PostgreSQL error 53300 (too_many_connections) appears when the server has reached its max_connections limit and cannot accept new client sessions.
PostgreSQL error 53300 (too_many_connections) signals that the database has hit its max_connections count, blocking new sessions. Reduce idle clients, increase max_connections, or add a connection pooler to resolve the issue.
PostgreSQL Error 53300
PostgreSQL raises error code 53300 with the message "too many connections" when a new client tries to connect but the server has already allocated all available connection slots defined by the max_connections parameter.
The failure stops the session before authentication and can cascade across applications that rely on the same database, so fast remediation is critical for uptime.
Exceeding max_connections is the direct trigger. Each backend process claims one slot.
When slots equal the configured ceiling plus reserved superuser slots, PostgreSQL rejects further attempts.
Surges in traffic, connection leaks in application code, and long-running idle sessions frequently exhaust the pool. Misconfigured connection pooling or monitoring tools that open persistent sessions can also consume capacity.
Free unused sessions first.
Superusers can connect using the reserved slots, inspect pg_stat_activity, and terminate idle connections with pg_terminate_backend().
If workload legitimately needs more slots, increase max_connections in postgresql.conf and reload or restart. For high scale, add PgBouncer or Pgpool-II to multiplex thousands of logical clients onto fewer physical connections.
Web applications without pooling often open one connection per HTTP request.
Deploying PgBouncer in transaction mode caps physical sessions while keeping response latency low.
Background jobs that forget to close connections leak slots. Add ensureClose() logic or use connection-pool libraries with timeout cleanup.
Set max_connections based on RAM (roughly shared_buffers * 3) and expected peak load. Monitor pg_stat_activity and alert at 75 percent usage.
Adopt connection pools.
Galaxy's desktop SQL editor automatically reuses sessions and shows active connection counts, helping developers keep usage low during query testing.
FATAL: remaining connection slots are reserved for non-replication superuser connections – occurs when max_connections is hit by non-superusers.
FATAL: sorry, too many clients already (MySQL 1040) – the MySQL counterpart solved by similar pooling and parameter tuning.
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By default, three slots are kept free for roles with SUPERUSER so administrators can still connect and fix issues.
You can increase max_connections only with a postmaster restart. A plain reload applies if your new value is below the compiled maximum and shared memory allows.
Balance against RAM because each backend consumes memory. Many installations run 200-500 slots and rely on pooling for higher logical concurrency.
Galaxy reuses editor sessions, shows active connection counts in its UI, and encourages connection pooling best practices via inline tips.
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