feat: Add Redis cache adapter

[vazarkevych]

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Add Redis cache adapter (growthbook-cache-redis)

Summary

Adds a new optional Gradle module, growthbook-cache-redis, that provides a Redis-backed implementation of the SDK's GbCacheManager. This enables sharing the GrowthBook feature cache across multiple instances/pods instead of keeping it in-process or on the local filesystem.

Two Redis clients are supported out of the box:

• Jedis — JedisGbCacheManager
• Lettuce — LettuceGbCacheManager

Both client dependencies are compileOnly, so consumers include only the client they actually use; nothing is forced onto the SDK's runtime classpath.

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Design

Shared base, thin adapters

AbstractRedisGbCacheManager contains the shared logic:

• key namespacing
• Redis hash ↔ entry mapping
• cache-miss vs failure handling
• FeatureCacheException wrapping

Each adapter only implements low-level Redis operations:

• writeHash
• readHash
• deleteByPrefix

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Storage format

Each cache entry is stored as a Redis hash with:

• data
• updatedAt (epoch millis)

RedisCacheEntry centralizes field names and mapping so both adapters use the same on-the-wire format.

getLastUpdatedMillis() is backed by updatedAt, enabling freshness-based cache refresh skipping in the SDK.

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Configuration (self-typed builder)

AbstractRedisCacheOptions provides configuration:

• keyPrefix
  Namespace for all keys (default: growthbook:features:).
  clearCache() removes only keys under this prefix via SCAN + DEL (no blocking KEYS).

• ttl
  Optional per-entry TTL applied on write. null means no expiration.

• clock
  Injectable clock for deterministic testing.

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Connection ownership

The caller owns the lifecycle of:

• JedisPool
• StatefulRedisConnection

The adapter only borrows connections and executes commands.

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Error handling

• Missing key → treated as cache miss (null)
• Real Redis failures → wrapped in FeatureCacheException with the affected key for context

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Usage

Jedis

GbCacheManager cache = JedisGbCacheManager.builder()
        .jedisPool(jedisPool)
        .keyPrefix("growthbook:features:")
        .ttl(Duration.ofMinutes(10))
        .buildManager();

Lettuce

GbCacheManager cache = LettuceGbCacheManager.builder()
        .connection(connection)
        .buildManager();

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Testing

Unit tests

• JedisGbCacheManagerTest
• LettuceGbCacheManagerTest
• Implemented with Mockito

Integration tests

• RedisCacheIntegrationTest
• Runs against real Redis via Testcontainers
• Located in a separate integrationTest source set
• Exposed via opt-in integrationTest Gradle task
• Not included in check, so standard builds do not require Docker

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Build / packaging

• New module registered in settings.gradle
• Targets Java 8 (sourceCompatibility / targetCompatibility = 1.8)
• Publishes sources and Javadoc jars to GitHub Packages
• Fully consistent with existing lib module