Aamir

Nov 26, 2023

How does connection pooling work in MongoDB when you cache the 'MongoClient' instance in a Lambda function?

Before understanding the connection pool for mongodb with lambda, will check what is the best way to connect the mongodb with aws lambda.

We should not create mongodb connection inside the aws lambda handler, because we don't want to create the connection everytime, when aws lambda handlers triggers. as creating the connection is a costly operation.

Below is the example:-

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

let cachedDb = null;

async function connectToDatabase(uri) {
  if (cachedDb) {
    return cachedDb;
  }

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

  try {
    await client.connect();
    console.log('Connected to MongoDB');
    cachedDb = client.db(); // Assuming you want to use the default database

    return cachedDb;
  } catch (error) {
    console.error('Error connecting to MongoDB:', error);
    throw error;
  }
}

// Lambda function handler
exports.handler = async (event, context) => {
  const uri = process.env.MONGODB_URI;

  try {
    const db = await connectToDatabase(uri);

    // Your MongoDB operations using the 'db' instance

    return { statusCode: 200, body: 'Success' };
  } catch (error) {
    console.error('Lambda function error:', error);
    return { statusCode: 500, body: 'Internal Server Error' };
  }
};

Now let see how mongodb connection pool works incase serverless when we cache the db connection.

Caching the MongoClient instance in a Lambda function for reuse across invocations is a common practice and can be effective in reducing connection overhead. In this scenario, the connection pooling mechanism is still relevant, but with some considerations:

-> Lambda functions may experience environment reuse, meaning that if a Lambda function is invoked in the same execution environment (within the same container), the cached MongoClient instance and its associated connection pool will persist.

-> This can lead to improved performance by reusing existing connections rather than establishing new ones.

-> The MongoClient instance manages the connection pool internally. If the cached MongoClient is reused in subsequent Lambda invocations within the same execution environment, it will reuse existing connections from the pool when available.

-> Keep in mind that the MongoClient instance should be cached for the lifetime of the Lambda execution environment to take advantage of connection reuse.

-> If you create a new MongoClient for every Lambda invocation, you lose the benefits of connection pooling.

Thanks for reading