Commonwealth Bank USD→AUD Rate Bot

A niche utility for Commonwealth Bank customers who hold a USD Foreign Currency Account and want to know the best time to convert to an AUD Smart Access account. It is currently a simple web service that polls Commonwealth Bank's published USD→AUD retail rate and optionally alerts you via Telegram when your target is reached.

How it works

• Fetches forex data from Commonwealth Bank's JSON feed on a fixed interval (default 60 minutes).
• Extracts the USD→AUD bsImt buy/sell rate.
• Stores the latest rate and timestamp in memory and exposes them via a lightweight HTTP endpoint with CORS enabled.
• Sends a Telegram message when the rate crosses the configured target.

Key components

• Poller and notifier: server/main.go — fetches the rate, updates shared state, and sends Telegram alerts based on config.Config.
• Configuration defaults: server/config/config.go — TargetRate (default 1.20) and CheckInterval (default 60m).
• API surface: server/main.go exposes GET /api/rate returning { rate, lastUpdated } with lastUpdated in RFC3339 format.
• Telegram integration: driven by environment variables (TELEGRAM_BOT_TOKEN, TELEGRAM_CHAT_ID) and github.com/joho/godotenv for local .env loading.

Run locally

1. Install Go 1.20+ (module targets Go 1.25.4).
2. From the server directory, create a .env file:

   TELEGRAM_BOT_TOKEN=your_bot_token
   TELEGRAM_CHAT_ID=your_chat_id

3. Start the service:

   cd server
   go run ./...

4. Fetch the latest rate:

   curl http://localhost:8080/api/rate

Telegram setup (token and chat ID)

1. Create a bot token:
   • In Telegram, search for @BotFather.
   • Start a chat and send /start if needed, then /newbot.
   • Follow the prompts to name your bot and choose a unique username ending in bot.
   • BotFather returns an HTTPS API token; place it in TELEGRAM_BOT_TOKEN.
2. Find your chat ID:
   • Add your new bot to a chat (or DM it) and send any message (e.g., "hi").
   • In a browser, open https://api.telegram.org/bot<YOUR_TOKEN>/getUpdates.
   • In the JSON response, locate message.chat.id — that numeric value is your TELEGRAM_CHAT_ID.
3. Test messaging:
   • Call https://api.telegram.org/bot<YOUR_TOKEN>/sendMessage?chat_id=<CHAT_ID>&text=hello in a browser or with curl to confirm delivery.

Configuration

• Update server/config/config.go to change TargetRate or CheckInterval (e.g., make checks more frequent or tighten the alert threshold).
• Telegram alerts fire only when the fetched rate is strictly greater than TargetRate.
• The service keeps rates in-memory; restarting clears history.

API

• GET /api/rate → 200 OK

  {
    "rate": 1.2345,
    "lastUpdated": "2025-01-24T12:34:56Z"
  }

Diagrams

Flow (polling and alerting)

flowchart TD
      start([Start]) --> load[Load .env and config]
      load --> initial[Immediate rate check]
      initial --> ticker[Wait for interval tick]
      ticker --> fetch[Fetch CommBank USD.json]
      fetch --> parse[Parse AUD bsImt rate]
      parse --> update[Update in-memory rate and timestamp]
      update --> check{Rate > TargetRate?}
      check -->|Yes| alert[Send Telegram alert]
      check -->|No| skip[Skip alert]
      alert --> serve[Expose GET /api/rate]
      skip --> serve
      serve --> ticker

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Sequence (serving rate and sending alert)

sequenceDiagram
      participant Client as API Client
      participant Service as Rate Bot
      participant CommBank as CommBank API
      participant Telegram as Telegram API

      Service->>CommBank: GET USD.json (scheduled)
      CommBank-->>Service: USD→AUD rates
      Service->>Service: Update rate cache
      Service->>Telegram: sendMessage (if rate > target)
      Telegram-->>Service: Delivery response

      Client->>Service: GET /api/rate
      Service-->>Client: { rate, lastUpdated }

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Future ideas

• Persist historical rates for trend analysis and notifications on improvements beyond a trailing window.
• Add some UI for user-configurable thresholds and more.
• Expand to additional currency pairs (GBP, EUR).