Pub/Sub
Distributed microservices often respond to events that occur in other microservices. This pattern is commonly referred to as event-driven architecture.
In the retail company scenario, the various microservices must communicate with each other and react to events from other microservices. Today, they manually store the addresses for other microservices, but this technique is challenging to manage, increases technical complexity, and doesn’t scale well.
Event aggregation using Pub/Sub
An event aggregator is a middleware solution that aggregates events across the entire solution in a scalable and straightforward to manage way.
Here, you learn how the Pub/Sub feature of Azure Cache for Redis can serve as a middleware to simplify the communication between components of your application. Pub/Sub can help your application components subscribe to other’s events and publish their own events.
The Pub/Sub feature of Azure Cache for Redis routes messages between application components. Microservices can use this feature to subscribe to messages or publish messages. Azure Cache for Redis handles the routing of messages to the appropriate destinations without each microservice knowing where each of their messages should go.
This feature can significantly simplify the requirement of microservices reacting to events across the entire solution.
Subscribing to channels
Clients can subscribe to a topic or range of topics using a string value. In Redis nomenclature, topics are referred to as channels. For example, a client that wants to subscribe to the staff.newhire channel can use the appropriate command with the staff.newhire string value.
Once the client is subscribed, all messages in the staff.newhire channel are sent to that specific client.
Diagram of a server application sending a message to Azure Cache for Redis and only the client subscribed to the staff.newhire channel receiving the message.
Redis includes a SUBSCRIBE command used to subscribe to one or more channels. This command is flexible enough to subscribe to a space-delimited list of channels.
Subscribe to a single known channel
The most common use case of the SUBSCRIBE command is to subscribe to a single channel. For example, the command can be used to subscribe to the staff.newhire channel.
SUBSCRIBE staff.newhire
Subscribe to multiple known channels
The SUBSCRIBE command can also be used to subscribe to multiple channels simultaneously. To subscribe to multiple channels, separate each channel with a single space in the channels list.
As an example of subscribing to multiple channels, use the SUBSCRIBE command to subscribe to the orders.delete and orders.new channels simultaneously.
SUBSCRIBE orders.new orders.delete
Subscribe to a pattern of channels
The PSUBSCRIBE command uses glob-style patterns to subscribe a client to any channel that matches the specific pattern. There are three primary operators that you can use in a glob-style pattern:
| Operator | Description | Example | Matches | Does not match |
|---|---|---|---|---|
? |
Matches any single character | l?arn |
learn, loarn |
larn, lern |
* |
Matches any content (including none) | lear* |
learn, learaeiou |
larn, lern |
[] |
Matches only characters within the list | le[ao]rn |
learn, leorn |
lern, leurn |
For example, use the PSUBSCRIBE command to subscribe to all channels that begin with a prefix of inventory.
PSUBSCRIBE inventory.*
Alternatively, as another example, use the PSUBSCRIBE command to subscribe to all channels with the suffix of .new.
PSUBSCRIBE *.new
In this example, the PSUBSCRIBE command is used to subscribe to all channels that include orders or staff as a whole word in the name.
PSUBSCRIBE *orders* *staff*
Unsubscribe from channels
Once a client no longer wishes to receive messages from a specific channel, the client should unsubscribe from that channel or pattern of channels.
Redis includes an UNSUBSCRIBE command to remove a client’s subscription from one or more channels. The command is flexible enough to support unsubscribing from multiple channels, like the SUBSCRIBE command.
To unsubscribe from a pattern of channels, Redis includes a PUNSUBSCRIBE command that functions similar to the PSUBSCRIBE command.
There are three common ways to unsubscribe from a channel or channels.
Unsubscribe from one or more known channels
The client can unsubscribe from specific channel or channels by providing one or more channels to the UNSUBSCRIBE command.
UNSUBSCRIBE staff.newhire
UNSUBSCRIBE orders.new orders.delete
Unsubscribe from one or more channel patterns
The client can also unsubscribe from one or more patterns by providing each of those patterns to the PUNSUBSCRIBE command.
PUNSUBSCRIBE inventory.*
PUNSUBSCRIBE inventory.* orders.* staff.*
Unsubscribe from all channels or patterns of channels
If the client wants to unsubscribe from all known subscriptions, the client can invoke the UNSUBSCRIBE command with no arguments.
UNSUBSCRIBE
The UNSUBSCRIBE command unsubscribes the client from all channels the client specifically subscribed to. The command doesn't unsubscribe the client from pattern-based subscriptions. The PUNSUBSCRIBE command is used to unsubscribe the client from pattern-based subscriptions. Invoke the PUNSUBSCRIBE command with no arguments to unsubscribe the client from all pattern-based subscriptions.
PUNSUBSCRIBE
Publishing a message to a channel
Any client can send messages by publishing the message to a channel. Azure Cache for Redis automatically routes the message to clients with matching subscriptions. For example, if a client publishes a message to the orders.new channel, Azure Cache for Redis routes the message to:
- Clients subscribed explicitly to the orders.new channel
- Clients subscribed to a pattern that matches orders.new (Example: orders.*)
Diagram of a server application sending a message to Azure Cache for Redis on the orders.new channel and multiple clients receiving the message based on both direct and pattern-based subscriptions.
Redis includes a PUBLISH command that takes in two arguments. The first argument is the channel's name where the message is published. The second argument is the string content of the message.
Consider these two examples. The PUBLISH command is used to send a message with the string content sad348957298s534gh to the orders.delete channel. The PUBLISH command is also used to send a message with content 02a67b49-9da1-487e-8b49-d5aad3f514ae to the staff.newhire channel.
PUBLISH orders.delete sad348957298s534gh
PUBLISH staff.newhire 02a67b49-9da1-487e-8b49-d5aad3f514ae
Tip
If a client subscribes to a pattern and a known channel with overlap, the client could potentially receive multiple messages. For example, consider a client subscribed to staff.* and staff.retire. If a message is published to the staff.retire channel; the client will receive the message twice. One receipt will be for the known channel name subscription, and the other will be for the pattern match subscription.