Understanding Visibility of Functions and Assets: Public, Protected and Private

A public Function or Asset is publicly accessible on the internet at a specific URL once deployed. For example, if you create a Function with the path /send/sms and deploy to an environment example-1234.twil.io, then your function will be publicly accessible at https://example-1234.twil.io/send/sms.

Similarly, a public Asset called ahoy.mp3 in the same Service would be accessible by anyone from https://example-1234.twil.io/ahoy.mp3.

A protected Function or Asset can be referenced via a URL as well, but requires a valid Twilio X-Twilio-Signature header in the request in order to be accessed. This empowers you to limit your Functions and Assets to only be accessible by Twilio webhooks such as an incoming call or SMS message, by Twilio Studio widgets such as the Run Function Widget or the Say/Play Widget, or by your own Functions.

This extra layer of protection makes Protected Assets particularly useful for storing sensitive information that needs to be referenced by your code or in a Studio Flow, for example, but not accessible by the public.

Private Functions and Assets are library files intended only for access via other Functions. These files will not be accessible by URL or exposed to the web; rather, they are packaged alongside your Service at build time.

For example, if your Function relies on a JSON file of data to read from, you can deploy that JSON file as a private Asset and read it from the Function.

A private Asset could even be a way for you to store a list of five-letter words for building your own Wordle.

Similarly, private Functions are a great way to reuse code or define private libraries that you want to keep out of reach from the wider web.

That's most of the fundamentals out of the way! Let's apply all of this knowledge and start building by creating your first Service, or by following one of the many examples.