通道与键空间通知

So far you have published to explicit channels with PUBLISH. Redis also offers keyspace notifications, which automatically emit events when keys change. Both deliver messages to subscribers, but the source differs.

With explicit channels, your application decides what to publish and when. The channel name is arbitrary text.

Keyspace notifications let Redis itself publish an event whenever a key is modified, expired, or evicted. You subscribe to special channels to observe data changes without your app having to publish anything.

They are off by default. Enable them with the notify-keyspace-events config flag, where letters select event classes.

• K keyspace events
• E keyevent events
• A all command/event types
• x expired, g generic, $ string

There are two channel families:

• Keyspace: __keyspace@0__:mykey tells you which event happened to a key
• Keyevent: __keyevent@0__:del tells you which keys had a given event

The @0 is the database number.

A common use case is reacting when a key expires, for example to clean up a session. Enable Ex and subscribe to the expired keyevent channel.

Set a session key with a TTL. When it expires, Redis publishes to the expired channel, and your worker reacts.

Keyspace notifications use Pub/Sub, so they are fire-and-forget. If no client is subscribed when the event fires, the message is lost. They are also not guaranteed for keys that expire while idle until a read or the background expiry cycle touches them.

Use explicit channels when your app controls the event semantics (chat, notifications, custom events). Use keyspace notifications when you want to react to data mutations you do not directly control, like cache invalidation or expiry cleanup.

Enabling A (all events) on a busy server generates a high volume of messages and adds overhead. Subscribe only to the specific event classes you need.

Explicit channels are app-driven; keyspace notifications are Redis-driven. Both ride on the same lightweight Pub/Sub transport, so neither persists messages.

Pick the correct statement about keyspace notifications.

You compared explicit Pub/Sub channels with keyspace notifications, learned how to enable notifications via notify-keyspace-events, the keyspace vs keyevent channel families, and the fire-and-forget delivery caveats. Reach for keyspace notifications to react to data changes without publishing them yourself.