Naive Mailbox

The first instinct — store undelivered messages in a Redis mailbox

If Bob is offline, hold his messages somewhere until he reconnects. Redis is already in the system. Why not use it?

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The naive approach

When the server detects Bob is offline, instead of dropping the message, it queues it in Redis:

RPUSH mailbox:bob → { conversation_id: conv_abc123, content: "hey", seq: 42, ... }
RPUSH mailbox:bob → { conversation_id: conv_abc123, content: "where are you?", seq: 43, ... }

When Bob reconnects, drain the mailbox:

LRANGE mailbox:bob → get all queued messages
→ push them to Bob over WebSocket
→ delete mailbox:bob

Simple. No new components. Reuses existing Redis.

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Why it fails — problem 1: memory

Redis is an in-memory store. Every byte in a Redis mailbox costs RAM. At WhatsApp scale:

100M DAU
Each offline user accumulates messages while away
Average offline duration: several hours
Messages per hour per active conversation: ~10-20

If even 10% of DAU goes offline with pending messages:
  10M users × 10 messages × 250 bytes = 25 GB just for one hour of offline messages

Redis is not designed to be a durable message store for millions of users. This blows up memory fast.

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Why it fails — problem 2: durability

Bob goes offline Friday evening. Redis restarts Saturday morning — a routine deploy, or a crash. AOF helps recover most data, but this mailbox is the only record of messages Bob hasn't received yet.

Bob's mailbox before restart:
  seq=42 "hey"
  seq=43 "where are you?"

Redis restarts → AOF replays → mailbox restored

BUT: what if AOF sync was slightly behind? 
     seq=43 is lost. Bob never receives it.

For a messaging app, losing a message silently is a critical failure. The mailbox is too important to trust to an in-memory store.

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The root cause

The naive mailbox stores full message content in Redis. But the full messages are already safely stored in DynamoDB. Duplicating them into Redis adds memory cost and durability risk with no real benefit.

The right question isn't "where do I store the messages" — it's "what's the minimum information I need to find them later?"