Redis Pub/Sub
"Fire and Forget" - Stateless Delivery
Publisher
Redis Broker
RAM Only (No Persistence)
Memory Clears Instantly
Sub A
Online
Sub B
Offline
Apache Kafka
"Distributed Log" - Stateful Storage
Producer
Kafka Broker
Append-Only Log (Disk)
Cons A
Online
Cons B
Offline
Architectural Comparison
Redis Pub/Sub
- Stateless: Messages are delivered and immediately removed from memory.
- Delivery: If a subscriber is offline, they miss the message forever.
- Latency: Extremely low, as it's purely in-memory and bypasses disk.
- Use Case: Real-time chat, gaming updates, cache invalidation.
Apache Kafka
- Stateful: Messages are persisted to disk as an immutable append-only log.
- Replayability: Consumers can "rewind" and read history (Event Sourcing).
- Throughput: High-performance sequential disk I/O allows massive volumes.
- Use Case: Activity tracking, log aggregation, complex event processing.
| Feature | Redis Pub/Sub | Apache Kafka |
|---|---|---|
| Data Storage | No. Ethereal/Ephemeral. | Yes. Persistent on disk. |
| Late Joiners | Receive only new messages. | Can read historical messages. |
| Scaling | Scales vertically; Cluster support. | Scales horizontally via Partitions. |
| Guarantees | Best effort delivery. | At-least-once, Exactly-once. |