How to Use Pub/Sub Channels in Redict #

SUBSCRIBE, UNSUBSCRIBE and PUBLISH implement the Publish/Subscribe messaging paradigm where (citing Wikipedia) senders (publishers) are not programmed to send their messages to specific receivers (subscribers). Rather, published messages are characterized into channels, without knowledge of what (if any) subscribers there may be. Subscribers express interest in one or more channels and only receive messages that are of interest, without knowledge of what (if any) publishers there are. This decoupling of publishers and subscribers allows for greater scalability and a more dynamic network topology.

For instance, to subscribe to channels “channel11” and “ch:00” the client issues a SUBSCRIBE providing the names of the channels:

SUBSCRIBE channel11 ch:00

Messages sent by other clients to these channels will be pushed by Redict to all the subscribed clients. Subscribers receive the messages in the order that the messages are published.

A client subscribed to one or more channels shouldn’t issue commands, although it can SUBSCRIBE and UNSUBSCRIBE to and from other channels. The replies to subscription and unsubscribing operations are sent in the form of messages so that the client can just read a coherent stream of messages where the first element indicates the type of message. The commands that are allowed in the context of a subscribed RESP2 client are:

PING
PSUBSCRIBE
PUNSUBSCRIBE
QUIT
RESET
SSUBSCRIBE
SUBSCRIBE
SUNSUBSCRIBE
UNSUBSCRIBE

However, if RESP3 is used (see HELLO), a client can issue any commands while in the subscribed state.

Please note that when using redis-cli, in subscribed mode commands such as UNSUBSCRIBE and PUNSUBSCRIBE cannot be used because redis-cli will not accept any commands and can only quit the mode with Ctrl-C.

Delivery semantics #

Redict’ 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 Redict 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 Redict Streams. Messages in streams are persisted, and support both at-most-once as well as at-least-once delivery semantics.

Format of pushed messages #

A message is an 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 Redict 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.

Database & Scoping #

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…).

Wire protocol example #

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

Pattern-matching subscriptions #

The Redict 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.

Messages matching both a pattern and a channel subscription #

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.

The meaning of the subscription count with pattern matching #

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.

Sharded Pub/Sub #

With Sharded Pub/Sub, 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.

Programming example #

Pieter Noordhuis provided a great example using EventMachine and Redis to create a multi user high performance web chat.

Client library implementation hints #

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.