Every single connection to Postgres is a new process, which requires a fork and new memory allocation (at least 10MB plus whatever you need for your query).
replyPgBouncer opens a pool of connections and then reuses them each time a client asks for a connection. This reduces latency (no more fork) and overhead (reuse memory).
If it's designed well, your application also opens a pool of connections and re-uses them each time the code needs a DB connection.
replyThe issue is language ecosystems that don't use client side connection pooling because they're single threaded (node, Python). So scaling up the number of web server threads means scaling the number of Postgres processes, which are expensive.
replyThis solves the latency but not the overhead--you'll end up with a full pool of connections for each running instance of your application.
replyHow many running instances do you need though? e.g. Scala web frameworks should be able to do thousands of RPS on a single core without the application developer really trying to optimize anything, and I always hear that even Ruby, Python, etc. are also fast enough to be IO bound so you should just need 2 copies for redundancy, right? Then give each like 8-16 connections.
replyIf I'm implementing my own connection pool, I'll certainly make the number of connections configurable.
replyThat's fine but doesn't address the issue that PgBouncer does. Your application connection pool can multiplex all the connections needed in one application. PgBouncer can multiplex the connections across all applications (whether different apps or many instances of the same app).
replyIf you handle database requests naively, every request to the database may open its own connection. This is a super simple approach but will exceed the amount of connections the database can or wants to handle concurrently.
replyOne solution is to increase your app complexity and introduce a layer that manages connection pooling or queuing.
Or you can just keep your app naive and simple and put pgbouncer transparently in front of your DB. Even for multiple apps, so instead of every app increasing in complexity, reimplementing connection handling, you just have pgbouncer.