ERROR 53300: Too many connections

Error Message#

ERROR: sorry, too many clients already
DETAIL: All connection slots are currently in use.
HINT: Consider increasing max_connections (currently 100).

Description#

This error occurs when you try to connect to a PostgreSQL server that has already reached its maximum number of concurrent connections. The server rejects new connections to prevent overloading system resources.

Causes#

• Application connection leaks (connections not properly closed)
• Insufficient connection pooling
• Database under heavy load with many clients
• max_connections parameter set too low for your workload
• Long-running queries or transactions holding connections

Solutions#

1. Implement connection pooling with tools like PgBouncer:

   # Example PgBouncer configuration in pgbouncer.ini
   [databases]
   mydb = host=localhost port=5432 dbname=mydb
   
   [pgbouncer]
   pool_mode = transaction
   max_client_conn = 1000
   default_pool_size = 20

2. Terminate idle connections to free up connection slots:

   -- Find idle connections
   SELECT pid, datname, usename, state
   FROM pg_stat_activity
   WHERE state = 'idle' AND current_timestamp - state_change > interval '10 minutes';
   
   -- Terminate specific connection
   SELECT pg_terminate_backend(pid);

3. Increase the maximum connections (requires server restart):

   -- Check current setting
   SHOW max_connections;
   
   -- In postgresql.conf:
   # max_connections = 200

4. Fix connection leaks in your application code:

   • Ensure connections are closed in finally blocks
   • Use connection pooling libraries
   • Set appropriate connection timeouts

Prevention#

• Use connection pooling in your application
• Monitor connection usage regularly
• Set appropriate timeouts for idle connections
• Design applications to use fewer connections (batch operations, shorter transactions)