Originally posted at Serverless on October 4th, 2018
It’s been 6 months since AWS Lambda added support Node.js 8.10. I’m super happy that I can finally use async/await to simplify my Lambda functions.
In the meantime, I have helped a few clients with their Node8 serverless projects. In doing so I have seen some recurring mistakes around async/await.
Still using callbacks
Many people are still using the callbacks in their async handler functions:
instead of the simpler alternative:
Not using promisify
Before Node8, bluebird filled a massive gap. It provided the utility to convert callback-based functions to promise-based. But Node8’s built-in util module has filled that gap with the promisify function.
For example, we can now transform the readFile function from the fs module like this:
No need to use bluebird anymore. That’s one less dependency, which helps reduce the cold start time for our functions.
Too sequential
async/await lets you write asynchronous code as if they're synchronous, which is awesome. No more dealing with callback hell!
On the flip side, we can also miss a trick and not perform tasks concurrently where appropriate.
Take the following code as example:
This function is easy to follow, but it’s hardly optimal. teamModel.fetch doesn't depend on the result of fixtureModel.fetchAll, so they should run concurrently.
Here is how you can improve it:
In this version, both fixtureModel.fetchAll and teamModel.fetch are started concurrently.
You also need to watch out when using map with async/await. The following will call teamModel.fetch one after another:
Instead, you should write it as the following:
In this version we map teamIds to an array of Promise. We can then use Promise.all to turn this array into a single Promise that returns an array of teams.
In this case, teamModel.fetch is called concurrently and can significantly improve execution time.
async/await inside forEach()
This is a tricky one, and can sometimes catch out even experienced Node.js developers.
The problem is that code like this doesn’t behave the way you’d expect it to:
When you run this you’ll get the following output:
See this post for a longer explanation about why this doesn’t work. For now, just remember to avoid using async/await inside a forEach!
Not using AWSSDK’s .promise()
Did you know that the AWS SDK clients support both callbacks and promises? To use async/await with the AWS SDK, add .promise() to client methods like this:
No more callback functions, yay!
Wrap-up
That’s it, 5 common mistakes to avoid when working with Node.js 8.10 in Lambda. For more tips on building production-ready serverless applications and operational best practices, check out my video course. ;-)
Further reading:
Originally published at https://www.serverless.com.
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