ERR max number of clients reached

The server already has as many connections as its limit allows.

Redis returns this error when a new client connects, but the server already has as many connections as its limit allows. The limit is the maxclients setting, which is 10,000 by default. Redis sends the error and then closes the new connection.

ERR max number of clients reached

In a cluster, the connections from other cluster nodes count toward the limit too, and the message is ERR max number of clients + cluster connections reached.

In redis-py, this error is ConnectionError: max number of clients reached.

How to fix it

1. Check how many clients are connected with INFO clients. connected_clients shows the number of connections, and maxclients shows the limit.
2. Find out where the connections come from with CLIENT LIST. Each line shows a connection's address in addr, its name in name, and how many seconds it has been idle in idle. Many connections from the same app that stay idle for a long time usually mean that the app doesn't close them.
3. Fix the cause:
   • Your app opens a new connection for each request. Create one client, or one connection pool, when your app starts, and reuse it. For example, see Connect with a connection pool for redis-py.
   • Many copies of your app each keep a large pool. The total number of connections is the pool size multiplied by the number of copies. Make the pools smaller.
   • Idle connections never close. By default, Redis never closes idle connections. Set timeout to the number of seconds after which Redis closes an idle connection. See Client timeouts.
   • You need more connections. Raise maxclients. Each connection also uses a file descriptor, so check the operating system's limit too. See Maximum concurrent connected clients.