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| const { MongoClient } = require('mongodb'); | |
| // In MongoDB 4.2 and earlier, CRUD operations in transactions must be on existing collections | |
| // See https://docs.mongodb.com/manual/core/transactions/#transactions-api for more information | |
| // Before running this script... | |
| // 1. Create a database named 'banking' | |
| // 2. Create a collection named 'accounts' in the database | |
| // 3. Create two documents in the 'accounts' collection: | |
| // {"_id":"account1", "balance":500} | |
| // {"_id":"account2", "balance":0} | |
| // 4: Optional: add schema validation to ensure an account balance cannot drop below 0. | |
| // See https://docs.mongodb.com/manual/core/schema-validation/ for details on how to | |
| // enable schema validation. Configuring schema validation in MongoDB Compass is an | |
| // easy way to add schema validation to an existing database: https://docs.mongodb.com/compass/current/validation/ | |
| // | |
| // { | |
| // $jsonSchema: { | |
| // properties: { | |
| // balance: { | |
| // minimum: 0, | |
| // description: 'account balance cannot be negative' | |
| // } | |
| // } | |
| // } | |
| // } | |
| async function main() { | |
| /** | |
| * Connection URI. Update <username>, <password>, and <your-cluster-url> to reflect your cluster. | |
| * See https://docs.mongodb.com/drivers/node/ for more details | |
| */ | |
| const uri = "mongodb+srv://<username>:<password>@<your-cluster-url>/banking?retryWrites=true&w=majority"; | |
| /** | |
| * The Mongo Client you will use to interact with your database | |
| * See https://mongodb.github.io/node-mongodb-native/3.6/api/MongoClient.html for more details | |
| */ | |
| const client = new MongoClient(uri); | |
| try { | |
| // Connect to the MongoDB cluster | |
| await client.connect(); | |
| // Transfer $100 from "account1" to "account2" | |
| await transferMoney(client, "account1", "account2", 100); | |
| } finally { | |
| // Close the connection to the MongoDB cluster | |
| await client.close(); | |
| } | |
| } | |
| main().catch(console.error); | |
| /** | |
| * Transfer money from one bank account to another using | |
| * @param {MongoClient} client A MongoClient that is connected to a cluster with the banking database | |
| * @param {String} account1 The _id of the account where money should be subtracted | |
| * @param {String} account2 The _id of the account where money should be added | |
| * @param {Number} amount The amount of money to be transferred | |
| */ | |
| async function transferMoney(client, account1, account2, amount) { | |
| /** | |
| * The accounts collection in the banking database | |
| */ | |
| const accountsCollection = client.db("banking").collection("accounts"); | |
| // Step 1: Start a Client Session | |
| // See https://mongodb.github.io/node-mongodb-native/3.6/api/MongoClient.html#startSession for the startSession() docs | |
| const session = client.startSession(); | |
| // Step 2: Optional. Define options for the transaction | |
| const transactionOptions = { | |
| readPreference: 'primary', | |
| readConcern: { level: 'local' }, | |
| writeConcern: { w: 'majority' } | |
| }; | |
| try { | |
| // Step 3: Use withTransaction to start a transaction, execute the callback, and commit (or abort on error) | |
| // Note: The callback for withTransaction MUST be async and/or return a Promise. | |
| // See https://mongodb.github.io/node-mongodb-native/3.6/api/ClientSession.html#withTransaction for the withTransaction() docs | |
| const transactionResults = await session.withTransaction(async () => { | |
| // Important:: You must pass the session to each of the operations | |
| // Remove the money from the first account | |
| const subtractMoneyResults = await accountsCollection.updateOne( | |
| { _id: account1 }, | |
| { $inc: { balance: amount * -1 } }, | |
| { session }); | |
| console.log(`${subtractMoneyResults.matchedCount} document(s) found in the accounts collection with _id ${account1}.`); | |
| console.log(`${subtractMoneyResults.modifiedCount} document(s) was/were updated to remove the money.`); | |
| if (subtractMoneyResults.modifiedCount !== 1) { | |
| await session.abortTransaction(); | |
| return; | |
| } | |
| // Add the money to the second account | |
| const addMoneyResults = await accountsCollection.updateOne( | |
| { _id: account2 }, | |
| { $inc: { balance: amount } }, | |
| { session }); | |
| console.log(`${addMoneyResults.matchedCount} document(s) found in the accounts collection with _id ${account2}.`); | |
| console.log(`${addMoneyResults.modifiedCount} document(s) was/were updated to add the money.`); | |
| if (addMoneyResults.modifiedCount !== 1) { | |
| await session.abortTransaction(); | |
| return; | |
| } | |
| }, transactionOptions); | |
| if (transactionResults) { | |
| console.log("The money was successfully transferred. Database operations from the transaction are now visible outside the transaction."); | |
| } else { | |
| console.log("The money was not transferred. The transaction was intentionally aborted."); | |
| } | |
| } catch (e) { | |
| console.log("The money was not transferred. The transaction was aborted due to an unexpected error: " + e); | |
| } finally { | |
| // Step 4: End the session | |
| await session.endSession(); | |
| } | |
| } |