Building a Telegram Bot with Cloudflare Workers, Durable Objects and Grammy

Background

Last month I visited my friend for a week or so who mentioned I drink way less water and he showed me the reading on his water purifier. On average, I drank like 1L of water.

I then started keeping track of how much water I drank, but then I used to miss a couple of days. So I thought, why not create an app to track this and also remind myself to drink water.

I present to you Drinky which does just that. If you’re interested, the GitHub repo is here. This blog post will go into why I went via the bot route, the tech stack I chose and the things I learned.

Has to be a new tech stack and low friction to use

I didn’t want to develop another web app in which I had to log in to. I thought of creating an Android app and to store the data locally but it would have been overkill and it would also be a hassle if I wanted to share it with somebody. I had already built a Telegram bot to notify when your Blender render is completed (Render Notifier) and I knew the Telegram API was also pretty nice to work with. This would also allow me to share the bot with anyone. So there it was. Telegram bot.

I was tired of using D1 due to its cold start time and drizzle not supporting its sessions API. Since I wanted to deploy everything to Cloudflare, I thought why not use Durable Objects? I had used it a while back to write a blog post, but not in a product as such and each user gets their own DB and since there won’t be any external DB calls as such, the latency between data fetching, updating, etc would be close to 0ms. So I chose DO for my database.

The last remaining part was which TS library to use to interact with Telegram API. I had read about telegraf.js somewhere so I checked it out, but it was not actively maintained. After searching for a bit, I went with grammY due to extensive documentation and it being actively maintained.

Some more new things

Some more new libs which I have used are:

• oxc – for linting and formatting
• Vitest – for testing
• AGENTS.md – for Gemini CLI
• drizzle (beta version) – for ORM

Things I learned

Setting webhook URL

Seemed pretty basic in hindsight, but no matter what, I was unable to listen to the messages sent to the bot. Added to the fact that I was using wrangler to develop locally (since I would be deploying to Cloudflare Workers), I was not sure what I was doing wrong. I eventually found out I had to set the webhook like so https://api.telegram.org/bot/setWebhook?url=. This would tell the bot to forward the messages to the webhook URL. To expose my localhost to the public internet, I used Cloudflare Tunnel.

Bot not responding to messages

I was now able to see the messages, but I was not able to respond to them. I regrettably had to ask Cursor to fix this to which it spat out below code.

app.post("/webhook", async (c) => {
    const update = await c.req.json();

    const bot = await setupBot({
        token: c.env.BOT_TOKEN,
        env: c.env,
        commands,
        callbacks,
    });

    await bot.handleUpdate(update);

    return c.json({ ok: true });
});

await bot.handleUpdate(update); is used to pass the updates to the bot instance. Apparently, this is needed in a serverless env where everything is stateless. Adding this fixed the issue

Commands can be registered dynamically

Initially I was under the impression that BotFather has to be used to update any commands which the bot can show to the user. But turns out you can set them via the API as well.

app.post("/webhook", async (c) => {
    const update = await c.req.json();

    const bot = await setupBot({
      token: c.env.BOT_TOKEN,
      env: c.env,
      commands,
      callbacks,
    });

    await bot.api.setMyCommands(
      commands.map((command) => ({
        command: command.name,
        description: command.description,
      })),
    );

    // This is required for grammy to work in Cloudflare Workers. Also, this has to go after the commands are registered.
    await bot.handleUpdate(update);

    return c.json({ ok: true });
});

As you can see, the handleUpdate method needs to be called after the commands are set. Otherwise, the data is not passed along.

Not everything has to be AI

There came a point where I had to have timezone identification in the bot. My first thought was to ask user where they’re located, send it to an LLM and then get the timezone in IANA format. Instead, I now ask the user for their location via Telegram’s API, send their lat and long to tz-lookup and get the timezone. I’m glad I did not go via the AI route and I’m ashamed I did not think of the tz-lookup solution first.

Conclusion

All in all, it was a lot of fun building this bot. I learned a lot and also solved my problem rather elegantly if I say so myself.