aws-serverless-express 连接到 mongo 并获取 mongo 文档

Question

I am trying to make a Lambda proxy with express using the aws-serverless-express node module. However I keep getting the error 'Error: socket hang up' and I don't know why.

Here is my lambda code:

 const app = require('./app');
 const awsServerlessExpress = require('aws-serverless-express');
 const server = awsServerlessExpress.createServer(app);

 exports.handler = (event, context, callback) => {
  console.log('EVENT: ' + JSON.stringify(event));
  awsServerlessExpress.proxy(server, event, context);
}

and here is my express code:

MongoClient.connect("mongodb:/mongoUriGoesHere", function(err, db) {
if(err) { return console.dir(err); }

let collection = db.collection('collection');

app.get('/hello', (req, res) => {
 collection.find().toArray()
  .then((documents) => /* Handle success (console.log) */)
  .catch((err) => /* Handle error */)
 })
});

module.exports = app;

This is my response from the lambda

https://i.stack.imgur.com/Y0ybB.png

I know lambda functions are supposed to be stateless when creating a connection to a db, but I don't know how to get this to work. I've also tried using mongoose and no luck. I can get it to work when there is no code that attemps to connect to the db, but this is my problem at the moment. Any answers would be greatly appreaciated. Been at this for a while and still can't figure it out!

Answer 1

Lambda is a little strange in how it works under the hood and is not documented as well as it should be.

Lambda has hot and cold states. If it's not invoked for 15 minutes it goes cold otherwise it reuses certain parts of your code and your setup every time it is invoked and refreshes the hot state timeout.

Lambda will not re-execute the bits of your code that sit outside your handler function. Which means that the database connection code you defined will only run once and the code will try to re-use that connection for however long the lambda is hot.

So if there is a connection timeout on MongoClient then it's going to time out your connection but the connection code is not going to re-execute meaning you will get an error saying the expected connection cannot be utilized.

Why does all express code not work in this way? It's because when you're defining your middleware eg app.get('/hello', (req, res) => {} this code is only instantiated inside of the lambda handler which means it will re-execute every time.

The only workaround for this problem that I could come up with is putting your connection command inside the express middleware like this:

app.use(async function (req, res, next) {
    await mongoose.connect(config.database.mongodb.url, dbConfig);

    res.on("finish", function () {
        if(mongoose.connection.readyState === 1){
            mongoose.connection.close();
        }
    });

    next();
});

This solution has drawbacks tho. If you have any asynchronous tasks running after you responded to the client then they'll fail because the db connection will be closed by then.

Have a look at https://mongoosejs.com/docs/lambda.html and see how they handle the connection. They reuse the same connection across all lambdas.

Answer 2

我认为你必须返回 awsServerlessExpress.proxy 对象