Transactions and consistency

In plain English

A replica set is several MongoDB servers keeping the same data so the system keeps working if one fails. A session, opened with startSession, is a way to run commands that belong together. withTransaction wraps a block of work so that either every change commits or none of them do. A write concern says how many copies must acknowledge a write before it is reported as successful, and a read concern says which state of the data you are willing to read. Multi-document transactions only work on a replica set, so a single standalone server cannot run them.

Step by step

1. 1Confirm you are connected to a replica set rather than a standalone server.
2. 2Open a session with db.getMongo().startSession().
3. 3Run several updates inside session.withTransaction.
4. 4Close the session with endSession when you are done.
5. 5Set writeConcern to majority for writes that must survive a crash.

const session = db.getMongo().startSession()

session.withTransaction(async () => {
  const accounts = session.getDatabase("bank").accounts
  await accounts.updateOne(
    { _id: "ada" },
    { $inc: { balance: -100 } }
  )
  await accounts.updateOne(
    { _id: "grace" },
    { $inc: { balance: 100 } }
  )
})

session.endSession()

db.accounts.updateOne(
  { _id: "ada" },
  { $inc: { balance: -10 } },
  { writeConcern: { w: "majority" } }
)

Common mistakes

• Running a multi-document transaction against a standalone server fails because transactions require a replica set.
• Doing slow network or human work inside a transaction holds locks and blocks other operations.
• Assuming a write is safely stored just because it was acknowledged, without setting a stronger write concern, is unsafe.
• Ending a session without committing or aborting the transaction leaves the session hanging in an uncertain state.