How keys map to hash slots and shards, how to add or remove shards and move slots safely with redis-cli, why multi-key commands fail across slots, and how to find hot keys and big keys that make one shard the bottleneck.
The redis-cli commands you reach for every week - connecting, health checks, finding keys safely, memory, clients, slow commands, replication, persistence, ACLs and cluster.
Redis is usually sub-millisecond. When p99 jumps, use SLOWLOG, the latency monitor and INFO to find the blocking command, fork, fsync or host problem, then fix it.
Redis deployments range from a single node to sharded clusters across zones. Decide by data size, throughput, availability needs and client support, understand what each topology changes for applications (multi-key commands, failover handling), and prefer managed services unless you have the skills to run Redis HA yourself.
A cache helps only if it stays correct and survives failures gracefully. Choose cache-aside, read-through or write-through per data type, set TTLs and invalidation rules, protect the database from stampedes when hot keys expire, and design the application to keep working when Redis is slow or down.
Redis keeps everything in RAM, so memory use is cost. Find what uses memory, store small objects in hashes with compact encodings, set TTLs on cache keys, choose the right eviction policy, watch fragmentation, and right-size the instance or cluster once usage is under control.
The Redis master is unreachable, clients get connection errors or READONLY, and Sentinel hasn't promoted a replica. Check Sentinel quorum and majority, replica eligibility and announced addresses, then force a failover and fix clients that bypass Sentinel.
Stop patching Redis VMs and Sentinel. Decide whether the data even needs migrating, pick the managed tier and clustering mode, move data with RDB import or dual-write where it matters, and change clients to handle TLS and cluster mode.
Read INFO memory, choose the right maxmemory-policy, find big keys and missing TTLs, and handle fragmentation on self-managed and managed Redis.
Run a single Redis or Valkey node safely. Covers when standalone is enough, RDB vs AOF persistence, maxmemory and eviction policies, kernel settings, ACLs and TLS, backups, and the signs that you need replication.
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