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  h2 Using Mongoose With AWS Lambda
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    [AWS Lambda](https://aws.amazon.com/lambda/) is a popular service for running
    arbitrary functions without managing individual servers. Using Mongoose in your
    AWS Lambda functions is easy. Here's a sample function that connects to a
    MongoDB instance and finds a single document:

    ```javascript
    const mongoose = require('mongoose');

    let conn = null;

    const uri = 'YOUR CONNECTION STRING HERE';

    exports.handler = async function(event, context) {
      // Make sure to add this so you can re-use `conn` between function calls.
      // See https://www.mongodb.com/blog/post/serverless-development-with-nodejs-aws-lambda-mongodb-atlas
      context.callbackWaitsForEmptyEventLoop = false;

      // Because `conn` is in the global scope, Lambda may retain it between
      // function calls thanks to `callbackWaitsForEmptyEventLoop`.
      // This means your Lambda function doesn't have to go through the
      // potentially expensive process of connecting to MongoDB every time.
      if (conn == null) {
        conn = mongoose.createConnection(uri, {
          // Buffering means mongoose will queue up operations if it gets
          // disconnected from MongoDB and send them when it reconnects.
          // With serverless, better to fail fast if not connected.
          bufferCommands: false, // Disable mongoose buffering
          bufferMaxEntries: 0 // and MongoDB driver buffering
        });
        
        // `await`ing connection after assigning to the `conn` variable
        // to avoid multiple function calls creating new connections
        await conn;
        conn.model('Test', new mongoose.Schema({ name: String }));
      }

      const M = conn.model('Test');

      const doc = await M.findOne();
      console.log(doc);

      return doc;
    };
    ```

    To import this function into Lambda, go [the AWS Lambda console](https://console.aws.amazon.com/lambda)
    and click "Create Function".

    <a href="https://i.imgur.com/FlAD0vT.png"><img src="https://i.imgur.com/FlAD0vT.png"></a>

    Create a function called "mongoose-test" with the below settings:

    <a href="https://i.imgur.com/YOYJzeO.png"><img src="https://i.imgur.com/YOYJzeO.png"></a>

    Copy the source code into a file called `lambda.js`. Then run `npm install mongoose co`.
    Finally, run `zip -r mongoose-test.zip node_modules/ lambda.js` to create a
    zip that you can upload to Lambda using the "Upload a Zip File" option under "Function code".
    Make sure you also change the "Handler" input to `lambda.handler` to match the `lambda.js` file's `handler` function.

    <a href="https://i.imgur.com/BXc9egq.png"><img src="https://i.imgur.com/BXc9egq.png"></a>

    Next, click the "Save" button and then the "Test" button. The "Test" button will
    ask you to create a new test event, just create one because your inputs don't matter
    for this example. Then, hit "Test" again to actually run your function:

    <a href="https://i.imgur.com/2UKtWYq.png"><img src="https://i.imgur.com/2UKtWYq.png"></a>

    If your MongoDB database goes down in between function calls, you may
    see the below error message:

    ```
    cannot find account after reload: could not find config for <hostname>
    ```

    Lambda's JavaScript framework recently added support for
    async/await as long as you're using Node 8.x, so make sure you're not
    using Node.js 6.x.

    *Want to learn how to check whether your favorite JavaScript frameworks, like [Express](http://expressjs.com/) or [React](https://reactjs.org/), work with async/await? Spoiler alert: neither Express nor React support async/await. Chapter 4 of Mastering Async/Await explains the basic principles for determining whether a framework supports async/await. [Get your copy!](http://asyncawait.net/?utm_source=mongoosejs&utm_campaign=lambda)*

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