Implementation Principles of Redis Pub/Sub (Ordinary Subscription)

Redis Pub/Sub is a messaging pattern where publishers send messages to channels and subscribers receive them.

The Redis server stores subscription relationships in a dictionary (hash table) where the key is the channel name and the value is a linked list of client objects subscribed to that channel.

Each client also keeps a dictionary of the channels it subscribes to, enabling quick lookup of a client’s subscriptions.

When a SUBSCRIBE command is processed (function subscribeCommand in pubsub.c ), the server adds the channel‑client pair to the server‑wide hash and records the channel in the client’s pubsub_channels dictionary.

Publishing a message (function publishCommand in pubsub.c ) looks up the channel in server.pubsub_channels , iterates over the client list, and writes the message into each client’s reply buffer via addReplyPubsubMessage . If the server runs in cluster mode, clusterPropagatePublish forwards the message to replicas.

Important caveats:

Messages are not persisted; offline subscribers miss any messages sent while they are disconnected.

Each client’s reply buffer has a size limit; exceeding it forces the server to close the client connection, requiring the client to resubscribe.

Because of these characteristics, Redis Pub/Sub is unsuitable as a durable message queue.