Alert runbooks /

RedisTooManyConnections

The number of clients connected to Redis is approaching the maxclients limit.

What it means

The alert fires when connected clients have stayed at a large fraction of maxclients for several minutes. Once the limit is reached, Redis accepts the TCP connection and immediately closes it with ERR max number of clients reached, which applications see as intermittent connection failures.

The effective maxclients can also be lower than configured: Redis lowers it at startup if the process file descriptor limit is too small, and logs a warning when it does.

Common causes

• Connection leaks: clients that create a new connection per request and never close it.
• Many application replicas, each with a large connection pool.
• Idle connections never reaped because timeout is 0 (the default).
• Blocked clients (BLPOP, XREAD BLOCK) holding connections for a long time.
• A low file descriptor limit shrinking the usable maxclients.

First checks

1. Current count and limit:

   redis-cli -h <host> INFO clients | grep -E '^(connected_clients|blocked_clients)'
   redis-cli -h <host> CONFIG GET maxclients
   redis-cli -h <host> CONFIG GET timeout
   

2. Group connections by source IP to find the heavy client:

   redis-cli -h <host> CLIENT LIST | awk '{print $2}' | cut -d= -f2 | cut -d: -f1 | sort | uniq -c | sort -rn | head
   

3. Look for long-idle connections (the idle= field is seconds since last command):

   redis-cli -h <host> CLIENT LIST | awk '{for(i=1;i<=NF;i++) if ($i ~ /^idle=/) {split($i,a,"="); if (a[2] > 3600) print}}' | head
   

4. Check whether growth follows a deploy or scale-out:

   redis_connected_clients
   

Fixing it

Fix the leaking client or reduce pool sizes per replica. Set CONFIG SET timeout <seconds> so idle connections are closed (do not do this if clients rely on long-lived idle Pub/Sub connections). Kill specific offenders with CLIENT KILL ADDR <ip:port>. Raise maxclients only after checking ulimit -n for the Redis process allows it.

Related alerts

• RedisRejectedConnections: fires once the limit is actually hit.
• RedisMemoryHigh: each client adds buffer memory.
• RedisDown: a full client table can lock out the exporter.