Redis Diagnose

Redis Diagnose

Diagnose and fix common issues with StackExchange.Redis.Extensions.

When to use

When the user reports:

• Timeout exceptions
• Connection failures
• Serialization errors
• Pool exhaustion
• Pub/Sub messages not being received
• Distributed lock failures
• Lua scripting errors
• Performance issues

Diagnostic Tree

RedisTimeoutException

Symptoms: StackExchange.Redis.RedisTimeoutException

Check in order:

1. SyncTimeout too low — default is 5000ms, increase for slow networks

   config.SyncTimeout = 10000; // 10 seconds
   

2. Pool size too small — default 5, increase for high-throughput

   config.PoolSize = 10;
   

3. Connection strategy — switch to LeastLoaded if using RoundRobin

   config.ConnectionSelectionStrategy = ConnectionSelectionStrategy.LeastLoaded;
   

4. Large values — enable compression to reduce payload size

   services.AddRedisCompression<LZ4Compressor>();
   

5. ThreadPool starvation — check with ThreadPool.GetAvailableThreads(), increase min threads

RedisConnectionException

Symptoms: SocketClosed, ConnectionFailed

Check:

1. Redis server reachable? — redis-cli -h <host> -p <port> ping
2. Firewall/NSG rules — port 6379 (or 6380 for TLS) open?
3. TLS misconfiguration — if using Ssl=true, check certificate callbacks
4. Sentinel misconfiguration — ServiceName must match Redis master name exactly
5. Azure Cache for Redis — ensure Managed Identity is configured if not using password

Serialization Errors

Symptoms: JsonException, InvalidOperationException, corrupt data

Check:

1. Type mismatch — GetAsync must use same T as AddAsync
2. String values are JSON-encoded — "hello" is stored as ""hello"", this is by design
3. Compression migration — enabling compression makes old (uncompressed) data unreadable
   • Error: InvalidOperationException: Failed to decompress data from Redis
   • Fix: flush the database or read old data without compression first
4. Value type quirk — GetAsync<int>() returns 0 (not null) for missing keys because default(int) is 0. Use GetAsync<int?>() to distinguish missing keys from actual zero values.

Pub/Sub Not Receiving Messages

Check:

1. KeyPrefix — channels are automatically prefixed. Don't add prefix manually.
2. Serializer mismatch — publisher and subscriber must use the same serializer
3. Handler exceptions — check logs for EventId 4001 errors. Handlers that throw don't crash but the message is lost.
4. Different connection pools — ensure pub and sub use the same IRedisDatabase instance

Pool Exhaustion / All Connections Down

Symptoms: All operations fail, logs show EventId 1003

Check:

1. Pool health — inject IRedisClient and call client.ConnectionPoolManager.GetConnectionInformation()
2. Use health check — register builder.Services.AddHealthChecks().AddRedisExtensionsHealthCheck() to monitor pool status automatically (returns Healthy/Degraded/Unhealthy)
3. Redis server overloaded — check INFO clients on Redis
4. Network partition — the pool skips disconnected connections automatically and logs warnings
5. Dispose pattern — ensure IRedisConnectionPoolManager is not disposed prematurely

IDistributedCache Issues

Symptoms: Data not found, expiration not working, migration issues

Check:

1. Registration order — AddRedisDistributedCache() must be called after AddStackExchangeRedisExtensions<T>()
2. KeyPrefix applies — IDistributedCache goes through IRedisDatabase.Database which uses WithKeyPrefix. Cache keys are prefixed automatically.
3. Migration from Microsoft provider — hash schema is compatible (data/absexp/sldexp fields), but key prefix format may differ (this library uses KeyPrefix, Microsoft uses InstanceName)
4. Sliding expiration not refreshing — Get and Refresh both refresh the TTL. Check that SlidingExpiration was set in DistributedCacheEntryOptions

Keyed DI Not Resolving

Symptoms: [FromKeyedServices("name")] returns null

Check:

1. Config must have a Name — RedisConfiguration.Name must be non-empty for keyed registration
2. Use eager overloads — keyed services are only registered with the overloads that receive RedisConfiguration directly, NOT the Func<IServiceProvider, ...> overload
3. Name must match exactly — [FromKeyedServices("cache")] must match config.Name = "cache" (case-sensitive)

Distributed Lock Issues

Symptoms: Lock not acquired, deadlocks, lock lost during processing

Check:

1. LockAcquireAsync returns null — increase maxRetries or retryDelay, the resource may be legitimately contended
2. Lock expires during processing — use lockObj.ExtendAsync(TimeSpan) to extend the TTL before it expires
3. Lock not released on error — always use await using pattern, never try/finally with manual release
4. Deadlock — ensure lock expiry is always set. If the holder crashes, the lock auto-expires
5. This is a single-instance lock — for Redis Cluster, consider Redlock algorithm

Lua Script Errors

Symptoms: RedisServerException, NOSCRIPT, wrong return type

Check:

1. NOSCRIPT error — the script was cached but the server restarted. SE.Redis retries with EVAL automatically, but check for transient failures
2. Wrong return type — Lua return 1 returns long, return "hello" returns string. Use explicit casts on RedisResult
3. CROSSSLOT error in Cluster — all KEYS[] must hash to the same slot. Use hash tags: {user}:counter, {user}:data
4. Script blocks Redis — Lua runs atomically, blocking the event loop. Keep scripts short (<1ms). Use read-only variant (ScriptEvaluateReadOnlyAsync) for read operations to route to replicas
5. Typed deserialization fails — ScriptEvaluateAsync<T> passes the raw bytes through ISerializer. Ensure the script returns a value serialized in the same format

Performance Issues

Check:

1. Enable logging — set log level to Debug to see connection selection

   { "Logging": { "LogLevel": { "StackExchange.Redis.Extensions.Core": "Debug" } } }
   

2. Check outstanding commands — pool info shows outstanding count per connection
3. Use compression for large objects — LZ4 adds ~1ms latency but reduces network 5-10x
4. Use AddAllAsync for bulk writes instead of loop of AddAsync
5. Use GetAllAsync for bulk reads (note: requires HashSet<string> for keys, not arrays)

Logging Reference

Enable with:

{ "Logging": { "LogLevel": { "StackExchange.Redis.Extensions.Core": "Information" } } }