MongoDB watch randomly crashes (Cursor session id is not the same as the operation context's session id)

Question

I am connecting to my MongoDB with the official node driver.

I am trying to watch changes, both on a specific collection and a specific document. Both ways are experiencing this error.

My code works perfectly fine most of the time, but sometimes while it's idle it randomly crashes my whole app. I would like to understand why this is happening, and catch the error so it doesn't break my whole app. Unfortunately, it never points to a specific line in my code.

Here is one of my codes (simplified):

let watcher = q.watch({ids: [documentId]}).on("change", (changeEvent) => { 
    //only listen to update operations
    if (changeEvent.operationType !== 'update') return;
    console.log('document was updated');
    watcher.cursor.close();
});

Which gave me this error:

node:events:371
      throw er; // Unhandled 'error' event
      ^

MongoServerError: Cursor session id (8c01dfe6-6a3e-479c-98ce-243d61ec24e4 - 6tctnoM1KQxUOgLaEIm7HYAvVSF3YGlrCMxn+SswizE=) is not the same as the operation context's session id (none)
    at MessageStream.messageHandler (C:\project\node_modules\mongodb\lib\cmap\connection.js:463:30)
    at MessageStream.emit (node:events:394:28)
    at processIncomingData (C:\project\node_modules\mongodb\lib\cmap\message_stream.js:108:16)
    at MessageStream._write (C:\project\node_modules\mongodb\lib\cmap\message_stream.js:28:9)
    at writeOrBuffer (node:internal/streams/writable:389:12)
    at _write (node:internal/streams/writable:330:10)
    at MessageStream.Writable.write (node:internal/streams/writable:334:10)
    at TLSSocket.ondata (node:internal/streams/readable:754:22)
    at TLSSocket.emit (node:events:394:28)
    at addChunk (node:internal/streams/readable:315:12)
Emitted 'error' event on ChangeStream instance at:
    at closeWithError (C:\project\node_modules\mongodb\lib\change_stream.js:355:22)
    at processError (C:\project\node_modules\mongodb\lib\change_stream.js:445:12)
    at Readable.<anonymous> (C:\project\node_modules\mongodb\lib\change_stream.js:364:33)
    at Readable.emit (node:events:394:28)
    at emitErrorNT (node:internal/streams/destroy:157:8)
    at emitErrorCloseNT (node:internal/streams/destroy:122:3)
    at processTicksAndRejections (node:internal/process/task_queues:83:21) {
  operationTime: Timestamp { low: 1, high: 1629924381, unsigned: true },
  ok: 0,
  code: 13,
  codeName: 'Unauthorized',
  '$clusterTime': {
    clusterTime: Timestamp { low: 1, high: 1629924381, unsigned: true },
    signature: {
      hash: Binary {
        sub_type: 0,
        buffer: Buffer(20) [Uint8Array] [
          104,  40, 136,  32, 223, 251,
          247,  64,  47, 185, 197, 229,
          247,  96, 219, 103, 110, 116,
           65, 183
        ],
        position: 20
      },
      keyId: Long { low: 5, high: 1629063232, unsigned: false }
    }
  }
}

I also have the other code, which occasionally crashes the same way:

queueCollection.watch([{ $match: { operationType: "insert" }}])
    .on("change", (changeEvent) => { 
        console.debug('document added',currentlyProcessing?'currentlyProcessing':'notCurrentlyProcessing');
        if (!currentlyProcessing) processNextQueueItem();
    });

If anyone could explain why this error is happening, or how to catch and fix the watcher when it does, that would be great.

It looks like, after some time, your code's session becomes invalid and emits an 'error' event which is not caught (`.on("error")`)?, crashing your program. — theusaf, Aug 25 '21 at 21:33
@theusaf ah thanks, that sounds like it work to catch the error, I am trying it now (though unfortunately I just have to wait for it to trigger since it happens randomly). do you have any idea how I should reestablish the watch though, preferably with the same `change` and `error` functions still attached? — stackers, Aug 25 '21 at 21:51
I don't have much experience with MongoDB, but perhaps calling some `connect()` or `login()` method again could fix it? Otherwise, recreating the client and watchers is the only other way I can think of currently. — theusaf, Aug 25 '21 at 21:55

score 0 · Accepted Answer · answered Sep 17 '21 at 19:24

I still don't know how to prevent it, but here's how to hack your way around it:

//watches the collection for any inserted documents
function startWatcher (collection) {
    console.log('starting watcher');
    let watcher = collection.watch([{ $match: { operationType: "insert" }}])
        .on("change", (changeEvent) => {
            /*whatever you want to do when event is fired*/
        })
        //changestream connection died (cursor session id is not the same as the operation contexts session id)
        .on('error', e => {
            console.error('watcher died');

            //close current connection
            watcher.cursor.close();

            //restart new watcher
            startWatcher(collection);
        });
}

startWatcher(db.collection('collectionname'));

MongoDB watch randomly crashes (Cursor session id is not the same as the operation context's session id)

1 Answers1