Atlas error: WriteConflict inside a transaction

WriteConflict in transactions

Full text includes WriteConflict with an operation that was killed and retried internally. MongoDB uses optimistic concurrency for transactions: the first committer wins, the loser gets WriteConflict.

Confirm

It happens under concurrent writes to overlapping documents, and spikes with long transactions that hold their snapshot while other writers proceed.

Fix

Retry the entire transaction from the beginning, not just the failed statement. The standard pattern:

async function runTxn(fn) {
  for (let attempt = 0; attempt < 3; attempt++) {
    const session = client.startSession();
    try {
      let result;
      await session.withTransaction(fn);
      return result;
    } catch (e) {
      const labels = e.errorLabels || [];
      if (labels.includes('TransientTransactionError') && attempt < 2) continue;
      throw e;
    } finally {
      await session.endSession();
    }
  }
}

Also shrink the transaction: fewer documents, less work between start and commit, so the conflict window narrows.

Verify

Run the conflicting workload concurrently. Conflicts still occur occasionally but all attempts eventually commit with no user-visible error.

Transaction conflict behavior and retry guidance (TransientTransactionError, UnknownTransactionCommitResult labels) per https://www.mongodb.com/docs/manual/core/transactions/, verified 2026-09-26.