Example of transaction in Mongo DB

transferBalance.js

// transfer balance from one account to another
const transferBalance = async (fromAccount, toAccount, amount) => {
    // get database client
    const client = await database.getClient();
    // create a database session 
    const session = client.startSession();
    // define transaction options 
    const transactionOptions = {
        readPreference: 'primary',
        readConcern: { level: 'local' },
        writeConcern: { w: 'majority' }
    };
    // since transaction can throw error it must be executed within a try catch bolck
    try {
        // withTransaction takes a function as argument, 
        // the function must be an async function
        // if the function throws any error then all operations are rollbacked
        // and transaction is aborted. 
        // we can also call the abortTransaction explicitly to rollback the transaction as well. 
        const transactionResult = await session.withTransaction(async () => {
            const accountCollection = client.db('mydb').collection('accounts');
            const transactionLogCollection = client.db('mydb').collection('transactionLog');
            // update fromAccount and decrement balance 
            let updateResult = await accountCollection.updateOne({"accountNo": fromAccount}, {
                    $inc: {balance: -1 * amount},
                    $set: {updateTimestamp: utils.getCurrentTimestamp()}
                }, {session}
            ); 
            // NOTE: WE MUST PASS THE session object to the above updateOne statement
            // so that it executes within the session. 
            if (updateResult.modifedCount !== 1){
                // update did not happen properly, abort transaction
                await session.abortTransaction();
                return;
            }
            // update toAccount and increment balance 
            updateResult = await accountCollection.updateOne({"accountNo": toAccount}, {
                    $inc: {balance: amount},
                    $set: {updateTimestamp: utils.getCurrentTimestamp()}
                }, {session}
            );
            // NOTE: WE MUST PASS THE session object to the above updateOne statement
            // so that it executes within the session. 
            if (updateResult.modifedCount !== 1){
                // update did not happen properly, abort transaction
                await session.abortTransaction();
                return;
            }
            // insert the data into the transactionLog 
            await transactionLogCollection.insertOne({
                fromAccount,
                toAccount,
                amount,
                createdTimestamp: utils.getCurrentTimestamp()
            });
        },transactionOptions);

        // if the transaction was aborted then transactionResult will have undefined value
        // otherwise it will hold some value which is the return value of the last statement
        // executed within the transaction 
        // check the transactionResult value here and return appropriate response 
        if(!transactionResult) {
            console.error("transaction is aborted");
            return false;
        } 
        return true;
    } catch(err){
        // log the error or return error response from here
        console.error(err);
        return false;
    } finally{
        // we must always call endSession
        // thats why it is put inside finally block
        await session.endSession();
    }
};