Aws Lambda retry behaviours on stream-based event sources

From the documentation, AWS Lambda will retry failed function on stream-based events sources.

By using Node.js, we can fail the function by many different ways, e.g. using callback to return error, throw exception directly, throw exception inside Promise, using Promise.reject. Then the questions is, what’s the proper way to let AWS Lambda know it needs a retry?

I did an quick test on following scenarios by setting up DynamoDB Stream and event mappings. It’s fun to have a guess which one will be retried and which one won’t.

Different ways to end the function

On Exception

module.exports.throwException = (event, context) => {
  console.log(JSON.stringify(event));
  throw new Error('something wrong');
};

Using callback to return Success

module.exports.success = (event, context, callback) => {
  console.log(JSON.stringify(event));
  callback(null, 'result');
};

Using callback to return Error

module.exports.returnFailure = (event, context, callback) => {
  console.log(JSON.stringify(event));
  callback('error occurred');
};

On Promise Reject

module.exports.throwException = (event, context) => {
  console.log(JSON.stringify(event));
  Promise.resolve('')
    .then(() => Promise.reject('something wrong'));
};

Returning Promise Reject

module.exports.throwException = (event, context) => {
  console.log(JSON.stringify(event));
  return Promise.resolve('')
    .then(() => Promise.reject('something wrong'));
};

Catch Promise Reject and using callback to return error

module.exports.throwException = (event, context, callback) => {
  console.log(JSON.stringify(event));
  Promise.resolve('')
    .then(() => Promise.reject('something wrong'))
    .catch((err) => callback(err))
};

Throw Error inside Promise

module.exports.throwException = (event, context) => {
  console.log(JSON.stringify(event));
  Promise.resolve('')
    .then(() => {
      throw new Error('something wrong');
    });
};

Catch Error using Promise and using callback to return error

module.exports.throwException = (event, context, callback) => {
  console.log(JSON.stringify(event));
  Promise.resolve('')
    .then(() => {
      throw new Error('something wrong');
    })
    .catch(err => callback(err));
};