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Fan Out

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sarmakska edited this page May 31, 2026 · 1 revision

Fan-Out

Besides email, the service can post each delivered event to Slack and Telegram. Both channels are best-effort: a fan-out failure is logged but never fails or retries the job, because email is the source of truth and re-sending the email to recover a chat notification would spam the inbox. Fan-out runs only after the email send succeeds.

Slack

Set SLACK_WEBHOOK_URL to an incoming webhook URL:

SLACK_WEBHOOK_URL=https://hooks.slack.com/services/T.../B.../...

Get one from Slack, Apps, Incoming Webhooks, Add to channel.

The message is posted as Slack Block Kit: a header block with the subject, a section block with the body converted to Slack mrkdwn (bold, headings and links translated), and a context block naming the source. The body is truncated to fit Slack's block limits.

Telegram

Both variables must be set for Telegram fan-out to activate:

TELEGRAM_BOT_TOKEN=123456:ABC-DEF...
TELEGRAM_CHAT_ID=123456789

Get a bot token

  1. Message @BotFather on Telegram.
  2. Send /newbot and follow the prompts.
  3. Copy the token it gives you into TELEGRAM_BOT_TOKEN.

Get a chat id

  1. Add the bot to the target chat or start a direct message with it.
  2. Resolve the chat id, for example by messaging @userinfobot, or by reading https://api.telegram.org/bot<token>/getUpdates after sending the bot a message.
  3. Put the id into TELEGRAM_CHAT_ID.

The message is sent with parse_mode: Markdown: a bold subject followed by the plain-text body, with characters that break Telegram's parser escaped, and link previews disabled.

How fan-out fits the flow

graph LR
  W[Worker delivers email] -->|ok| E[Resend]
  W -->|ok| S{Slack set?}
  W -->|ok| T{Telegram set?}
  S -->|yes| SL[Slack Block Kit]
  T -->|yes| TG[Telegram sendMessage]
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Fan-out calls run in parallel and their failures are swallowed after a warning. The job is considered delivered as soon as the email send succeeds.

Verifying fan-out

Send a test webhook and watch stdout. A successful fan-out is silent; a failure logs Slack fan-out failed: or Telegram fan-out failed: with the reason. The notifier is fully injectable, so the test suite covers both channels, including the rule that both Telegram variables must be present before it fires.

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