Functions FAQ

Why does my Function return 'runtime application timed out'?

There are two possible reasons for why your Function has completed with the error: 'runtime application timed out'.

The most common reason is that your Function has exceeded the 10-second execution time limit. You can determine this by looking at the execution logs on the Function instance page. The last log line after execution will tell you how many milliseconds the Function took to execute. If the processing time is greater than 10,000 milliseconds then your Function was terminated by Twilio.

The other more subtle reason your Function ended with an application timeout is because of an incorrect invocation of callback(). If the callback() method is not being called or is unreachable, your Function will continue executing until it reaches the time limit and ultimately fails. A very common mistake is to forget to capture the catch() rejected state of a Promise and calling callback() there as well. The Function Execution documentation provides extensive details on the functionality and usage of the callback() method. Below are several examples of how to correctly use callback() to complete execution and emit a response.

Why isn't my code running?

The most common reason we have seen that a Function appears not to run is the misuse of callback(). Your Function invocation terminates as soon as the callback() is reached. If your request is asynchronous, for example, an API call to a Twilio resource, then the callback() must be placed inside the success response of the request.

How do I construct Voice TwiML?

You can generate Voice TwiML using the Twilio Node library which comes packaged within your Function.

How do I construct Messaging TwiML?

You can generate Messaging TwiML using the Twilio Node library which comes packaged within your Function.

How do I return JSON?

Simply return your object as outlined in the Function Execution documentation.

How do I send CORS headers?

You can send CORS headers by using the Twilio Response object described in the Function Execution documentation.

Can I execute my Functions on a schedule?

Currently, Functions are event-driven and can only be invoked by HTTP.

Can I see examples of existing Functions?

Absolutely! In addition to our Programmable SMS and Programmable Voice quickstarts, you can find a variety of templates within our catalog within the Create Function page.

How many Functions can I create?

During the beta period, we currently have a limit of 100 functions per account via the UI. To create more Functions, consider using the Functions and Assets API.

What limits are in place on Function invocation?

Functions are limited to 30 concurrent invocations - meaning that if you have more than 30 Functions being invoked at the same time, you will begin to see new Function invocations return with a 429 status code. To keep below the threshold optimize Functions to return as fast as possible - avoid artificial timeouts and use asynchronous calls to external systems rather than waiting on a large number of API calls to complete.

Can I create temporary files when executing a Function?

Yes. See this blog post for a description and samples on how to store files for one off usage.