Sink Health Monitoring: Slack & Discord Notification Channels

[joejohnson123]

Overview

Add Slack and Discord as notification channel types with rich formatting.

Depends on: #4 (Notification Dispatcher & Webhook Channel)

Slack Channel: Sequin.Monitors.Channels.Slack

Implementation

• POST to Slack incoming webhook URL
• Use Block Kit for rich formatting
• Color coding: red (#E74C3C) for triggered, green (#2ECC71) for resolved

Message Format (Block Kit)

{
  "attachments": [{
    "color": "#E74C3C",
    "blocks": [
      {
        "type": "header",
        "text": {"type": "plain_text", "text": "🚨 Monitor Alert: High failure rate"}
      },
      {
        "type": "section",
        "fields": [
          {"type": "mrkdwn", "text": "*Sink:*\nmy-webhook-sink"},
          {"type": "mrkdwn", "text": "*Metric:*\nFailed Messages"},
          {"type": "mrkdwn", "text": "*Current Value:*\n150"},
          {"type": "mrkdwn", "text": "*Threshold:*\n100"}
        ]
      },
      {
        "type": "context",
        "elements": [{"type": "mrkdwn", "text": "Triggered at 2026-02-26 13:00 UTC"}]
      },
      {
        "type": "actions",
        "elements": [{
          "type": "button",
          "text": {"type": "plain_text", "text": "View Sink"},
          "url": "https://app.sequinstream.com/sinks/..."
        }]
      }
    ]
  }]
}

Recovery Message

• Green color, ✅ emoji in header
• Shows how long the alert was active
• Same field layout

Discord Channel: Sequin.Monitors.Channels.Discord

Implementation

• POST to Discord webhook URL
• Use embed format for rich messages
• Color coding: red (0xE74C3C) for triggered, green (0x2ECC71) for resolved

Message Format (Embed)

{
  "embeds": [{
    "title": "🚨 Monitor Alert: High failure rate",
    "color": 15158332,
    "fields": [
      {"name": "Sink", "value": "my-webhook-sink", "inline": true},
      {"name": "Metric", "value": "Failed Messages", "inline": true},
      {"name": "Current Value", "value": "150", "inline": true},
      {"name": "Threshold", "value": "100", "inline": true}
    ],
    "footer": {"text": "Triggered at 2026-02-26 13:00 UTC"},
    "url": "https://app.sequinstream.com/sinks/..."
  }]
}

Files to Create

• lib/sequin/monitors/channels/slack.ex
• lib/sequin/monitors/channels/discord.ex
• test/sequin/monitors/channels/slack_test.exs
• test/sequin/monitors/channels/discord_test.exs

Implementation Notes

• Both use simple HTTP POST to webhook URLs (no OAuth needed)
• Slack webhook URL format: https://hooks.slack.com/services/T.../B.../...
• Discord webhook URL format: https://discord.com/api/webhooks/{id}/{token}
• Use Req for HTTP requests with 10s timeout
• Format metric names as human-readable (:failed_messages → "Failed Messages")
• For anomaly monitors, show detected value vs expected range instead of threshold

Acceptance Criteria

• Slack messages render with correct Block Kit format
• Discord messages render with correct embed format
• Color coding works for both triggered and resolved states
• "View Sink" link is included and correct
• Both handle HTTP errors gracefully
• Tests verify payload structure