Postgres Heroku Connection limit details - Rails application - ROR

max_connection defines the number of Postgres instances running simultaneously to serve queries. max_connections open up multiple connection for same postgres user. It can increase performance in some cases but this can lead to poor performance like these:

• Disk contention: If you are using disk(not RAM) then more tables and indexes to be accessed at the same time, causing heavier seeking all over the disk
• RAM: This leads more RAM usage.
• Synchronization: You need to handle synchronization of multiple instances which will effect the overall throughput.

If you are user base increases, then you may face too many clients error due to limited number of connections. Then there is a concept of

The solution is to two fold:

• pool connections: If Postgres is failing to handle the situation, you can use tools like pgbouncer, repmgr and PgPool to handle pool connection before going to Postgres.
• Application optimization: Try to open as less connections as possible and close the connection as soon as work is done.

You can study further on this from following links:

• https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server

• http://www.postgresql.org/docs/current/static/runtime-config-connection.html#GUC-MAX-CONNECTIONS

You can study particularly for Ruby here: https://devcenter.heroku.com/articles/concurrency-and-database-connections#maximum-database-connections