Redis cache backend support limits and default configuration in Contao

Redis Cache Configuration in Contao

In Contao 4.x, Redis integration is handled via the underlying Symfony Cache component. Because Redis is an optional backend, there are no "hard-coded" framework defaults for connection strings; instead, the system relies on the configuration provided in the environment files or YAML packages.

Default Configuration and Selection Logic

Contao does not automatically "switch" between file-based and Redis caching based on availability. The selection is explicit: if the framework.cache.app (or a specific cache pool) is configured to use the Redis adapter, Symfony will attempt to use it. If the Redis DSN is missing or the required PHP extension (phpredis) is not installed, the system will typically throw a service error rather than silently falling back to the filesystem, unless a fallback chain is explicitly defined in the configuration.

Achieving Cache Consistency

To ensure consistent behavior across multiple installations—especially when sharing a single Redis instance—you must explicitly define a unique namespace. Without this, different Contao installations will overwrite each other's keys, leading to unpredictable cache hits and misses.

The following settings are required for a stable, consistent Redis deployment:

• Redis DSN: Explicitly define the host, port, and password (e.g., redis://redis-server:6379).
• Namespace/Prefix: Use a unique prefix for each installation to isolate data.
• Eviction Policy: Since the framework often relies on the server's global settings, ensure your Redis server is configured with an appropriate maxmemory-policy (such as allkeys-lru) to prevent the cache from crashing the server when full.

Verification Steps

To verify which adapter is currently active and whether Redis is receiving traffic, use these commands:

# Check the active container services
bin/console debug:container | grep cache

# Monitor Redis traffic in real-time
redis-cli monitor

Diagnostic Requirement

To provide a more specific configuration snippet, please specify the exact version of Contao 4.x you are using, as the transition between Symfony 4 and 5 changed the YAML syntax for cache pools.