How to diagnose and fix the 53300 too_many_connections error code in Postgres.

The 53300 error code in PostgreSQL indicates a too_many_connections error. This error occurs when a user tries to establish a new database connection and the number of current connections exceeds the maximum number of connections that the PostgreSQL server is configured to allow.

To diagnose and fix this issue, consider the following steps:

Diagnose the Issue

  1. Check Current Connections: Use the following query to check the current active connections to the PostgreSQL server: SELECT * FROM pg_stat_activity;
  2. Check Maximum Allowed Connections: Look at the max_connections setting to see the maximum number of connections allowed: SHOW max_connections;

Fix the Issue

  1. Close Unnecessary Connections: If there are idle or unneeded connections, consider closing them to free up space for new connections. You can identify idle connections by looking at the state column in the pg_stat_activity view.
  2. Increase max_connections: If you consistently hit the connection limit and have the resources to handle more connections, you can increase the max_connections setting in the postgresql.conf file: max_connections = <new_limit> After changing this setting, you must restart the PostgreSQL server for the changes to take effect.
  3. Implement Connection Pooling: If increasing max_connections is not feasible or does not solve the problem, consider implementing connection pooling. Connection pooling allows you to manage a pool of connections that can be reused, reducing the number of connections that need to be opened and closed and thus the total number required. Popular connection poolers for PostgreSQL include PgBouncer and Pgpool-II.
  4. Optimize Application Logic: Review your application code to ensure it is efficiently opening and closing connections. Connections should be closed as soon as the work requiring the connection is completed.


Example 1: Closing Idle Connections

If you find idle connections that can be closed, you might terminate them using the pg_terminate_backend function:

SELECT pg_terminate_backend(pid)
FROM pg_stat_activity
WHERE -- conditions to identify idle connections, such as:
      state = 'idle'
  AND state_change < current_timestamp - INTERVAL '10 minutes';

Example 2: Increase max_connections

To increase the max_connections setting, edit the postgresql.conf file:

max_connections = 200  -- (or another appropriate value)

Then restart the PostgreSQL server:

sudo service postgresql restart

Example 3: Use Connection Pooling

To set up PgBouncer, a popular connection pooler, you would:

  1. Install PgBouncer.
  2. Configure PgBouncer by setting up a pgbouncer.ini file with the necessary connection information and pool size.
  3. Start the PgBouncer service.

Your applications would then connect to PostgreSQL through PgBouncer instead of directly to the database server.

General Tips

  • Regularly monitor your database connections and performance to anticipate and address issues before they lead to errors.
  • Be cautious when terminating connections and changing configuration settings to avoid disrupting active users and applications.
  • Consider setting up alerts for when the number of connections approaches the limit, allowing you to take proactive measures.

By managing your database connections carefully and making configuration changes as necessary, you can prevent the 53300 too many connections error in PostgreSQL and ensure that your database server runs smoothly.

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