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
• Development or debugging tools maintaining idle connections

Solutions#

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

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

2. Implement connection pooling with tools like PgBouncer or Pgpool-II:

   # 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

3. Terminate idle connections to free up connection slots:

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

4. Check for connection leaks in your application code:

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

Prevention#

1. Use connection pooling in your application:

   • Most modern frameworks and libraries support connection pooling
   • Configure pool size based on workload patterns

2. Set appropriate timeouts:

   -- Close idle connections after a period of inactivity
   SET idle_in_transaction_session_timeout = '5min';
   SET statement_timeout = '30s';

3. Monitor connection usage:

   SELECT count(*), datname, usename, state
   FROM pg_stat_activity
   GROUP BY datname, usename, state
   ORDER BY count(*) DESC;

4. Design applications to use fewer connections:

   • Batch operations where possible
   • Reduce transaction durations
   • Use asynchronous processing for long-running operations