethclient
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[go-api-docs]:https://pkg.go.dev/github.com/ethereum/go-ethereum@v1.10.19/ethclient/gethclient
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[web3go-link]:https://github.com/MariusVanDerWijden/web3go
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