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PostgreSQL

PG 13 - 17Connection & Pool

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FATAL: remaining connection slots are reserved for non-replication superuser connections

PostgreSQL "remaining connection slots are reserved": The Final Warning Before Outage

The relationship of reserved connection slots to max_connections, leveraging the superuser recovery window, and tracking down connection leaks.

Reviewed by:Senior Database Architect & SRE (dataiotech Labs)

Tested Versions: PostgreSQL 13 / 14 / 15 / 16 / 17

First 60 Seconds Emergency Protocol:

The error string is a symptom, not the root cause. Do not restart the server blindly. Run the diagnostic queries below to pinpoint the actual failure mode.

▸Symptoms

plaintext

FATAL: remaining connection slots are reserved for non-replication superuser connections

This message is the final recovery window before a complete connection lockout. The slots defined by superuser_reserved_connections (default: 3) are still available for administrative access.

▸Emergency Triage Steps

1Connect as Superuser immediately using psql -U postgres.
2Identify Top Consumers:

sql

   SELECT usename, application_name, client_addr, state, count(*)
   FROM pg_stat_activity
   GROUP BY 1, 2, 3, 4
   ORDER BY 5 DESC;

1Terminate Idle Sessions:

sql

   SELECT pg_terminate_backend(pid)
   FROM pg_stat_activity
   WHERE state = 'idle' AND pid <> pg_backend_pid();

1Enforce Role Connection Limits:

sql

ALTER ROLE app_user CONNECTION LIMIT 40;

1Architectural Fix: Introduce PgBouncer in transaction mode and configure idle_in_transaction_session_timeout.

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