written by Matthias Endler
Every once in a while my buddies and I meet for dinner. I value these evenings, but the worst part is scheduling these events!
  • We send out a message to the group.
  • We wait for a response.
  • We decide on a date.
  • Someone sends out a calendar invite.
  • Things finally happen.
None of that is fun except for the dinner. Being the reasonable person you are, you would think: “Why don’t you just use a scheduling app?”. I have tried many of them. None of them are any good. They are all…too much! Just let me send out an invite and whoever wants can show up.
  • I don’t want to have to create an account for your calendar/scheduling/whatever app.
  • I don’t want to have to add my friends.
  • I don’t want to have to add my friends’ friends.
  • I don’t want to have to add my friends’ friends’ friends.
  • You get the idea: I just want to send out an invite and get no response from you.

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The nerdy, introvert engineer’s solution

💡 What we definitely need is yet another calendar app which allows us to create events and send out an invite with a link to that event! You probably didn’t see that coming now, did you? Oh, and I don’t want to use Google Calendar to create the event because I don’t trust them. Like any reasonable person, I wanted a way to create calendar entries from my terminal. That’s how I pitched the idea to my buddies last time. The answer was: “I don’t know, sounds like a solution in search of a problem.” But you know what they say: Never ask a starfish for directions.

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Show, don’t tell

That night I went home and built a website that would create a calendar entry from GET parameters. It allows you to create a calendar event from the convenience of your command line: 
> curl https://zerocal.shuttle.app?start=2022-11-04+20:00&duration=3h&title=Birthday&description=paaarty
BEGIN:VCALENDAR
VERSION:2.0
PRODID:ICALENDAR-RS
CALSCALE:GREGORIAN
BEGIN:VEVENT
DTSTAMP:20221002T123149Z
CLASS:CONFIDENTIAL
DESCRIPTION:paaarty
DTEND:20221002T133149Z
DTSTART:20221002T123149Z
SUMMARY:Birthday
UID:c99dd4bb-5c35-4d61-9c46-7a471de0e7f4
END:VEVENT
END:VCALENDAR
You can then save that to a file and open it with your calendar app. 
curl https://zerocal.shuttle.app?start=2022-11-04+20:00&duration=3h&title=Birthday&description=paaarty > birthday.ics
open birthday.ics
In a sense, it’s a “serverless calendar app”, haha. There is no state on the server, it just generates a calendar event on the fly and returns it.

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How I built it

You probably noticed that the URL contains “shuttle.app”. That’s because I’m using shuttle.dev to host the website. Shuttle is a hosting service for Rust projects and I wanted to try it out for a long time. To initialize the project using the awesome axum web framework, I’ve used 
cargo install cargo-shuttle
shuttle init --template axum --name zerocal zerocal
and I was greeted with everything I needed to get started: 
use axum::{routing::get, Router};

async fn hello_world() -> &'static str {
  "Hello, world!"
}

#[shuttle_runtime::main]
async fn axum() -> shuttle_axum::ShuttleAxum {
  let router = Router::new().route("/hello", get(hello_world));

  Ok(router.into())
}
Let’s quickly commit the changes: 
git add .gitignore Cargo.toml src/
git commit -m "Hello World"
Then: 
shuttle deploy
Now let’s head over to the returned project URL: Hello World! Deploying the first version took less than 5 minutes. Nice! We’re all set for our custom calendar app.

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Writing the app

To create the calendar event, I used the icalendar crate (shout out to hoodie for creating this nice library!). iCalendar is a standard for creating calendar events that is supported by most calendar apps. 
cargo add icalendar
cargo add chrono # For date and time parsing
Let’s create a demo calendar event: 
let event = Event::new()
  .summary("test event")
  .description("here I have something really important to do")
  .starts(Utc::now())
  .ends(Utc::now() + Duration::days(1))
  .done();
Simple enough.

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How to return a file!?

Now that we have a calendar event, we need to return it to the user. But how do we return it as a file? There’s an example of how to return a file dynamically in axum here. 
async fn calendar() -> impl IntoResponse {
  let ical = Calendar::new()
    .push(
      // add an event
      Event::new()
        .summary("It works! 😀")
        .description("Meeting with the Rust community")
        .starts(Utc::now() + Duration::hours(1))
        .ends(Utc::now() + Duration::hours(2))
        .done(),
    )
    .done();

  CalendarResponse(ical)
}
Some interesting things to note here:
  • Every calendar file is a collection of events so we wrap the event in a Calendar object, which represents the collection.
  • impl IntoResponse is a trait that allows us to return any type that implements it.
  • CalendarResponse is a newtype wrapper around Calendar that implements IntoResponse.
Here is the CalendarResponse implementation: 
/// Newtype wrapper around Calendar for `IntoResponse` impl
#[derive(Debug)]
pub struct CalendarResponse(pub Calendar);

impl IntoResponse for CalendarResponse {
  fn into_response(self) -> Response {
    let mut res = Response::new(boxed(Full::from(self.0.to_string())));
    res.headers_mut().insert(
      header::CONTENT_TYPE,
      HeaderValue::from_static("text/calendar"),
    );
    res
  }
}
We just create a new Response object and set the Content-Type header to the correct MIME type for iCalendar files: text/calendar. Then we return the response.

