Redis Monitoring: Cache Hit Rates, Memory Pressure, and Eviction Strategies
Redis is the silent performance multiplier of most web applications. A healthy Redis with a 95%+ cache hit rate means your database handles a fraction of the read load it would otherwise need to. A degraded Redis means every user request is slow.
When Redis is healthy, your app is fast. When it is not, every request hits your database and your API slows to a crawl. Monitoring Redis is monitoring the speed of your entire application.
The 5 Redis metrics that matter
1. Cache hit rate — (hits / (hits + misses)). Below 90% means most requests are cache misses.
2. Memory usage % — Alert at 80%. At 100%, Redis starts evicting keys according to your eviction policy.
3. Eviction rate — Any eviction means Redis is full and discarding data. This usually causes a cascade to the database.
4. Connected clients — Spike means connection leak; drop means the pool is exhausted.
5. Command latency — GET/SET should be <1ms. If p99 is >10ms, something is wrong.
Health endpoint for Redis
Create a health endpoint that checks Redis: ```python @app.route('/health/redis') def redis_health(): try: redis_client.ping() info = redis_client.info() hit_rate = info['keyspace_hits'] / max(info['keyspace_hits'] + info['keyspace_misses'], 1) return {'status': 'ok', 'hit_rate': hit_rate}, 200 except Exception as e: return {'status': 'error'}, 503 ```
Monitor this endpoint on AlertsDock.
Eviction policy selection
Choose your eviction policy deliberately: - allkeys-lru — evict least-recently-used keys when full. Good for pure caching. - volatile-lru — evict only keys with an expiry set. Good for mixed cache/persistent data. - noeviction — return errors when full. Good for session storage (you don't want sessions silently lost).
For pure caching: allkeys-lru. For session storage: noeviction with aggressive alerting at 70% memory.
Monitoring Redis replication
If you run Redis replicas: - Alert when replica-master replication offset exceeds 100k bytes (significant lag) - Alert when replica count drops below expected (replica died) - Test read-from-replica with a synthetic check
Redis as a critical monitoring dependency
If your app uses Redis for session storage, rate limiting, or caching, a Redis outage is a service outage. Monitor Redis availability separately with its own AlertsDock monitor and configure it as a critical alert.
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Uptime Monitoring
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