How rails database connection pool works

Question

I am learning rails database connection pool concept. In rails application I have defined pool size of 5.

my understanding about connection pool size is as below.

1. When server start rails automatically creates n number of connection defined in the database.yml file. In my case it will create 5 connection since pool size is 5.

2. On every http request if there is need to access database then rails will use available connection from the connection pool to serve the request.

But my question is if I hit 1000 request at a time then most of the request will not get access to database connection because my connection pool size is only 5.

Is my above understanding about rails connection pool is right??

Thanks,

Answer 1

Purpose:
Database connections are not thread safe; so ActiveRecord uses separate database connection for each thread.

Limiting factor:
Total database connections is limited by the database server you use (e.g Posgres: default is typically 100 or lesser), by your app server's configuration (number of processes/threads available) and Active Record's configuration : Connection Pool defaults to 5 .

Pool size:
Active Record's pool size is for a single process. A thread uses a connection from this pool and releases it automatically afterwards. (unless you spawn a thread yourself, then you'll have to manually release it). If your application is running on multiple processes, you will have 5 database connections for each of them. If your server is hit by 1000 requests concurrently, it will distribute the requests among these connections, when it gets full, rest of the requests wait for their turn.

Read more at:
https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/ConnectionPool.html