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Add date parsing

This part is a bit hacky, so feel free to glance over it. We need to parse the date and duration from the query string. I used dateparser, because it supports sooo many different date formats. 
async fn calendar(Query(params): Query<HashMap<String, String>>) -> impl IntoResponse {
  let mut event = Event::new();
  event.class(Class::Confidential);

  if let Some(title) = params.get("title") {
    event.summary(title);
  } else {
    event.summary(DEFAULT_EVENT_TITLE);
  }
  if let Some(description) = params.get("description") {
    event.description(description);
  } else {
    event.description("Powered by zerocal.shuttle.app");
  }

  if let Some(start) = params.get("start") {
    let start = dateparser::parse(start).unwrap();
    event.starts(start);
    if let Some(duration) = params.get("duration") {
      let duration = humantime::parse_duration(duration).unwrap();
      let duration = chrono::Duration::from_std(duration).unwrap();
      event.ends(start + duration);
    }
  }

  if let Some(end) = params.get("end") {
    let end = dateparser::parse(end).unwrap();
    event.ends(end);
    if let Some(duration) = params.get("duration") {
      if params.get("start").is_none() {
        let duration = humantime::parse_duration(duration).unwrap();
        let duration = chrono::Duration::from_std(duration).unwrap();
        event.starts(end - duration);
      }
    }
  }

  let ical = Calendar::new().push(event.done()).done();

  CalendarResponse(ical)
}
Would be nice to support more date formats like now and tomorrow, but I’ll leave that for another time. Let’s test it: 
> shuttle run # This starts a local dev server
> curl 127.0.0.1:8000?start=2022-11-04+20:00&duration=3h&title=Birthday&description=Party
*🤖 bleep bloop, calendar file created*
Nice, it works! Opening it in the browser creates a new event in the calendar: And for all the odd people who don’t use a terminal to create a calendar event, let’s also add a form to the website.

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Add a form

<form>
  <table>
    <tr>
      <td>
        <label for="title">Event Title</label>
      </td>
      <td>
        <input type="text" id="title" name="title" value="Birthday" />
      </td>
    </tr>
    <tr>
      <td>
        <label for="desc">Description</label>
      </td>
      <td>
        <input type="text" id="desc" name="desc" value="Party" />
      </td>
    </tr>
    <tr>
      <td><label for="start">Start</label></td>
      <td>
        <input type="datetime-local" id="start" name="start" />
      </td>
    </tr>
    <tr>
      <td><label for="end">End</label></td>
      <td>
        <input type="datetime-local" id="end" name="end" />
      </td>
    </tr>
  </table>
</form>
I modified the calendar function a bit to return the form if the query string is empty: 
async fn calendar(Query(params): Query<HashMap<String, String>>) -> impl IntoResponse {
  // if query is empty, show form
  if params.is_empty() {
    return Response::builder()
      .status(200)
      .body(boxed(Full::from(include_str!("../static/index.html"))))
      .unwrap();
  }

  // ...
}
After some more tweaking, we got ourselves a nice little form in all of its web 1.0 glory. Serverless Calendar app And that’s it! We now have a little web app that can create calendar events. Well, almost. We still need to deploy it.

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Deploying

shuttle deploy
Right, that’s all. It’s that easy. Thanks to the folks over at shuttle.dev for making this possible. The calendar app is now available at https://zerocal.shuttle.app. Now I can finally send my friends a link to a calendar event for our next pub crawl. They’ll surely appreciate it.yeahyeah

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From zero to calendar in 100 lines of Rust

Boy it feels great to be writing plain HTML again. Building little apps never gets old. Check out the source code on GitHub and help me make it better! 🙏 Here are some ideas:
  • Add support for more human-readable date formats (e.g. now, tomorrow).
  • Add support for recurring events. Add support for timezones.
  • Add location support (e.g. location=Berlin or location=https://zoom.us/test). Add Google calendar short-links (https://calendar.google.com/calendar/render?action=TEMPLATE&dates=20221003T224500Z%2F20221003T224500Z&details=&location=&text=).
  • Add example bash command to create a calendar event from the command line.
  • Shorten the URL (e.g. zerocal.shuttle.app/2022-11-04T20:00/3h/Birthday/Party)?
Check out the issue tracker and feel free to open a PR!