Experience Neptune Gremlin connections problem on calling AWS lambda handlers` callback

Question

I am using gremlin@3.3.5 for my Node.js 8.10 application with AWS Lambdas. The process works all fine for a single invocation. Here is my very sample code.

const gremlin = require('gremlin');
const DriverRemoteConnection = gremlin.driver.DriverRemoteConnection;
const Graph = gremlin.structure.Graph;

exports.handler = (event, context, callback) => {
    dc = new DriverRemoteConnection('wss://your-neptune-endpoint:8182/gremlin');

    const graph = new Graph();
    const g = graph.traversal().withRemote(dc);
    try {
        const result = await g.V().limit(1).count().next();
        dc.close();
        callback(null, { result: result });
    } catch (exception) {
        callback('Error');
        throw error;
    }
}

When I run this process for single invocation, it appears to work all fine, but soon as I try to run a batch process of operations (something like 100,000 requests / hr), I am experiencing in CloudWatch log metrics that my connections are not closed successfully. I have tried a number of implementation of this, like callbackWaitForEventLoopEmpty, but that seizes the lambda. When I remove callback (or return similarly), this process works fine with batch operations too. But I do want to return data from this lambda with information that is passed to my step function to trigger another lambda based on that information.

Answer 1

After doing some research, I have found out the problem was with how gremlin package was handling the event of closing a connection didn't favor serverless architecture. When triggered driver.close(). When driver is instantiated, it creates instance of client, which inside itself creates instance of connection, which creates instance of websocket using ws library. Now ws.close() event gracefully closes all the events, which doesn't wait for event to be called before my callback is called and that event remains open and leaks. So after explicitly calling dc._client._connection.ws.terminate() on connection instance and then dc.close() closes connection immediately.