How do you turn a handful of isolated messages to and from the same party into a true conversation? You need some way to remember state between each message that is exchanged. This is because SMS is a stateless protocol. Building traditional web applications has this same hurdle, as HTTP is also a stateless protocol. This problem has been solved for web applications through the use of HTTP cookies and, rather than reinvent the wheel, the Twilio Messaging API uses the same solution.
If you haven't written your own SMS webhooks with Node.js before, you may want to first check out our guide, Receive and Reply to SMS and MMS Messages in Node.js. Ready to go? Let's get started!
Just like in web applications, a cookie is a small file that your application can store on Twilio's servers to keep track of information, such as a username or account. For Twilio SMS, cookies are scoped to the "conversation" between two parties -- you can have a unique cookie for each To/From phone number pair. For example, you can store a unique cookie for any messages sent between 415-555-2222 and 415-555-1111, which will be different than the cookie used between 415-555-3333 and 415-555-1111.
In web apps, you write a cookie to keep "statefulness" between separate requests from the same browser. Similarly, SMS messages are independent communications between two parties, so Twilio allows you to tie them together as a logical session via cookies. This means you can use server-side sessions to keep track of application state between requests. How cool is that? Twilio will expire the cookies for that conversation after four hours of inactivity, as if the user "closed the browser."
The cookies let you share state across multiple messages allowing you to treat separate messages as a conversation, and store data about the conversation in the cookies for future reference.
You can store the data directly in a cookie, or you can use a session state management framework. The code sample below does the latter, using the express-session module for Node.js and Express.
Let's try using session counters to see if a particular user has messaged us before. If they're a new sender, we'll thank them for the new message. If they've sent us messages before, we'll specify how many messages we've gotten from them.
It's almost too easy, isn't it? If you're looking for a more complex example of using cookies to track a conversation, take a look at our Employee Directory tutorial.