What is the difference between a message queue and pub-sub?

A queue distributes each message to exactly one of its consumers, which suits jobs that should be done once. Pub-sub delivers a copy of each message to every subscriber, which suits events that multiple independent systems care about. Most brokers support both shapes: SQS is a queue, SNS is pub-sub, and Kafka does both through consumer groups.

Does pub-sub guarantee my subscribers receive messages in order?

Not globally. Brokers preserve order only within a lane, a Kafka partition key or an SQS FIFO message group, and messages in different lanes interleave arbitrarily. Choose a key that matches your invariants, like order ID or user ID, and build subscribers to tolerate occasional reordering anyway, using sequence numbers or versions to discard stale events.

When is fire-and-forget pub-sub like Redis acceptable?

When missing a message is harmless because the next one supersedes it or the state can be re-fetched. Cache invalidation broadcasts, presence updates, and live dashboard ticks all qualify. Anything with business meaning, orders, payments, audit events, needs a durable broker where a subscriber that was down gets its backlog on return.

Communicationchapter 3 of 4 · 3 lessons

Pub/Sub Pattern

One-to-many broadcast. Publishers emit events, any number of subscribers receive them.

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An order gets placed. Your checkout code calls the notification service, then the loyalty service, then analytics, then the fraud check.

A new team needs to know about orders too, so somebody edits checkout and adds a fifth call. Then a sixth. Checkout is now the file every team has to touch, and it is on the critical path of a purchase.

Publish it instead. Checkout says an order was placed, once, and stops. Every team subscribes on their own, without checkout knowing they exist.

Six months later there are seven subscribers and checkout has not changed a line. That decoupling is the actual product, and the broadcasting is only how you get it.

Lessons

3 in this chapter

Queues vs TopicsA queue delivers each message to one worker; a topic delivers a copy to every subscriber.2 min
Fan-Out MechanicsOne event in, N durable deliveries out, with each subscriber's failures kept private.3 min
Ordering and Delivery CaveatsGlobal order does not exist, duplicates do, and your subscribers have to be built for both.2 min