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Design a messaging system using MongoDB and Redis (similar to Slack).
Architecture: 1) MongoDB: channels: {_id, name, members:[userId...], type}. messages: {_id, channelId, userId, text, attachments, reactions, createdAt}. 2) Index: messages.createIndex({channelId:1, createdAt:-1}). 3) Redis for real-time: SUBSCRIBE channel:{channelId} — WebSocket subscribers listen. On new message: PUBLISH channel:{channelId} {messageId, userId, text, ts}. 4) Recent messages cache:
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