valkey-pubsub - Man Page

Valkey’ Pub/Sub exhibits at-most-once message delivery semantics. As the name suggests, it means that a message will be delivered once if at all. Once the message is sent by the Valkey server, there’s no chance of it being sent again. If the subscriber is unable to handle the message (for example, due to an error or a network disconnect) the message is forever lost.

If your application requires stronger delivery guarantees, you may want to learn about valkey-streams-intro(7) Streams. Messages in streams are persisted, and support both at-most-once as well as at-least-once delivery semantics.

A message is an valkey-protocol(7) array-reply with three elements.

The first element is the kind of message:

• subscribe: means that we successfully subscribed to the channel given as the second element in the reply. The third argument represents the number of channels we are currently subscribed to.
• unsubscribe: means that we successfully unsubscribed from the channel given as second element in the reply. The third argument represents the number of channels we are currently subscribed to. When the last argument is zero, we are no longer subscribed to any channel, and the client can issue any kind of Valkey command as we are outside the Pub/Sub state.
• message: it is a message received as a result of a PUBLISH command issued by another client. The second element is the name of the originating channel, and the third argument is the actual message payload.

Pub/Sub has no relation to the key space. It was made to not interfere with it on any level, including database numbers.

Publishing on db 10, will be heard by a subscriber on db 1.

If you need scoping of some kind, prefix the channels with the name of the environment (test, staging, production...).

SUBSCRIBE first second
*3
$9
subscribe
$5
first
:1
*3
$9
subscribe
$6
second
:2

At this point, from another client we issue a PUBLISH operation against the channel named second:

> PUBLISH second Hello

This is what the first client receives:

*3
$7
message
$6
second
$5
Hello

Now the client unsubscribes itself from all the channels using the UNSUBSCRIBE command without additional arguments:

UNSUBSCRIBE
*3
$11
unsubscribe
$6
second
:1
*3
$11
unsubscribe
$5
first
:0

The Valkey Pub/Sub implementation supports pattern matching. Clients may subscribe to glob-style patterns to receive all the messages sent to channel names matching a given pattern.

For instance:

PSUBSCRIBE news.*

Will receive all the messages sent to the channel news.art.figurative, news.music.jazz, etc. All the glob-style patterns are valid, so multiple wildcards are supported.

PUNSUBSCRIBE news.*

Will then unsubscribe the client from that pattern. No other subscriptions will be affected by this call.

Messages received as a result of pattern matching are sent in a different format:

• The type of the message is pmessage: it is a message received as a result from a PUBLISH command issued by another client, matching a pattern-matching subscription. The second element is the original pattern matched, the third element is the name of the originating channel, and the last element is the actual message payload.

Similarly to SUBSCRIBE and UNSUBSCRIBE, PSUBSCRIBE and PUNSUBSCRIBE commands are acknowledged by the system sending a message of type psubscribe and punsubscribe using the same format as the subscribe and unsubscribe message format.

A client may receive a single message multiple times if it’s subscribed to multiple patterns matching a published message, or if it is subscribed to both patterns and channels matching the message. This is shown by the following example:

SUBSCRIBE foo
PSUBSCRIBE f*

In the above example, if a message is sent to channel foo, the client will receive two messages: one of type message and one of type pmessage.

In subscribe, unsubscribe, psubscribe and punsubscribe message types, the last argument is the count of subscriptions still active. This number is the total number of channels and patterns the client is still subscribed to. So the client will exit the Pub/Sub state only when this count drops to zero as a result of unsubscribing from all the channels and patterns.

From Redis OSS 7.0, sharded Pub/Sub is introduced in which shard channels are assigned to slots by the same algorithm used to assign keys to slots. A shard message must be sent to a node that owns the slot the shard channel is hashed to. The cluster makes sure the published shard messages are forwarded to all nodes in the shard, so clients can subscribe to a shard channel by connecting to either the master responsible for the slot, or to any of its replicas. SSUBSCRIBE, SUNSUBSCRIBE and SPUBLISH are used to implement sharded Pub/Sub.

Sharded Pub/Sub helps to scale the usage of Pub/Sub in cluster mode. It restricts the propagation of messages to be within the shard of a cluster. Hence, the amount of data passing through the cluster bus is limited in comparison to global Pub/Sub where each message propagates to each node in the cluster. This allows users to horizontally scale the Pub/Sub usage by adding more shards.

Because all the messages received contain the original subscription causing the message delivery (the channel in the case of message type, and the original pattern in the case of pmessage type) client libraries may bind the original subscription to callbacks (that can be anonymous functions, blocks, function pointers), using a hash table.

When a message is received an O(1) lookup can be done to deliver the message to the registered callback.