mongodb/node-mongodb-native · error · MongoTransactionError

Transactions do not support unacknowledged write concern

Error message

Transactions do not support unacknowledged write concern

What it means

Thrown by the Transaction constructor when the resolved write concern has w === 0 (unacknowledged). MongoDB transactions require that every operation be acknowledged so the server can report commit/abort status; an unacknowledged write concern makes transaction semantics impossible. The driver rejects this at transaction creation time via MongoTransactionError.

Solutions

1. Remove w:0 from the transaction options or use an acknowledged write concern (w:1, w:'majority', or omit it).
2. If the client-level writeConcern is w:0, pass an explicit acknowledged write concern to startTransaction/withTransaction.
3. Do not use unacknowledged writes for any operation that participates in a transaction.

Example fix

// before
session.startTransaction({ writeConcern: { w: 0 } });

// after
session.startTransaction({ writeConcern: { w: 'majority' } });

function isAcknowledgedWC(wc: unknown): boolean {
  if (!wc || typeof wc !== 'object') return true; // default is acknowledged
  const w = (wc as any).w;
  return w !== 0 && w !== '0';
}
if (!isAcknowledgedWC(txnOptions.writeConcern)) {
  throw new Error('Refusing to start transaction with unacknowledged write concern');
}

function hasAcknowledgedWriteConcern(opts: { writeConcern?: any }): boolean {
  const w = opts.writeConcern?.w;
  return w === undefined || w === 1 || w === 'majority' || (typeof w === 'number' && w > 0);
}

try {
  session.startTransaction({ writeConcern: { w: 'majority' } });
} catch (e) {
  if (e instanceof MongoTransactionError && /unacknowledged/.test(e.message)) {
    session.startTransaction(); // retry with default acknowledged write concern
  } else throw e;
}

When it happens

Trigger: Calling session.withTransaction(callback, { writeConcern: { w: 0 } }) or session.startTransaction({ writeConcern: { w: 0 } }). Also when the MongoClient-level writeConcern is set to w:0 and a transaction is started without overriding it.

Common situations: Reusing a fire-and-forget write concern (w:0) from high-throughput insert paths when running transactional code. Copy-pasting client options across contexts where transactional guarantees are needed.

Related errors

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AI-assisted analysis of mongodb/node-mongodb-native@dce7939f86 (2026-08-11). Data as JSON: /api/errors/361822ad2ec3f1e9. Report an issue: GitHub.

Appendix: source

Thrown at src/transactions.ts:93

/**
 * @internal
 */
export class Transaction {
  state: TxnState;
  options: TransactionOptions;
  _pinnedServer?: Server;
  _recoveryToken?: Document;

  /** Create a transaction */
  constructor(options?: TransactionOptions) {
    options = options ?? {};
    this.state = TxnState.NO_TRANSACTION;
    this.options = {};

    const writeConcern = WriteConcern.fromOptions(options);
    if (writeConcern) {
      if (writeConcern.w === 0) {
        throw new MongoTransactionError('Transactions do not support unacknowledged write concern');
      }

      this.options.writeConcern = writeConcern;
    }

    if (options.readConcern) {
      this.options.readConcern = ReadConcern.fromOptions(options);
    }

    if (options.readPreference) {
      this.options.readPreference = ReadPreference.fromOptions(options);
    }

    if (options.maxCommitTimeMS) {
      this.options.maxTimeMS = options.maxCommitTimeMS;
    }

    // TODO: This isn't technically necessary

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