docs: add redis docs: used dbs, setup tips

which redis db is used by what
which databases are available, what's the limit
mention memory overcommitting and maxmemory settings

Author: sebix

docs/admin/configuration/redis.md

@@ -0,0 +1,44 @@
+# Redis Pipeline (Message broker)
+<!-- comment
+   SPDX-FileCopyrightText: 2025 Sebastian Wagner, Intevation GmbH <sebix@sebix.at>
+   SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
+The default IntelMQ Pipeline (the message exchange between bots) is [Redis](https://redis.io/) or its OpenSource successor [Valkey](https://valkey.io/).
+
+For AMQP (RabbitMQ) see [Using AMQP Message Broker](../beta-features.md#using-amqp-message-broker) in the section about Beta features.
+
+## Usage of databases
+
+You can use any redis database for any purpose. There are no hardcoded defaults or other requirements in IntelMQ.
+
+It's also possible to use the same database for two different bots. Just make sure, that their data doesn't collide.
+
+These are some of the usages of redis databases using the IntelMQ default values:
+
+- 2: Pipeline (Queues)
+- 4: IntelMQ Tests
+- 6: Deduplicator Expert
+- 7: Reverse DNS Expert
+- 8: RDAP Expert, Aggregate Expert
+- 10: RIPE Expert
+- 12: Shadowserver Reports API Collector
+- 15: SMTP Batch Output
+
+By default, Redis/valkey have a maximum of 16 databases (0-15). In it the Redis/valkey server configuration file, this value can be increased.
+While the number of maximum databases [is unlimited](https://redis.io/docs/latest/embeds/how-many-databases-software/), the practical limit is INT_32.
+
+## Setup tips
+
+SWAP space should be bigger or equal to your memory. See also [Hardware requirements](../hardware-requirements.md).
+
+### Redis memory overcommitting
+
+It is [recommended](https://redis.io/docs/latest/operate/oss_and_stack/management/admin/#linux) to enable memory overcommitting for Redis.
+
+Run `sysctl vm.overcommit_memory=1` to set it for the current session.
+To enable it permanently, create a file with `vm.overcommit_memory = 1` in `/etc/sysctl.d/intelmq.conf`.
+
+### Maxmemory
+
+To limit the maximum memory used by Redis and also to mitigate possible downsides of memory overcommitting, you can set a maximum memory usage in the Redis/Valkey server configuration with setting `maxmemory <bytes>`.

mkdocs.yml

@@ -116,6 +116,7 @@ nav:
           - IntelMQ: 'admin/configuration/intelmq.md'
           - IntelMQ API: 'admin/configuration/intelmq-api.md'
           - IntelMQ Manager: 'admin/configuration/intelmq-manager.md'
+          - Redis: 'admin/configuration/redis.md'
       - Management:
           - IntelMQ: 'admin/management/intelmq.md'
           - IntelMQ API: 'admin/management/intelmq-api.md'