The best way to understand how Twilio Functions works is to start by understanding the HTTP request flow and how Twilio ultimately executes your code. Twilio Functions is designed to handle as much of the infrastructure work of building a web app as possible so that you can focus on application logic instead. For Twilio Functions, this work begins at the front door, handling the HTTP request for your Function and continues through the pipeline up to your code.

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1. Every Function invocation begins with an HTTP client issuing a request to your Function. For most Functions, the client is the Twilio Voice or Messaging API responding to an incoming phone call or text message.
2. The Twilio Functions Gateway receives this request and attempts to validate it. If your Function has Signature Validation enabled, the Function Gateway will attempt to validate the signature. If the request or the Twilio Signature are invalid, the request is rejected with anHTTP 400 response.
3. After the Function Gateway has accepted the request, it will normalize the request into a payload for your Function. The normalized request is provided to your Function as two arguments:context andevent. Once the payload has been constructed, it is used to invoke your Function.
4. At invocation, your Function begins executing thehandler code that you have provided. When your Function completes executing, it can emit a response using thecallback method. The emitted response will be transmitted to the Function Gateway.
5. Upon receiving the result from your Function, the Function Gateway will construct an HTTP response and return it to the client that issued the request.