Pub/Sub, Streams, and how both differ from Kafka

Key points

• Pub/Sub is fire and forget: PUBLISH delivers only to clients subscribed right now and returns the count.
• A subscriber that is disconnected for two seconds loses every message sent in those two seconds, permanently.
• Streams arrived in Redis 5.0: XADD appends entries with time-ordered IDs that stay until you trim them.
• XREADGROUP plus XACK gives at-least-once delivery, and unacknowledged entries sit in the pending list.
• XAUTOCLAIM, added in 6.2, reassigns entries a crashed consumer never acknowledged to a live one.
• Trim on write with XADD key MAXLEN ~ 10000 *, because an untrimmed stream grows until it fills memory.

Example

# Pub/Sub - nobody listening means nobody gets it
PUBLISH orders.created "991"
(integer) 0        # zero subscribers, message gone

# Streams - stored, grouped, acknowledged
XADD orders MAXLEN ~ 100000 * id 991 total 240
XGROUP CREATE orders billing $ MKSTREAM
XREADGROUP GROUP billing worker-1 COUNT 10 STREAMS orders >
XACK orders billing 1722688200000-0
XPENDING orders billing        # what is claimed but not yet acknowledged

Pub/Sub for live notifications you can afford to lose, Streams for work queues, Kafka when the log must outlive memory.