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Type: Improvement
Resolution: Unresolved
Priority: Major - P3
Fix Version/s: None
Affects Version/s: 3.6.0
Component/s: None
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I am following the example in https://docs.mongodb.com/manual/core/transactions/ with a deployment using 4.0 replica set with mmapv1 which does not support transactions.

const { MongoClient } = require("mongodb");

const uri = "mongodb://localhost:14071/?retrywrites=false&directconnection=false"

const client = new MongoClient(uri,{useUnifiedTopology:true});

async function run() {
  await client.connect();

 // Prereq: Create collections.

  await client.db('mydb1').collection('foo').insertOne({ abc: 0 }, { w: 'majority' });

  await client.db('mydb2').collection('bar').insertOne({ xyz: 0 }, { w: 'majority' });

  // Step 1: Start a Client Session
  const session = client.startSession();

  // Step 2: Optional. Define options to use for the transaction
  const transactionOptions = {
    readPreference: 'primary',
    readConcern: { level: 'local' },
    writeConcern: { w: 'majority' }
  };

  // Step 3: Use withTransaction to start a transaction, execute the callback, and commit (or abort on error)
  // Note: The callback for withTransaction MUST be async and/or return a Promise.
  try {
    await session.withTransaction(async () => {
      const coll1 = client.db('mydb1').collection('foo');
      const coll2 = client.db('mydb2').collection('bar');

      // Important:: You must pass the session to the operations

      await coll1.insertOne({ abc: 1 }, { session });
      await coll2.insertOne({ xyz: 999 }, { session });
    }, transactionOptions);
  } finally {
    await session.endSession();
    await client.close();
  }
}
run().catch(console.dir);

I get the following diagnostics:

serene% node test.js
MongoError: This MongoDB deployment does not support retryable writes. Please add retryWrites=false to your connection string.
    at getMMAPError (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/core/topologies/shared.js:413:18)
    at handler (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/core/sdam/topology.js:944:15)
    at /home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/cmap/connection_pool.js:354:13
    at handleOperationResult (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/core/sdam/server.js:558:5)
    at commandResponseHandler (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/core/wireprotocol/command.js:115:25)
    at MessageStream.messageHandler (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/cmap/connection.js:266:11)
    at MessageStream.emit (events.js:315:20)
    at processIncomingData (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/cmap/message_stream.js:144:12)
    at MessageStream._write (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/cmap/message_stream.js:42:5)
    at doWrite (_stream_writable.js:403:12) {
  originalError: MongoError: Transaction numbers are only allowed on storage engines that support document-level locking
      at MessageStream.messageHandler (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/cmap/connection.js:266:20)
      at MessageStream.emit (events.js:315:20)
      at processIncomingData (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/cmap/message_stream.js:144:12)
      at MessageStream._write (/home/w/apps/tests/retryable-writes-node/node_modules/mongodb/lib/cmap/message_stream.js:42:5)
      at doWrite (_stream_writable.js:403:12)
      at writeOrBuffer (_stream_writable.js:387:5)
      at MessageStream.Writable.write (_stream_writable.js:318:11)
      at Socket.ondata (_stream_readable.js:717:22)
      at Socket.emit (events.js:315:20)
      at addChunk (_stream_readable.js:295:12) {
    operationTime: Timestamp { _bsontype: 'Timestamp', low_: 2, high_: 1596646405 },
    ok: 0,
    code: 20,
    codeName: 'IllegalOperation',
    '$clusterTime': { clusterTime: [Timestamp], signature: [Object] }
  }
}

Note that the error message instructs me to add retrywrites=false to the connection string, which I have already done. This is a very confusing situation to deal with as a user of the driver.

The Ruby driver provides the "Please add retryWrites=false to your connection string." instruction when issuing inserts and retrywrites=false is not set (in which case setting retrywrites=false would in fact fix the insert not working), but not when issuing transactions where it doesn't help.

I expect the node driver to:

Not suggest to users that they add retrywrites=false to their connection string when this option is already requested by the users.
Not suggest to users that they add retrywrites=false when this will not make the operation succeed, specifically when they are attempting transactions.

Test code:
https://github.com/p-mongo/tests/tree/master/retryable-writes-node
https://github.com/p-mongo/tests/tree/master/retryable-writes-ruby

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