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概述

On Ethereum, before being included in a block, transactions remain in what is called a pending transaction queue, tx pool, or mempool - they all mean the same thing. Miners then select a subset of all pending transactions from this queue to mine - there are a lot of benefits to being able to access and analyze this information for traders, people who want to save fees on gas, and more.

In this guide, we will learn how to stream pending transactions from Ethereum and similar chains with ethers.js.

Prefer a video walkthrough? Follow along with Noah and learn how to stream pending transactions with Ethers.js in 8 minutes.

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先决条件

• Node.js installed on your system
• 一个文本编辑器
• 终端（又称命令行）
• An Ethereum node

What is a Pending Transaction?

To write or update any on the Ethereum network, someone needs to create, sign and send a transaction. Transactions are how the external world communicates with the Ethereum network. When sent to the Ethereum network, a transaction stays in a queue known as mempool where transactions wait to be processed by miners - the transactions in this waiting state are known as pending transactions. The small fee required to send a transaction is known as a gas; Transactions get included in a block by miners, and they are prioritized based on the amount of gas price they include which goes to the miner.

You can get more information on mempool and pending transactions here.

Why do we want to see pending transactions?

By examining pending transactions, one can do the following:

• Estimate gas: We can theoretically look at pending transactions to predict the next block's optimal gas price.
• For Trading analytics: We can analyze pending transactions on decentralized exchanges. To predict market trends using the analysis.
• Front running: In DeFi, you can preview upcoming oracle related transactions related to price and potentially issue a liquidation for a vault on MKR, COMP, and other protocols.

There can be many use cases for streaming pending transactions - we won't cover them all here.

We’ll use ethers.js to stream these pending transactions with WebSockets. Let’s see how to install ethers.js before writing our code.

Installing ethers.js

Our first step here would be to check if node.js is installed on the system or not. To do so, copy-paste the following in your terminal/cmd:

$ node -v

如果尚未安装，您可以从官方网站下载 NodeJS 的 LTS 版本。

Now that we have node.js installed let’s install the ethers.js library using npm (Node Package Manager), which comes with Node.js.

$ npm i ethers@5.7

Make sure that your Ethers.js installation version is 5.7

此步骤中最常见的问题是 `node-gyp` 出现内部故障。您可以按照此处的 node-gyp 安装说明进行操作。

注意：如果您遇到 node-gyp 相关问题，请确保您的 Python 版本与上述说明中列出的兼容版本之一一致。 

Another common issue is a stale cache; clear your npm cache by simply typing the below into your terminal:

$ npm 清除缓存

如果一切顺利，ethers.js 就会安装到您的系统上。

设置您的 Quicknode 以太坊端点

We could use pretty much any Ethereum client, such as Geth or OpenEthereum (fka Parity), for our purposes today. Since to stream incoming new pending transactions, a node connection must be stable and reliable; it’s a challenging task to maintain a node, we'll just create a free Quicknode account here and easily set up an Ethereum endpoint. After you've created your free Ethereum endpoint, copy your WSS (WebSocket) Provider endpoint:

稍后你会用到这个，所以请复制并保存下来。

Streaming pending transactions

Create a short script file pending.js, which will have a transaction filter on incoming pending transactions. Copy-paste the following in the file:

var ethers = require("ethers");
var url = "ADD_YOUR_ETHEREUM_NODE_WSS_URL";

var init = function () {
  var customWsProvider = new ethers.providers.WebSocketProvider(url);
  
  customWsProvider.on("pending", (tx) => {
    customWsProvider.getTransaction(tx).then(function (transaction) {
      console.log(transaction);
    });
  });

  customWsProvider._websocket.on("error", async () => {
    console.log(`Unable to connect to ${ep.subdomain} retrying in 3s...`);
    setTimeout(init, 3000);
  });
  customWsProvider._websocket.on("close", async (code) => {
    console.log(
      `Connection lost with code ${code}! Attempting reconnect in 3s...`
    );
    customWsProvider._websocket.terminate();
    setTimeout(init, 3000);
  });
};

init();

So go ahead and replace `ADD_YOUR_ETHEREUM_NODE_WSS_URL` with the WSS  (WebSocket) provider from the section above. 

上述代码的说明。

第 1 行：导入 ethers 库。

Line 2: Setting our Ethereum node URL.

Line 4: Creating the init function.

Line 5: Instantiating an ethers WebSocketProvider instance.

Line 7: Creating an event listener for pending transactions that will run each time a new transaction hash is sent from the node.

Line 8-10: Getting the whole transaction using the transaction hash obtained from the previous step and printing the transaction in the console.

Line 13-16: A function to restart the WebSocket connection if the connection encounters an error.

Line 17-21: A function to restart the WebSocket connection if the connection ever dies.

Line 24: Calling the init function.

Now, let’s run our script.

$ node pending

If everything goes right, you must see incoming pending transactions. Something like this

Use Ctrl+c to stop the script.

结论

Here we saw how to get pending transactions from the Ethereum network using ethers,js. Learn more about Event filters and Transaction filters in ethers.js in their documentation.

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