A PostgreSQL server has a connection limit for all clients. If there are too many client connections, it may lead to random connection closing. You have to thoroughly calculate client instance count in the cluster and serverside connection limit to get maximum client pool size. Moreover, you have to recalculate the limit in case of cluster resize.
It is a whole thing to maintain the limit all the time and it is error-prone. A wrong setup might lead to incidents.
That's why we have "connlimit-mode: auto" that override the components::Postgres static config option max_pool_size.
The PostgreSQL driver allows one to automatically determine the required max connection limit. It is calculated as follows:
server_max_connections is calculated based on SHOW max_connections answer and on the current user connection limit. The service identifies instanses based on the following algorithm. The service creates u_clients table and regularly writes down information about itself. After that the service reads the table and identifies alive instances. reserved is set to be 5 to reflect administrative scripts connections, migration scripts, etc.
The limit is promptly changed after service topology change: node addition/removal, service instance stop, etc.
The feature can be turned on/off in runtime via the dynamic config variable POSTGRES_CONNLIMIT_MODE_AUTO_ENABLED. If disabled dynamically, the previous connection limit settings are used.
If you want to disable the feature permanently for some clusters (e.g. in case the current user may not create u_clients table), you can do it in static config for components::Postgres:
This error tells that there are no available connections in the pool. But it doesn't mean that the service exceeded the limit of the number of connections.
The pg-driver will try to create a new connection asynchronously (with the deadline from the initial request). If another connection is released from another request, driver will take it. If request spikes are expected, you might increase the min_pool_size option (default 4) in components::Postgres config to prepare some available connections for these request spikes. Also, if there are logs such as Connecting: X. Max concurrent connecting: X., you might increase the connecting_limit option (default unlimited) in components::Postgres config. Also, you can change these options by the dynamic config POSTGRES_CONNECTION_POOL_SETTINGS
recent-errors-threshold (default 2) in the dynamic config POSTGRES_CONNECTION_SETTINGS. Errors accounted for the purposes of recent-errors-threshold are: 1) A timeout error when creating a connection 2) A ConnectionError (aborted connections and pipeline mode errors)