Issue

The database is experiencing slowness and locks, and in addition the following error message appears in the logs although it is unclear what it means:

ConnectionBus: Opening JDBC connection to jdbc:postgresql://[...] failed with SQLState: 53300 Error code: 0 Message: "FATAL: remaining connection slots are reserved for non-replication superuser and rds_superuser connections", retrying...(2/4)

Environment

Applications hosted in Mendix Cloud

Cause

The error message states that the remaining available connection slots in the PostgreSQL database are reserved for specific connections, namely "non-replication superuser" and "rds_superuser" connections. This usually means that the database has reached its configured maximum number of connections that regular users can make, and the remaining slots are reserved for administrative and replication purposes. In short, the DB is exhausting its connections.

Solution/Workaround

The standard configuration sets a maximum of 50 connections per instance. If the environment is running out of connections, the number of connections can be increased. This setting is editable through a custom runtime setting described in Advanced Custom Settings in Mendix Runtime.

Additional information

• Mendix Documentation: Advanced Custom Settings in Mendix Runtime.
• Other Documentation: https://repost.aws/knowledge-center/rds-postgresql-error-connection-slots