Re-throw errors on Firebase

Jun 27, 2018

TL;DR jump to the conclusion.

Behold 3 Firebase triggers.

The main differences are:

  • deletemeSwallow does not re-throw the error in the catch block, i.e. it “swallows” the error
  • deletemeRethrow re-throws the error in the catch block
  • deletemeNoCatch has no catch block, so any errors will propagate up the call-stack naturally
exports.deletemeSwallow = functions.pubsub.topic('deleteme').onPublish((payload) => {
  return new Promise(resolve => {
    console.log('I am deletemeSwallow')
    if (Buffer.from(payload.data, 'base64').toString() === 'fail') {
      throw new Error('You want me to fail')
    }
    resolve(payload)
  }).then(payload => {
    return console.log('superfluous then block', payload)
  }).catch(err => {
    console.error('I am just going to swallow the error', err)
    // throw err   <-- Gulp!
  })
})

exports.deletemeRethrow = functions.pubsub.topic('deleteme').onPublish((payload) => {
  return new Promise(resolve => {
    console.log('I am deletemeRethrow')
    if (Buffer.from(payload.data, 'base64').toString() === 'fail') {
      throw new Error('You want me to fail')
    }
    resolve(payload)
  }).then(payload => {
    return console.log('superfluous then block', payload)
  }).catch(err => {
    console.error('I am going to rethrow the error', err)
    throw err
  })
})

exports.deletemeNoCatch = functions.pubsub.topic('deleteme').onPublish((payload) => {
  return new Promise(resolve => {
    console.log('I am deletemeNoCatch')
    if (Buffer.from(payload.data, 'base64').toString() === 'fail') {
      throw new Error('You want me to fail')
    }
    resolve(payload)
  }).then(payload => {
    return console.log('superfluous then block', payload)
  })
})

Happy path

gcloud pubsub topics publish deleteme --message "as you were, soldier"

Given a happy path, this is what we see in the logs - all functions exit with 'OK'.

success

Forced failure

gcloud pubsub topics publish deleteme --message "fail"

Given a forced failure, this is what we see in the logs:

  • deletemeNocatch exited with 'error', which is correct considering the error propagated up the call-stack unhindered
  • deletemeRethrow exited with 'error', which is correct considering the error was re-thrown as we’re not recovering from it in the catch block
  • deletemeSwallow incorrectly exited with 'OK' because clearly the catch block does not recover from the error - it just swallows the error.

forced failure

Conclusion

If an error happens in your Firebase trigger business logic, and you catch at any point in your Promise chain, you must either recover from the error, or re-throw the error so that Firebase can exit with the error status.