AWS Lambda with Knex JS and RDS Postgres

Question

I have been doing some research and I'm not able to find a good answer about using Knex JS within a Lambda function:

How do I use Knex with AWS Lambda? #1875

Serverless URL Shortener with Apex and AWS Lambda

Use Promise.all() in AWS lambda

Here is what I have in my index.js:

const knex = require('knex')({
  client: 'pg',
  connection: {...},
});

exports.handler = (event, context, callback) => {
  console.log('event received: ', event);
  console.log('knex connection: ', knex);

  knex('goals')
    .then((goals) => {
      console.log('received goals: ', goals);
      knex.client.destroy();
      return callback(null, goals);
    })
    .catch((err) => {
      console.log('error occurred: ', err);
      knex.client.destroy();
      return callback(err);
    });
};

I am able to connect and execute my code fine locally, but I'm running into an interesting error when it's deployed to AWS - the first call is always successful, but anything after fails. I think this is related to the knex client being destroyed, but then trying to be used again on the next call. If I re-upload my index.js, it goes back to working for one call, and then failing.

I believe this can be resolved somehow using promises but this my first time working with Lambda so I'm not familiar with how it's managing the connection to RDS on subsequent calls. Thanks in advance for any suggestions!

Answer 1

For me, it worked on my local machine but not after deploying. I was kind of be mislead.

It turns out the RDS inbound source is not open to my Lambda function. Found solution at AWS Lambda can't connect to RDS instance, but I can locally?: either changing RDS inbound source to 0.0.0.0/0 or use VPC.

After updating RDS inbound source, I can use Lambda with Knex successfully.

The Lambda runtime I am using is Node.js 8.10 with packages:

knex: 0.17.0
pg: 7.11.0

The code below using async also just works for me

const Knex = require('knex');

const pg = Knex({ ... });

module.exports.submitForm = async (event) => {
  const {
    fields,
  } = event['body-json'] || {};

  return pg('surveys')
    .insert(fields)
    .then(() => {
      return {
        status: 200
      };
    })
    .catch(err => {
      return {
        status: 500
      };
    });
};

Hopefully it will help people who might meet same issue in future.

Answer 2

The most reliable way of handling database connections in AWS Lambda is to connect and disconnect from the database within the invocation process itself.

In your code above, since you disconnected already after the first invocation, the second one does not have a connection anymore.

To fix it, just move your instantiation of knex.

exports.handler = (event, context, callback) => {
  console.log('event received: ', event);

  // Connect
  const knex = require('knex')({
    client: 'pg',
    connection: {...},
  });

  console.log('knex connection: ', knex);

  knex('goals')
    .then((goals) => {
      console.log('received goals: ', goals);
      knex.client.destroy();
      return callback(null, goals);
    })
    .catch((err) => {
      console.log('error occurred: ', err);
      // Disconnect
      knex.client.destroy();
      return callback(err);
    });
};

There are ways to reuse an existing connection but success rates for that varies widely depending on database server configuration and production load.