# Using Redis or Valkey {#using-redis-or-valkey}


Shlink can take advantage of a shared cache server like [redis](https://redis.io/) or [valkey](https://valkey.io/) in multiple contexts.

- For [caching purposes](#caching-purposes).
- To store [shared locks](#shared-locks).
- To publish [real-time updates](https://sourcey-shlink.pages.dev/documentation/advanced/real-time-updates/index.md#redis-pubsub) via Pub/Sub.


### Caching purposes {#caching-purposes}


Shlink can persist certain things in a faster storage for quick access.

By default, it uses APCu if available (when using a [classic web server](https://sourcey-shlink.pages.dev/documentation/supported-runtimes/classic-web-server/index.md)) or in-memory (when using [Roadrunner](https://sourcey-shlink.pages.dev/documentation/supported-runtimes/serve-with-roadrunner/index.md) as the runtimes), but it can be provided with some shared cache server access configuration.

When doing so, Shlink will use it as the cache persistence.


### Shared locks {#shared-locks}




> [!IMPORTANT]
> **Note**
>
> **Locks** are used to make sure certain tasks are not run more than once in parallel. For example, geolocating visits.



If you plan to run multiple Shlink instances, it's important to make sure the so-called "locks" are shared between all of them.

A shared cache server is a good way to do so. If Shlink is provided with access configuration, it will use it to store shared locks.


## Supported capabilities {#supported-capabilities}


Shlink supports multiple shared cache server capabilities. They can be enabled by providing different connection URIs:

- **Single instance** ([Redis](https://redis.io/docs/latest/operate/oss_and_stack/install/install-stack/) / [Valkey](https://valkey.io/topics/installation/)), when providing just one server (for example, `tcp://my_redis_server:6379`).
- **Cluster** ([Redis](https://redis.io/docs/latest/operate/oss_and_stack/management/scaling/) / [Valkey](https://valkey.io/topics/cluster-tutorial/)), when providing multiple servers (for example, `tcp://one_redis_server:6379,tcp://another_redis_server:6379`).
- **Sentinels** ([Redis](https://redis.io/docs/latest/operate/oss_and_stack/management/sentinel/) / [Valkey](https://valkey.io/topics/sentinel/)), when providing multiple servers, together with a sentinel service name.
- Database selection (since Shlink 4.2.1), by providing the database index in the URI path (for example, `tcp://my_redis_server:6379/5`).
- Credentials, that can be provided as part of the server URIs (whether they are redis instances or sentinels):
  - ACL, with username and password ([Redis](https://redis.io/docs/latest/operate/oss_and_stack/management/security/acl/) / [Valkey](https://valkey.io/topics/acl/)): `tcp://the_username:the_password@my_redis_server:6379`
  - Legacy, with just password ([Redis](https://redis.io/docs/latest/operate/oss_and_stack/management/security/) / [Valkey](https://valkey.io/topics/security/)): `tcp://:the_password@my_redis_server:6379`.
  - Separately via config or env vars, when using sentinels and redis instances have their own auth.


## Providing configuration {#providing-configuration}


As with any other configuration params, there are two main ways to provide shared cache server options:

- [Environment variables](https://sourcey-shlink.pages.dev/documentation/environment-variables/index.md#redis-integration), which allow you to enable the integration however you like.
- The [installation tool](https://sourcey-shlink.pages.dev/documentation/command-line-interface/installation-tool/index.md), which will, at some point, ask you if you want to integrate with redis, and request you to provide all the capabilities above if so.

  ```shell
   [...]

  INTEGRATIONS
  ============

   Do you want to use a redis instance, redis cluster or redis sentinels as a shared cache for Shlink? (recommended if you run a cluster of Shlink instances) (yes/no) [no]:
   > yes

   Provide a comma-separated list of URIs (redis servers/sentinel instances):
   > tcp://one_redis_server:6379,tcp://another_redis_server:6379

   Provide the name of the sentinel service (leave empty if not using redis sentinel):
   >

   Do you want Shlink to publish real-time updates in this Redis instance/cluster? (yes/no) [no]:
   > yes

   [...]
  ```