transactions.md

Transactions

Just like in SQL, MongoDB also offers transactions! Have a look at the sql section for the theory behind why transactions are important as that is the same.

In this section we will go into how to use transactions in MongoDB. The MongoDB team does a good of explaining how to do it in the following video:

{% hyf-youtube src="https://www.youtube.com/watch?v=bdS03tgD2QQ" %}

Let's have a look at some example code. In this function we want to transfer credits from one account to another:

async function transferCredits(fromAccountId, toAccountId, amount) {
  const accountsCollection = client.db("billing").collection("accounts");
  const session = client.startSession();

  try {
    await session.withTransaction(async () => {
      // Remove from fromUser
      await accountsCollection.updateOne(
        { _id: fromAccountId },
        { $inc: { credits: amount * -1 } },
        { session }
      );

      // Add to toUser
      await accountsCollection.updateOne(
        { _id: toAccountId },
        { $inc: { credits: amount } },
        { session }
      );
    });
  } catch (err) {
    await session.abortTransaction();
  } finally {
    await session.endSession();
  }
}

There is a lot happening there so the important things in a row:

• You need to always have a session, the client can provide you one
• You always need to end the session, the finally block helps a lot to make sure that happens
• If something goes wrong you need to abort the transaction to tell MongoDB to not apply anything! The catch block makes this nice and simple!
• Whenever you are doing a mutation you need to provide the session to the function. That is how MongoDB knows that this is part of a transaction so that it doesn't just apply it straight away!