Debugging
Redis is single-threaded (for command execution), so any blocking operation stops the server from processing other commands. This leads to:
- High latency
- Timeouts in applications
- Replication lag
- Slow clients
- Stuck transactions
- Load imbalance
Understanding and debugging blocking operations is crucial for Redis performance and stability.
Common Blocking Commands
A blocking operation is any Redis command that waits for something instead of responding immediately.
| Command | Blocking Reason |
|---|---|
BLPOP, BRPOP, BRPOPLPUSH | Waits for elements in a list |
XREAD BLOCK | Waits for new messages in a stream |
XREADGROUP BLOCK | Waits for pending stream entries |
WAIT | Waits for replication |
DEBUG SLEEP | Explicitly blocks Redis execution |
| Long Lua scripts | Redis cannot process other commands until script finishes |
| Slow commands scheduled by clients | Eg. KEYS *, LRANGE biglist 0 -1, HGETALL huge_hash |
To debug blocking
| Tool | Purpose |
|---|---|
CLIENT LIST | Find blocked clients |
SLOWLOG | Find slow/expensive commands |
MONITOR | Live command stream |
INFO clients | Count blocked clients |
LATENCY DOCTOR | Analyze latency spikes |